forked from seankross/the-unix-workbench
-
Notifications
You must be signed in to change notification settings - Fork 0
/
nephology.html
895 lines (855 loc) · 62.2 KB
/
nephology.html
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
599
600
601
602
603
604
605
606
607
608
609
610
611
612
613
614
615
616
617
618
619
620
621
622
623
624
625
626
627
628
629
630
631
632
633
634
635
636
637
638
639
640
641
642
643
644
645
646
647
648
649
650
651
652
653
654
655
656
657
658
659
660
661
662
663
664
665
666
667
668
669
670
671
672
673
674
675
676
677
678
679
680
681
682
683
684
685
686
687
688
689
690
691
692
693
694
695
696
697
698
699
700
701
702
703
704
705
706
707
708
709
710
711
712
713
714
715
716
717
718
719
720
721
722
723
724
725
726
727
728
729
730
731
732
733
734
735
736
737
738
739
740
741
742
743
744
745
746
747
748
749
750
751
752
753
754
755
756
757
758
759
760
761
762
763
764
765
766
767
768
769
770
771
772
773
774
775
776
777
778
779
780
781
782
783
784
785
786
787
788
789
790
791
792
793
794
795
796
797
798
799
800
801
802
803
804
805
806
807
808
809
810
811
812
813
814
815
816
817
818
819
820
821
822
823
824
825
826
827
828
829
830
831
832
833
834
835
836
837
838
839
840
841
842
843
844
845
846
847
848
849
850
851
852
853
854
855
856
857
858
859
860
861
862
863
864
865
866
867
868
869
870
871
872
873
874
875
876
877
878
879
880
881
882
883
884
885
886
887
888
889
890
891
892
893
894
895
<!DOCTYPE html>
<html >
<head>
<meta charset="UTF-8">
<meta http-equiv="X-UA-Compatible" content="IE=edge">
<title>Chapter 7 Nephology | The Unix Workbench</title>
<meta name="description" content="Chapter 7 Nephology | The Unix Workbench">
<meta name="generator" content="bookdown and GitBook 2.6.7">
<meta property="og:title" content="Chapter 7 Nephology | The Unix Workbench" />
<meta property="og:type" content="book" />
<meta property="og:url" content="https://github.com/seankross/the-unix-workbench" />
<meta property="og:image" content="https://github.com/seankross/the-unix-workbenchimages/cover.png" />
<meta name="github-repo" content="seankross/the-unix-workbench" />
<meta name="twitter:card" content="summary" />
<meta name="twitter:title" content="Chapter 7 Nephology | The Unix Workbench" />
<meta name="twitter:image" content="https://github.com/seankross/the-unix-workbenchimages/cover.png" />
<meta name="author" content="Sean Kross">
<meta name="date" content="2019-01-22">
<meta name="viewport" content="width=device-width, initial-scale=1">
<meta name="apple-mobile-web-app-capable" content="yes">
<meta name="apple-mobile-web-app-status-bar-style" content="black">
<link rel="prev" href="git-and-github.html">
<link rel="next" href="start-building.html">
<script src="libs/jquery-2.2.3/jquery.min.js"></script>
<link href="libs/gitbook-2.6.7/css/style.css" rel="stylesheet" />
<link href="libs/gitbook-2.6.7/css/plugin-table.css" rel="stylesheet" />
<link href="libs/gitbook-2.6.7/css/plugin-bookdown.css" rel="stylesheet" />
<link href="libs/gitbook-2.6.7/css/plugin-highlight.css" rel="stylesheet" />
<link href="libs/gitbook-2.6.7/css/plugin-search.css" rel="stylesheet" />
<link href="libs/gitbook-2.6.7/css/plugin-fontsettings.css" rel="stylesheet" />
<!-- Global site tag (gtag.js) - Google Analytics -->
<script async src="https://www.googletagmanager.com/gtag/js?id=UA-47412721-2"></script>
<script>
window.dataLayer = window.dataLayer || [];
function gtag(){dataLayer.push(arguments);}
gtag('js', new Date());
gtag('config', 'UA-47412721-2');
</script>
<style type="text/css">
a.sourceLine { display: inline-block; line-height: 1.25; }
a.sourceLine { pointer-events: none; color: inherit; text-decoration: inherit; }
a.sourceLine:empty { height: 1.2em; }
.sourceCode { overflow: visible; }
code.sourceCode { white-space: pre; position: relative; }
div.sourceCode { margin: 1em 0; }
pre.sourceCode { margin: 0; }
@media screen {
div.sourceCode { overflow: auto; }
}
@media print {
code.sourceCode { white-space: pre-wrap; }
a.sourceLine { text-indent: -1em; padding-left: 1em; }
}
pre.numberSource a.sourceLine
{ position: relative; left: -4em; }
pre.numberSource a.sourceLine::before
{ content: attr(data-line-number);
position: relative; left: -1em; text-align: right; vertical-align: baseline;
border: none; pointer-events: all; display: inline-block;
-webkit-touch-callout: none; -webkit-user-select: none;
-khtml-user-select: none; -moz-user-select: none;
-ms-user-select: none; user-select: none;
padding: 0 4px; width: 4em;
color: #aaaaaa;
}
pre.numberSource { margin-left: 3em; border-left: 1px solid #aaaaaa; padding-left: 4px; }
div.sourceCode
{ }
@media screen {
a.sourceLine::before { text-decoration: underline; }
}
code span.al { color: #ff0000; font-weight: bold; } /* Alert */
code span.an { color: #60a0b0; font-weight: bold; font-style: italic; } /* Annotation */
code span.at { color: #7d9029; } /* Attribute */
code span.bn { color: #40a070; } /* BaseN */
code span.bu { } /* BuiltIn */
code span.cf { color: #007020; font-weight: bold; } /* ControlFlow */
code span.ch { color: #4070a0; } /* Char */
code span.cn { color: #880000; } /* Constant */
code span.co { color: #60a0b0; font-style: italic; } /* Comment */
code span.cv { color: #60a0b0; font-weight: bold; font-style: italic; } /* CommentVar */
code span.do { color: #ba2121; font-style: italic; } /* Documentation */
code span.dt { color: #902000; } /* DataType */
code span.dv { color: #40a070; } /* DecVal */
code span.er { color: #ff0000; font-weight: bold; } /* Error */
code span.ex { } /* Extension */
code span.fl { color: #40a070; } /* Float */
code span.fu { color: #06287e; } /* Function */
code span.im { } /* Import */
code span.in { color: #60a0b0; font-weight: bold; font-style: italic; } /* Information */
code span.kw { color: #007020; font-weight: bold; } /* Keyword */
code span.op { color: #666666; } /* Operator */
code span.ot { color: #007020; } /* Other */
code span.pp { color: #bc7a00; } /* Preprocessor */
code span.sc { color: #4070a0; } /* SpecialChar */
code span.ss { color: #bb6688; } /* SpecialString */
code span.st { color: #4070a0; } /* String */
code span.va { color: #19177c; } /* Variable */
code span.vs { color: #4070a0; } /* VerbatimString */
code span.wa { color: #60a0b0; font-weight: bold; font-style: italic; } /* Warning */
</style>
<link rel="stylesheet" href="style.css" type="text/css" />
</head>
<body>
<div class="book without-animation with-summary font-size-2 font-family-1" data-basepath=".">
<div class="book-summary">
<nav role="navigation">
<ul class="summary">
<li><a href="./">The Unix Workbench</a></li>
<li class="divider"></li>
<li class="chapter" data-level="" data-path="index.html"><a href="index.html"><i class="fa fa-check"></i>Title Page</a></li>
<li class="chapter" data-level="" data-path="dedication.html"><a href="dedication.html"><i class="fa fa-check"></i>Dedication</a></li>
<li class="chapter" data-level="" data-path="acknowledgements.html"><a href="acknowledgements.html"><i class="fa fa-check"></i>Acknowledgements</a></li>
<li class="chapter" data-level="" data-path="introduction.html"><a href="introduction.html"><i class="fa fa-check"></i>Introduction</a></li>
<li class="chapter" data-level="1" data-path="what-is-unix.html"><a href="what-is-unix.html"><i class="fa fa-check"></i><b>1</b> What is Unix?</a></li>
<li class="chapter" data-level="2" data-path="getting-unix.html"><a href="getting-unix.html"><i class="fa fa-check"></i><b>2</b> Getting Unix</a><ul>
<li class="chapter" data-level="2.1" data-path="getting-unix.html"><a href="getting-unix.html#mac-ubuntu-users"><i class="fa fa-check"></i><b>2.1</b> Mac & Ubuntu Users</a></li>
<li class="chapter" data-level="2.2" data-path="getting-unix.html"><a href="getting-unix.html#windows"><i class="fa fa-check"></i><b>2.2</b> Windows</a></li>
</ul></li>
<li class="chapter" data-level="3" data-path="command-line-basics.html"><a href="command-line-basics.html"><i class="fa fa-check"></i><b>3</b> Command Line Basics</a><ul>
<li class="chapter" data-level="3.1" data-path="command-line-basics.html"><a href="command-line-basics.html#hello-terminal"><i class="fa fa-check"></i><b>3.1</b> Hello Terminal!</a><ul>
<li class="chapter" data-level="3.1.1" data-path="command-line-basics.html"><a href="command-line-basics.html#summary"><i class="fa fa-check"></i><b>3.1.1</b> Summary</a></li>
<li class="chapter" data-level="3.1.2" data-path="command-line-basics.html"><a href="command-line-basics.html#exercises"><i class="fa fa-check"></i><b>3.1.2</b> Exercises</a></li>
</ul></li>
<li class="chapter" data-level="3.2" data-path="command-line-basics.html"><a href="command-line-basics.html#navigating-the-command-line"><i class="fa fa-check"></i><b>3.2</b> Navigating the Command Line</a><ul>
<li class="chapter" data-level="3.2.1" data-path="command-line-basics.html"><a href="command-line-basics.html#summary-1"><i class="fa fa-check"></i><b>3.2.1</b> Summary</a></li>
<li class="chapter" data-level="3.2.2" data-path="command-line-basics.html"><a href="command-line-basics.html#exercises-1"><i class="fa fa-check"></i><b>3.2.2</b> Exercises</a></li>
</ul></li>
<li class="chapter" data-level="3.3" data-path="command-line-basics.html"><a href="command-line-basics.html#creation-and-inspection"><i class="fa fa-check"></i><b>3.3</b> Creation and Inspection</a><ul>
<li class="chapter" data-level="3.3.1" data-path="command-line-basics.html"><a href="command-line-basics.html#summary-2"><i class="fa fa-check"></i><b>3.3.1</b> Summary</a></li>
<li class="chapter" data-level="3.3.2" data-path="command-line-basics.html"><a href="command-line-basics.html#exercises-2"><i class="fa fa-check"></i><b>3.3.2</b> Exercises</a></li>
</ul></li>
<li class="chapter" data-level="3.4" data-path="command-line-basics.html"><a href="command-line-basics.html#migration-and-destruction"><i class="fa fa-check"></i><b>3.4</b> Migration and Destruction</a><ul>
<li class="chapter" data-level="3.4.1" data-path="command-line-basics.html"><a href="command-line-basics.html#summary-3"><i class="fa fa-check"></i><b>3.4.1</b> Summary</a></li>
<li class="chapter" data-level="3.4.2" data-path="command-line-basics.html"><a href="command-line-basics.html#exercises-3"><i class="fa fa-check"></i><b>3.4.2</b> Exercises</a></li>
</ul></li>
</ul></li>
<li class="chapter" data-level="4" data-path="working-with-unix.html"><a href="working-with-unix.html"><i class="fa fa-check"></i><b>4</b> Working with Unix</a><ul>
<li class="chapter" data-level="4.1" data-path="working-with-unix.html"><a href="working-with-unix.html#self-help"><i class="fa fa-check"></i><b>4.1</b> Self-Help</a><ul>
<li class="chapter" data-level="4.1.1" data-path="working-with-unix.html"><a href="working-with-unix.html#summary-4"><i class="fa fa-check"></i><b>4.1.1</b> Summary</a></li>
<li class="chapter" data-level="4.1.2" data-path="working-with-unix.html"><a href="working-with-unix.html#exercises-4"><i class="fa fa-check"></i><b>4.1.2</b> Exercises</a></li>
</ul></li>
<li class="chapter" data-level="4.2" data-path="working-with-unix.html"><a href="working-with-unix.html#get-wild"><i class="fa fa-check"></i><b>4.2</b> Get Wild</a><ul>
<li class="chapter" data-level="4.2.1" data-path="working-with-unix.html"><a href="working-with-unix.html#summary-5"><i class="fa fa-check"></i><b>4.2.1</b> Summary</a></li>
<li class="chapter" data-level="4.2.2" data-path="working-with-unix.html"><a href="working-with-unix.html#exercises-5"><i class="fa fa-check"></i><b>4.2.2</b> Exercises</a></li>
</ul></li>
<li class="chapter" data-level="4.3" data-path="working-with-unix.html"><a href="working-with-unix.html#search"><i class="fa fa-check"></i><b>4.3</b> Search</a><ul>
<li class="chapter" data-level="4.3.1" data-path="working-with-unix.html"><a href="working-with-unix.html#regular-expressions"><i class="fa fa-check"></i><b>4.3.1</b> Regular Expressions</a></li>
<li class="chapter" data-level="4.3.2" data-path="working-with-unix.html"><a href="working-with-unix.html#metacharacters"><i class="fa fa-check"></i><b>4.3.2</b> Metacharacters</a></li>
<li class="chapter" data-level="4.3.3" data-path="working-with-unix.html"><a href="working-with-unix.html#character-sets"><i class="fa fa-check"></i><b>4.3.3</b> Character Sets</a></li>
<li class="chapter" data-level="4.3.4" data-path="working-with-unix.html"><a href="working-with-unix.html#escaping-anchors-odds-and-ends"><i class="fa fa-check"></i><b>4.3.4</b> Escaping, Anchors, Odds, and Ends</a></li>
<li class="chapter" data-level="4.3.5" data-path="working-with-unix.html"><a href="working-with-unix.html#find"><i class="fa fa-check"></i><b>4.3.5</b> <code>find</code></a></li>
<li class="chapter" data-level="4.3.6" data-path="working-with-unix.html"><a href="working-with-unix.html#summary-6"><i class="fa fa-check"></i><b>4.3.6</b> Summary</a></li>
<li class="chapter" data-level="4.3.7" data-path="working-with-unix.html"><a href="working-with-unix.html#exercises-6"><i class="fa fa-check"></i><b>4.3.7</b> Exercises</a></li>
</ul></li>
<li class="chapter" data-level="4.4" data-path="working-with-unix.html"><a href="working-with-unix.html#configure"><i class="fa fa-check"></i><b>4.4</b> Configure</a><ul>
<li class="chapter" data-level="4.4.1" data-path="working-with-unix.html"><a href="working-with-unix.html#history"><i class="fa fa-check"></i><b>4.4.1</b> History</a></li>
<li class="chapter" data-level="4.4.2" data-path="working-with-unix.html"><a href="working-with-unix.html#customizing-bash"><i class="fa fa-check"></i><b>4.4.2</b> Customizing Bash</a></li>
<li class="chapter" data-level="4.4.3" data-path="working-with-unix.html"><a href="working-with-unix.html#summary-7"><i class="fa fa-check"></i><b>4.4.3</b> Summary</a></li>
</ul></li>
<li class="chapter" data-level="4.5" data-path="working-with-unix.html"><a href="working-with-unix.html#differentiate"><i class="fa fa-check"></i><b>4.5</b> Differentiate</a><ul>
<li class="chapter" data-level="4.5.1" data-path="working-with-unix.html"><a href="working-with-unix.html#summary-8"><i class="fa fa-check"></i><b>4.5.1</b> Summary</a></li>
</ul></li>
<li class="chapter" data-level="4.6" data-path="working-with-unix.html"><a href="working-with-unix.html#pipes"><i class="fa fa-check"></i><b>4.6</b> Pipes</a><ul>
<li class="chapter" data-level="4.6.1" data-path="working-with-unix.html"><a href="working-with-unix.html#summary-9"><i class="fa fa-check"></i><b>4.6.1</b> Summary</a></li>
<li class="chapter" data-level="4.6.2" data-path="working-with-unix.html"><a href="working-with-unix.html#exercises-7"><i class="fa fa-check"></i><b>4.6.2</b> Exercises</a></li>
</ul></li>
<li class="chapter" data-level="4.7" data-path="working-with-unix.html"><a href="working-with-unix.html#make"><i class="fa fa-check"></i><b>4.7</b> Make</a><ul>
<li class="chapter" data-level="4.7.1" data-path="working-with-unix.html"><a href="working-with-unix.html#summary-10"><i class="fa fa-check"></i><b>4.7.1</b> Summary</a></li>
</ul></li>
</ul></li>
<li class="chapter" data-level="5" data-path="bash-programming.html"><a href="bash-programming.html"><i class="fa fa-check"></i><b>5</b> Bash Programming</a><ul>
<li class="chapter" data-level="5.1" data-path="bash-programming.html"><a href="bash-programming.html#math"><i class="fa fa-check"></i><b>5.1</b> Math</a><ul>
<li class="chapter" data-level="5.1.1" data-path="bash-programming.html"><a href="bash-programming.html#summary-11"><i class="fa fa-check"></i><b>5.1.1</b> Summary</a></li>
<li class="chapter" data-level="5.1.2" data-path="bash-programming.html"><a href="bash-programming.html#exercises-8"><i class="fa fa-check"></i><b>5.1.2</b> Exercises</a></li>
</ul></li>
<li class="chapter" data-level="5.2" data-path="bash-programming.html"><a href="bash-programming.html#variables"><i class="fa fa-check"></i><b>5.2</b> Variables</a><ul>
<li class="chapter" data-level="5.2.1" data-path="bash-programming.html"><a href="bash-programming.html#summary-12"><i class="fa fa-check"></i><b>5.2.1</b> Summary</a></li>
<li class="chapter" data-level="5.2.2" data-path="bash-programming.html"><a href="bash-programming.html#exercises-9"><i class="fa fa-check"></i><b>5.2.2</b> Exercises</a></li>
</ul></li>
<li class="chapter" data-level="5.3" data-path="bash-programming.html"><a href="bash-programming.html#user-input"><i class="fa fa-check"></i><b>5.3</b> User Input</a><ul>
<li class="chapter" data-level="5.3.1" data-path="bash-programming.html"><a href="bash-programming.html#summary-13"><i class="fa fa-check"></i><b>5.3.1</b> Summary</a></li>
<li class="chapter" data-level="5.3.2" data-path="bash-programming.html"><a href="bash-programming.html#exercises-10"><i class="fa fa-check"></i><b>5.3.2</b> Exercises</a></li>
</ul></li>
<li class="chapter" data-level="5.4" data-path="bash-programming.html"><a href="bash-programming.html#logic-and-ifelse"><i class="fa fa-check"></i><b>5.4</b> Logic and If/Else</a><ul>
<li class="chapter" data-level="5.4.1" data-path="bash-programming.html"><a href="bash-programming.html#conditional-execution"><i class="fa fa-check"></i><b>5.4.1</b> Conditional Execution</a></li>
<li class="chapter" data-level="5.4.2" data-path="bash-programming.html"><a href="bash-programming.html#conditional-expressions"><i class="fa fa-check"></i><b>5.4.2</b> Conditional Expressions</a></li>
<li class="chapter" data-level="5.4.3" data-path="bash-programming.html"><a href="bash-programming.html#if-and-else"><i class="fa fa-check"></i><b>5.4.3</b> If and Else</a></li>
<li class="chapter" data-level="5.4.4" data-path="bash-programming.html"><a href="bash-programming.html#summary-14"><i class="fa fa-check"></i><b>5.4.4</b> Summary</a></li>
<li class="chapter" data-level="5.4.5" data-path="bash-programming.html"><a href="bash-programming.html#exercises-11"><i class="fa fa-check"></i><b>5.4.5</b> Exercises</a></li>
</ul></li>
<li class="chapter" data-level="5.5" data-path="bash-programming.html"><a href="bash-programming.html#arrays"><i class="fa fa-check"></i><b>5.5</b> Arrays</a><ul>
<li class="chapter" data-level="5.5.1" data-path="bash-programming.html"><a href="bash-programming.html#summary-15"><i class="fa fa-check"></i><b>5.5.1</b> Summary</a></li>
<li class="chapter" data-level="5.5.2" data-path="bash-programming.html"><a href="bash-programming.html#exercises-12"><i class="fa fa-check"></i><b>5.5.2</b> Exercises</a></li>
</ul></li>
<li class="chapter" data-level="5.6" data-path="bash-programming.html"><a href="bash-programming.html#braces"><i class="fa fa-check"></i><b>5.6</b> Braces</a><ul>
<li class="chapter" data-level="5.6.1" data-path="bash-programming.html"><a href="bash-programming.html#summary-16"><i class="fa fa-check"></i><b>5.6.1</b> Summary</a></li>
<li class="chapter" data-level="5.6.2" data-path="bash-programming.html"><a href="bash-programming.html#exercises-13"><i class="fa fa-check"></i><b>5.6.2</b> Exercises</a></li>
</ul></li>
<li class="chapter" data-level="5.7" data-path="bash-programming.html"><a href="bash-programming.html#loops"><i class="fa fa-check"></i><b>5.7</b> Loops</a><ul>
<li class="chapter" data-level="5.7.1" data-path="bash-programming.html"><a href="bash-programming.html#for"><i class="fa fa-check"></i><b>5.7.1</b> <code>for</code></a></li>
<li class="chapter" data-level="5.7.2" data-path="bash-programming.html"><a href="bash-programming.html#while"><i class="fa fa-check"></i><b>5.7.2</b> <code>while</code></a></li>
<li class="chapter" data-level="5.7.3" data-path="bash-programming.html"><a href="bash-programming.html#nesting"><i class="fa fa-check"></i><b>5.7.3</b> Nesting</a></li>
<li class="chapter" data-level="5.7.4" data-path="bash-programming.html"><a href="bash-programming.html#summary-17"><i class="fa fa-check"></i><b>5.7.4</b> Summary</a></li>
<li class="chapter" data-level="5.7.5" data-path="bash-programming.html"><a href="bash-programming.html#exercises-14"><i class="fa fa-check"></i><b>5.7.5</b> Exercises</a></li>
</ul></li>
<li class="chapter" data-level="5.8" data-path="bash-programming.html"><a href="bash-programming.html#functions"><i class="fa fa-check"></i><b>5.8</b> Functions</a><ul>
<li class="chapter" data-level="5.8.1" data-path="bash-programming.html"><a href="bash-programming.html#writing-functions"><i class="fa fa-check"></i><b>5.8.1</b> Writing Functions</a></li>
<li class="chapter" data-level="5.8.2" data-path="bash-programming.html"><a href="bash-programming.html#getting-values-from-functions"><i class="fa fa-check"></i><b>5.8.2</b> Getting Values from Functions</a></li>
<li class="chapter" data-level="5.8.3" data-path="bash-programming.html"><a href="bash-programming.html#summary-18"><i class="fa fa-check"></i><b>5.8.3</b> Summary</a></li>
<li class="chapter" data-level="5.8.4" data-path="bash-programming.html"><a href="bash-programming.html#exercises-15"><i class="fa fa-check"></i><b>5.8.4</b> Exercises</a></li>
</ul></li>
<li class="chapter" data-level="5.9" data-path="bash-programming.html"><a href="bash-programming.html#writing-programs"><i class="fa fa-check"></i><b>5.9</b> Writing Programs</a><ul>
<li class="chapter" data-level="5.9.1" data-path="bash-programming.html"><a href="bash-programming.html#the-unix-philosophy"><i class="fa fa-check"></i><b>5.9.1</b> The Unix Philosophy</a></li>
<li class="chapter" data-level="5.9.2" data-path="bash-programming.html"><a href="bash-programming.html#making-programs-executable"><i class="fa fa-check"></i><b>5.9.2</b> Making Programs Executable</a></li>
<li class="chapter" data-level="5.9.3" data-path="bash-programming.html"><a href="bash-programming.html#environmental-variables"><i class="fa fa-check"></i><b>5.9.3</b> Environmental Variables</a></li>
<li class="chapter" data-level="5.9.4" data-path="bash-programming.html"><a href="bash-programming.html#summary-19"><i class="fa fa-check"></i><b>5.9.4</b> Summary</a></li>
<li class="chapter" data-level="5.9.5" data-path="bash-programming.html"><a href="bash-programming.html#exercises-16"><i class="fa fa-check"></i><b>5.9.5</b> Exercises</a></li>
</ul></li>
</ul></li>
<li class="chapter" data-level="6" data-path="git-and-github.html"><a href="git-and-github.html"><i class="fa fa-check"></i><b>6</b> Git and GitHub</a><ul>
<li class="chapter" data-level="6.1" data-path="git-and-github.html"><a href="git-and-github.html#what-are-git-and-github"><i class="fa fa-check"></i><b>6.1</b> What are Git and GitHub?</a></li>
<li class="chapter" data-level="6.2" data-path="git-and-github.html"><a href="git-and-github.html#setting-up-git-and-github"><i class="fa fa-check"></i><b>6.2</b> Setting Up Git and GitHub</a></li>
<li class="chapter" data-level="6.3" data-path="git-and-github.html"><a href="git-and-github.html#getting-started-with-git"><i class="fa fa-check"></i><b>6.3</b> Getting Started with Git</a><ul>
<li class="chapter" data-level="6.3.1" data-path="git-and-github.html"><a href="git-and-github.html#summary-20"><i class="fa fa-check"></i><b>6.3.1</b> Summary</a></li>
<li class="chapter" data-level="6.3.2" data-path="git-and-github.html"><a href="git-and-github.html#exercises-17"><i class="fa fa-check"></i><b>6.3.2</b> Exercises</a></li>
</ul></li>
<li class="chapter" data-level="6.4" data-path="git-and-github.html"><a href="git-and-github.html#important-git-features"><i class="fa fa-check"></i><b>6.4</b> Important Git Features</a><ul>
<li class="chapter" data-level="6.4.1" data-path="git-and-github.html"><a href="git-and-github.html#getting-help-logs-and-diffs"><i class="fa fa-check"></i><b>6.4.1</b> Getting Help, Logs, and Diffs</a></li>
<li class="chapter" data-level="6.4.2" data-path="git-and-github.html"><a href="git-and-github.html#ignoring-files"><i class="fa fa-check"></i><b>6.4.2</b> Ignoring Files</a></li>
<li class="chapter" data-level="6.4.3" data-path="git-and-github.html"><a href="git-and-github.html#summary-21"><i class="fa fa-check"></i><b>6.4.3</b> Summary</a></li>
<li class="chapter" data-level="6.4.4" data-path="git-and-github.html"><a href="git-and-github.html#exercises-18"><i class="fa fa-check"></i><b>6.4.4</b> Exercises</a></li>
</ul></li>
<li class="chapter" data-level="6.5" data-path="git-and-github.html"><a href="git-and-github.html#branching"><i class="fa fa-check"></i><b>6.5</b> Branching</a><ul>
<li class="chapter" data-level="6.5.1" data-path="git-and-github.html"><a href="git-and-github.html#summary-22"><i class="fa fa-check"></i><b>6.5.1</b> Summary</a></li>
<li class="chapter" data-level="6.5.2" data-path="git-and-github.html"><a href="git-and-github.html#exercises-19"><i class="fa fa-check"></i><b>6.5.2</b> Exercises</a></li>
</ul></li>
<li class="chapter" data-level="6.6" data-path="git-and-github.html"><a href="git-and-github.html#github"><i class="fa fa-check"></i><b>6.6</b> GitHub</a><ul>
<li class="chapter" data-level="6.6.1" data-path="git-and-github.html"><a href="git-and-github.html#markdown"><i class="fa fa-check"></i><b>6.6.1</b> Markdown</a></li>
<li class="chapter" data-level="6.6.2" data-path="git-and-github.html"><a href="git-and-github.html#pull-requests"><i class="fa fa-check"></i><b>6.6.2</b> Pull Requests</a></li>
<li class="chapter" data-level="6.6.3" data-path="git-and-github.html"><a href="git-and-github.html#pages"><i class="fa fa-check"></i><b>6.6.3</b> Pages</a></li>
<li class="chapter" data-level="6.6.4" data-path="git-and-github.html"><a href="git-and-github.html#forking"><i class="fa fa-check"></i><b>6.6.4</b> Forking</a></li>
<li class="chapter" data-level="6.6.5" data-path="git-and-github.html"><a href="git-and-github.html#summary-23"><i class="fa fa-check"></i><b>6.6.5</b> Summary</a></li>
<li class="chapter" data-level="6.6.6" data-path="git-and-github.html"><a href="git-and-github.html#exercises-20"><i class="fa fa-check"></i><b>6.6.6</b> Exercises</a></li>
</ul></li>
</ul></li>
<li class="chapter" data-level="7" data-path="nephology.html"><a href="nephology.html"><i class="fa fa-check"></i><b>7</b> Nephology</a><ul>
<li class="chapter" data-level="7.1" data-path="nephology.html"><a href="nephology.html#introduction-to-cloud-computing"><i class="fa fa-check"></i><b>7.1</b> Introduction to Cloud Computing</a></li>
<li class="chapter" data-level="7.2" data-path="nephology.html"><a href="nephology.html#setting-up-digitalocean"><i class="fa fa-check"></i><b>7.2</b> Setting Up DigitalOcean</a></li>
<li class="chapter" data-level="7.3" data-path="nephology.html"><a href="nephology.html#connecting-to-the-cloud"><i class="fa fa-check"></i><b>7.3</b> Connecting to the Cloud</a><ul>
<li class="chapter" data-level="7.3.1" data-path="nephology.html"><a href="nephology.html#summary-24"><i class="fa fa-check"></i><b>7.3.1</b> Summary</a></li>
</ul></li>
<li class="chapter" data-level="7.4" data-path="nephology.html"><a href="nephology.html#cloud-computing-basics"><i class="fa fa-check"></i><b>7.4</b> Cloud Computing Basics</a><ul>
<li class="chapter" data-level="7.4.1" data-path="nephology.html"><a href="nephology.html#moving-files-in-and-out-of-the-cloud"><i class="fa fa-check"></i><b>7.4.1</b> Moving Files In and Out of the Cloud</a></li>
<li class="chapter" data-level="7.4.2" data-path="nephology.html"><a href="nephology.html#talking-to-other-servers"><i class="fa fa-check"></i><b>7.4.2</b> Talking to Other Servers</a></li>
<li class="chapter" data-level="7.4.3" data-path="nephology.html"><a href="nephology.html#automating-tasks"><i class="fa fa-check"></i><b>7.4.3</b> Automating Tasks</a></li>
<li class="chapter" data-level="7.4.4" data-path="nephology.html"><a href="nephology.html#summary-25"><i class="fa fa-check"></i><b>7.4.4</b> Summary</a></li>
<li class="chapter" data-level="7.4.5" data-path="nephology.html"><a href="nephology.html#exercises-21"><i class="fa fa-check"></i><b>7.4.5</b> Exercises</a></li>
</ul></li>
<li class="chapter" data-level="7.5" data-path="nephology.html"><a href="nephology.html#shutting-down-a-server"><i class="fa fa-check"></i><b>7.5</b> Shutting Down a Server</a></li>
</ul></li>
<li class="chapter" data-level="8" data-path="start-building.html"><a href="start-building.html"><i class="fa fa-check"></i><b>8</b> Start Building</a><ul>
<li class="chapter" data-level="8.1" data-path="start-building.html"><a href="start-building.html#next-steps"><i class="fa fa-check"></i><b>8.1</b> Next Steps</a></li>
<li class="chapter" data-level="8.2" data-path="start-building.html"><a href="start-building.html#giving-feedback"><i class="fa fa-check"></i><b>8.2</b> Giving Feedback</a></li>
<li class="chapter" data-level="8.3" data-path="start-building.html"><a href="start-building.html#using-this-book"><i class="fa fa-check"></i><b>8.3</b> Using this Book</a></li>
</ul></li>
<li class="divider"></li>
<li><a href="https://github.com/rstudio/bookdown" target="blank">Published with bookdown</a></li>
</ul>
</nav>
</div>
<div class="book-body">
<div class="body-inner">
<div class="book-header" role="navigation">
<h1>
<i class="fa fa-circle-o-notch fa-spin"></i><a href="./">The Unix Workbench</a>
</h1>
</div>
<div class="page-wrapper" tabindex="-1" role="main">
<div class="page-inner">
<section class="normal" id="section-">
<div id="nephology" class="section level1">
<h1><span class="header-section-number">Chapter 7</span> Nephology</h1>
<blockquote>
<p>I saw a city in the clouds. - Dagobahnian proverb</p>
</blockquote>
<div id="introduction-to-cloud-computing" class="section level2">
<h2><span class="header-section-number">7.1</span> Introduction to Cloud Computing</h2>
<p>Nephology is the study of clouds. Few modern technology concepts (other than
maybe data science and artificial intelligence) have been hyped as loudly as “the
cloud.” The cloud is simply a computer which we can access over the internet.
In this chapter we’ll set up a cloud computer and we’ll learn the basics of
interacting with one.</p>
<p>To get the most out of this chapter you’re going to need your credit or
debit card, or a <a href="https://www.paypal.com">PayPal</a> account. We’re going to be
using <a href="https://m.do.co/c/530d6cfa2b37">DigitalOcean</a>, a company which we
can rent cloud computers from. Throughout this chapter I might refer to cloud
computers as <strong>servers</strong> (computers connected to the internet) or as
<strong>droplets</strong>, which is a marketing term DigitalOcean uses to refer to their
servers (a droplet is <em>not</em> a technical term). Renting from DigitalOcean won’t
cost you any money since I’m giving you a coupon for two free months of service!
There are several companies that offer similar services compared to
DigitalOcean, but in my opinion they have the best user interface and the most
transparent pricing model.</p>
<p><strong>Warning:</strong> <em>At the end of this chapter we will discuss how to shut down any
servers we’ve started on DigitalOcean. If you don’t shut down your server
after two months then your account will be charged real money for using
DigitalOcean. Please be sure to shut down any servers you start after you are
finished using them.</em></p>
</div>
<div id="setting-up-digitalocean" class="section level2">
<h2><span class="header-section-number">7.2</span> Setting Up DigitalOcean</h2>
<p>To get started with DigitalOcean we need to rent a server from their website.
<a href="https://m.do.co/c/530d6cfa2b37">Click this link</a> to sign up for DigitalOcean
in order to get two free months of server use. (If you don’t use this link then
you don’t get two free months). Click <strong>Sign Up</strong> in the upper right corner,
then enter your email address and choose your password.</p>
<p><img src="images/do-main.png" /></p>
<p>Check your email for a message from DigitalOcean and click the enclosed link to
confirm your email. Then you’ll need to enter your credit or debit card
information, or your Paypal account details. As long as you close down all of
your servers less than two months after you start them you will not be charged.
After entering in your payment information you should see this screen:</p>
<p><img src="images/do-create-drop.png" /></p>
<p>Click the big blue <strong>Create a new Droplet</strong> button which should then bring you
to a screen where you can customize the server you’re going to be renting. Make
sure you have Ubuntu chosen as your distribution, and select the $5 per month
size option:</p>
<p><img src="images/do-distro-size.png" /></p>
<p>Scroll down the page and then select the region that is geographically closest
to you. Some regions have multiple data centers, it doesn’t matter which data
center you pick. Currently I’m in Baltimore, Maryland, USA, so I’m going to pick
the number 1 data center in New York:</p>
<p><img src="images/do-region.png" /></p>
<p>Finally at the bottom of the page click the big green <strong>Create</strong> button in order
to start your server!</p>
<p><img src="images/do-create-confirm.png" /></p>
<p>It will take a minute to launch the server, but once launched
you should receive an email from Digital Ocean with the details about your
new server. Included in this email you should find the IP address of your
server, the default username (which should be <code>root</code>) and a
randomly generated password that you will need to connect to
your server for the first time. Once you’ve received this email open up a new
terminal.</p>
</div>
<div id="connecting-to-the-cloud" class="section level2">
<h2><span class="header-section-number">7.3</span> Connecting to the Cloud</h2>
<p>We can connect to computers on the internet with the <code>ssh</code> program, which
stands for <strong>S</strong>ecure <strong>Sh</strong>ell. The <code>ssh</code> command provides a command line
interface to whichever computer we point it to. A computer that is connected to
the internet has an address (just like a house has an address) which is
specified by an <strong>IP address</strong>. The command for connecting to a computer with
<code>ssh</code> generally looks like this:</p>
<pre class="sourceCode bash"><code class="sourceCode bash"><span class="fu">ssh</span> [username]@[IP address]</code></pre>
<p>Let’s connect to our DigitalOcean server using <code>ssh</code>. Enter the following
command in the terminal substituting the IP address you received from
DigitalOcean for the IP address I’m using in this example:</p>
<pre class="sourceCode bash"><code class="sourceCode bash"><span class="fu">ssh</span> [email protected]</code></pre>
<pre><code>## The authenticity of host '159.203.134.88 (159.203.134.88)' can't be established.
## ECDSA key fingerprint is SHA256:UhtoIx/3c6/MmAIE+H8w5oGE06PsbXdzRRsAUhKtjhs.
## Are you sure you want to continue connecting (yes/no)?</code></pre>
<p>Type <code>yes</code> and then press Enter.</p>
<pre><code>## Warning: Permanently added '159.203.134.88' (ECDSA) to the list of known hosts.
## [email protected]'s password:</code></pre>
<p>This password should be in the email you received from DigitalOcean. Copy and
paste the password into the terminal, then press Enter.</p>
<pre><code>## You are required to change your password immediately (root enforced)
## Welcome to Ubuntu 16.04.2 LTS (GNU/Linux 4.4.0-78-generic x86_64)
##
## * Documentation: https://help.ubuntu.com
## * Management: https://landscape.canonical.com
## * Support: https://ubuntu.com/advantage
##
## Get cloud support with Ubuntu Advantage Cloud Guest:
## http://www.ubuntu.com/business/services/cloud
##
## 0 packages can be updated.
## 0 updates are security updates.
##
##
##
## The programs included with the Ubuntu system are free software;
## the exact distribution terms for each program are described in the
## individual files in /usr/share/doc/*/copyright.
##
## Ubuntu comes with ABSOLUTELY NO WARRANTY, to the extent permitted by
## applicable law.
##
## Changing password for root.
## (current) UNIX password:</code></pre>
<p>We now need to create a new password for this server. First paste in the old
password and press Enter. Then think of a new, strong password and enter it into
the console. Then enter the new password again to confirm. After entering in the
new password we should have a prompt! Press enter a few times to make sure that
you get the prompt back each time. We’re in!</p>
<p>Now we have access to all of the Unix commands we would normally have:</p>
<pre class="sourceCode bash"><code class="sourceCode bash"><span class="bu">pwd</span></code></pre>
<pre><code>## /root</code></pre>
<p>In order to disconnect from the server and return to your machine use <code>logout</code>.</p>
<pre class="sourceCode bash"><code class="sourceCode bash"><span class="bu">logout</span></code></pre>
<pre><code>## Connection to 159.203.134.88 closed.</code></pre>
<p>To reconnect to the server use <code>ssh</code> again:</p>
<pre class="sourceCode bash"><code class="sourceCode bash"><span class="fu">ssh</span> [email protected]</code></pre>
<pre><code>## [email protected]'s password:</code></pre>
<p>Enter your password and you should get the prompt back for your cloud server.</p>
<div id="summary-24" class="section level3">
<h3><span class="header-section-number">7.3.1</span> Summary</h3>
<ul>
<li><code>ssh</code> connects you to computers that are connected to the internet. The
template for the command to connect is <code>ssh [username]@[IP address]</code>.</li>
<li>To disconnect from an <code>ssh</code> session use the <code>logout</code> command.</li>
</ul>
</div>
</div>
<div id="cloud-computing-basics" class="section level2">
<h2><span class="header-section-number">7.4</span> Cloud Computing Basics</h2>
<div id="moving-files-in-and-out-of-the-cloud" class="section level3">
<h3><span class="header-section-number">7.4.1</span> Moving Files In and Out of the Cloud</h3>
<p>So now that we have a cloud computer, what can we do with it? One thing we
can do is store and retrieve files from a cloud computer. The program <code>scp</code>
allows us to copy local files to a server and it allows us to copy files on
a server to our local computer. First let’s connect to our server so we can
create a file there:</p>
<pre class="sourceCode bash"><code class="sourceCode bash"><span class="fu">ssh</span> [email protected]</code></pre>
<pre><code>## [email protected]'s password:
## # (Enter your password)</code></pre>
<pre class="sourceCode bash"><code class="sourceCode bash"><span class="fu">mkdir</span> textfiles
<span class="bu">echo</span> <span class="st">"From the server"</span> <span class="op">></span> textfiles/server-file.txt
<span class="bu">logout</span></code></pre>
<pre><code>## Connection to 159.203.134.88 closed.</code></pre>
<p>Now that we’re back at the prompt on our local machine let’s try getting
<code>server-file.txt</code> from our server. The arguments for copying files from a server
with <code>scp</code> have the following general structure:</p>
<pre class="sourceCode bash"><code class="sourceCode bash"><span class="fu">scp</span> [username]@[IP address]:path/to/file/on/server path/on/my/computer</code></pre>
<p>This copies the file located on the server at <code>path/to/file/on/server</code> to a
local path at <code>path/on/my/computer</code>. In the same way we can copy an entire
folder from a server using the <code>-r</code> flag:</p>
<pre class="sourceCode bash"><code class="sourceCode bash"><span class="fu">scp</span> -r [username]@[IP address]:path/to/folder/on/server folder/on/my/computer</code></pre>
<p>Let’s try doing this now from our local computer. Enter your password when
asked to do so:</p>
<pre class="sourceCode bash"><code class="sourceCode bash"><span class="bu">cd</span>
<span class="bu">pwd</span></code></pre>
<pre><code>## /Users/sean/</code></pre>
<pre class="sourceCode bash"><code class="sourceCode bash"><span class="fu">mkdir</span> Cloud
<span class="bu">cd</span> Cloud
<span class="fu">scp</span> [email protected]:/root/textfiles/server-file.txt downloaded.txt</code></pre>
<pre><code>## [email protected]'s password:
## server-file.txt 100% 16 1.2KB/s 00:00</code></pre>
<pre class="sourceCode bash"><code class="sourceCode bash"><span class="fu">cat</span> downloaded.txt</code></pre>
<pre><code>## From the server</code></pre>
<p>It worked! Now let’s try uploading a file to our server. The arguments for doing
this are just the swapped arguments for downloading a file from a server:</p>
<pre class="sourceCode bash"><code class="sourceCode bash"><span class="fu">scp</span> path/on/my/computer [username]@[IP address]:path/to/file/on/server </code></pre>
<p>Let’s create a file and upload it to our server:</p>
<pre class="sourceCode bash"><code class="sourceCode bash"><span class="bu">echo</span> <span class="st">"from local computer"</span> <span class="op">></span> to-upload.txt
<span class="fu">scp</span> to-upload.txt [email protected]:/root/textfiles/uploaded-file.txt</code></pre>
<pre><code>## [email protected]'s password:
## to-upload.txt 100% 20 1.8KB/s 00:00</code></pre>
<p>Now let’s log in to our server and we’ll see if it’s there:</p>
<pre class="sourceCode bash"><code class="sourceCode bash"><span class="fu">ssh</span> [email protected]
<span class="fu">cat</span> textfiles/uploaded-file.txt</code></pre>
<pre><code>## from local computer</code></pre>
<p>Looks like it worked! Keeping files in the cloud allows you to work with the
same files in the same workspace as long as you have access to a terminal and
<code>ssh</code>.</p>
</div>
<div id="talking-to-other-servers" class="section level3">
<h3><span class="header-section-number">7.4.2</span> Talking to Other Servers</h3>
<p>There are tons of servers out there on the internet! The way you’re probably
used to talking to a server is through a web browser, but there are other ways
we can talk to servers on the command line. One of the most popular command line
programs for talking to other servers is <code>curl</code>. The <code>curl</code> command allows us
to send requests and information to other servers.</p>
<p>One easy task that we can use <code>curl</code> for is downloading files that are available
online. For example, this entire book and all of the files associated with it
are hosted on a server! You can find the Markdown file for one of the first
chapters of this book <a href="http://seankross.com/the-unix-workbench/01-What-is-Unix.md">here</a>.
To download a file with <code>curl</code>, we simply need to provide the <code>-O</code> flag and the
URL of the file:</p>
<pre class="sourceCode bash"><code class="sourceCode bash"><span class="ex">curl</span> -O http://website.org/textfile.txt</code></pre>
<p>Let’s try downloading the Markdown file from my website:</p>
<pre class="sourceCode bash"><code class="sourceCode bash"><span class="ex">curl</span> -O http://seankross.com/the-unix-workbench/01-What-is-Unix.md</code></pre>
<pre><code>## % Total % Received % Xferd Average Speed Time Time Time Current
## Dload Upload Total Spent Left Speed
## 100 1198 100 1198 0 0 13681 0 --:--:-- --:--:-- --:--:-- 13770</code></pre>
<pre class="sourceCode bash"><code class="sourceCode bash"><span class="fu">head</span> -n 5 01-What-is-Unix.md</code></pre>
<pre><code>## # What is Unix?
##
## Unix is an operating system and a set of tools. The tool we'll be using the
## most in this book is a shell, which is a computer program that provides a
## command line interface. You've probably seen a command line interface in the</code></pre>
<p>Looks like we got the file! The <code>curl</code> command is also commonly used for
communicating with <strong>API</strong>s. API stands for <strong>a</strong>pplication
<strong>p</strong>rogramming <strong>i</strong>nterface. APIs are a set of rules which allow us to
communicate with computer programs or with servers on the web. GitHub has a
<a href="https://developer.github.com/v3/">large API</a> which allows us to find out
information about GitHub’s users and repositories. Let’s use <code>curl</code> to look
at what programming languages are used by some of my repositories. Let’s start
with the repository for this book:</p>
<pre class="sourceCode bash"><code class="sourceCode bash"><span class="ex">curl</span> https://api.github.com/repos/seankross/the-unix-workbench/languages</code></pre>
<pre><code>{
"CSS": 2615,
"TeX": 22
}</code></pre>
<p>It looks like most of the repository is dedicated to making the book website
look pretty! Take a look at the URL in the <code>curl</code> command above, and let’s
dissect it a little bit. The API itself is located at <code>https://api.github.com/</code>.
Then each word in the rest of the url acts as a sort of argument. We’re
interested in <code>repos</code> in this case, specifically a repo belonging to the
username <code>seankross</code> called <code>the-unix-workbench</code>, and we want to know about
which <code>languages</code> are used in that repo. Let’s take a look at one more of my
repositories just to see how the response can be different:</p>
<pre class="sourceCode bash"><code class="sourceCode bash"><span class="ex">curl</span> https://api.github.com/repos/seankross/lego/languages</code></pre>
<pre><code>{
"R": 4197,
"Shell": 442
}</code></pre>
<p>Use of <code>curl</code>, especially when coupled with using APIs can become very
complicated and much more advanced content has been written on the subject.
Let’s get a little more in depth by looking through some of the examples from
<a href="http://httpbin.org/">httpbin.org</a>. This website allows us to send requests to
it with <code>curl</code>, and it will return to us a structured version of whatever
information we sent. This is useful for debugging our <code>curl</code> commands. First
let’s send a request which should return our IP address:</p>
<pre class="sourceCode bash"><code class="sourceCode bash"><span class="ex">curl</span> http://httpbin.org/ip</code></pre>
<pre><code>{
"origin": "159.203.134.88"
}</code></pre>
<p>Looks like we’re getting the response we expect. Before we go on I should
clarify: <code>curl</code> sends <strong>HTTP requests</strong>. HTTP is a technology for sending
information over a network, and HTTP powers much of how the internet works.
There are different categories of HTTP requests, and the categories are often
called <strong>verbs</strong>. When we use <code>curl</code> without any flags we are sending a
<strong>GET</strong> request (GET is an HTTP verb). A GET request is a message that says to
a server: “Hi, I live at [IP address]. Would you mind sending some information
about yourself to that IP?” In the case above we asked for our own IP address,
which httpbin.org knew to just send back to us.</p>
<p>Let’s send a general HTTP GET request to <a href="http://httpbin.org/get" class="uri">http://httpbin.org/get</a>:</p>
<pre class="sourceCode bash"><code class="sourceCode bash"><span class="ex">curl</span> http://httpbin.org/get</code></pre>
<pre><code>{
"args": {},
"headers": {
"Accept": "*/*",
"Connection": "close",
"Host": "httpbin.org",
"User-Agent": "curl/7.47.0"
},
"origin": "159.203.134.88",
"url": "http://httpbin.org/get"
}</code></pre>
<p>The text that we get back from the request specifies four information groups:
<code>args</code>, <code>headers</code>, <code>origin</code>, and <code>url</code>. The <code>origin</code> shows our own IP address,
and <code>url</code> shows where we sent the request. The <code>headers</code> group shows some
interesting information, including the <code>User-Agent</code> which shows that httpbin.org
knows that we sent this request with <code>curl</code>. Notice that the <code>args</code> group is
empty. The <code>args</code> group is short for <em>arguments</em>, which hints at the fact that
we can provide arguments in an HTTP request, just like arguments we would use
for a function, or the arguments we used in the GitHub API.</p>
<p>In the general case we can provide arguments to an HTTP API by putting a
question mark (<code>?</code>) after the API’s URL. Let’s try this out:</p>
<pre class="sourceCode bash"><code class="sourceCode bash"><span class="ex">curl</span> http://httpbin.org/get?Baltimore</code></pre>
<pre><code>{
"args": {
"Baltimore": ""
},
"headers": {
"Accept": "*/*",
"Connection": "close",
"Host": "httpbin.org",
"User-Agent": "curl/7.47.0"
},
"origin": "159.203.134.88",
"url": "http://httpbin.org/get?Baltimore"
}</code></pre>
<p>Looks like <code>"Baltimore"</code> showed up in <code>args</code>! For most HTTP APIs we need to
give names to our arguments, unlike most arguments in Bash. We can specify
an argument’s name with the template <code>[argument name]=[argument value]</code>. Let’s
take a look at a simple example:</p>
<pre class="sourceCode bash"><code class="sourceCode bash"><span class="ex">curl</span> http://httpbin.org/get?city=Baltimore</code></pre>
<pre><code>{
"args": {
"city": "Baltimore"
},
"headers": {
"Accept": "*/*",
"Connection": "close",
"Host": "httpbin.org",
"User-Agent": "curl/7.47.0"
},
"origin": "159.203.134.88",
"url": "http://httpbin.org/get?city=Baltimore"
}</code></pre>
<p>Now we can see that in <code>args</code> there’s a correspondence between <code>city</code> and
<code>Baltimore</code>. We can add more named arguments by separating them with an
ampersand (<code>&</code>):</p>
<pre class="sourceCode bash"><code class="sourceCode bash"><span class="ex">curl</span> <span class="st">"http://httpbin.org/get?city=Baltimore&state=Maryland"</span></code></pre>
<pre><code>{
"args": {
"city": "Baltimore",
"state": "Maryland"
},
"headers": {
"Accept": "*/*",
"Connection": "close",
"Host": "httpbin.org",
"User-Agent": "curl/7.47.0"
},
"origin": "159.203.134.88",
"url": "http://httpbin.org/get?city=Baltimore&state=Maryland"
}</code></pre>
<p>Perhaps you could imagine building a server that accepts HTTP requests, and
sends back different information depending on what arguments are provided (for
example, send back a weather report given a location). Building these kinds of
servers is an advanced topic that is outside the scope of this book, but there
are lots of resources out there if you’re interested in building your own HTTP
API on a web server.</p>
</div>
<div id="automating-tasks" class="section level3">
<h3><span class="header-section-number">7.4.3</span> Automating Tasks</h3>
<p>One of the most compelling features about any web server is that it’s always
powered on and always connected to the internet. This means that we can instruct
our server to perform tasks automatically, without us needing to enter a command
into a shell. One of the most commonly used programs for executing <em>other programs</em>
with a regular frequency is called <code>cron</code>. Let’s take a look at how to use
<code>cron</code> to schedule a program to be run.</p>
<p>If you’re not already connected to the server use <code>ssh</code> to connect.</p>
<pre class="sourceCode bash"><code class="sourceCode bash"><span class="fu">ssh</span> [email protected]</code></pre>
<p>The <code>cron</code> program is part of a family of programs called <strong>daemons</strong>. A daemon
is a program that is always running in the background of our computer. First,
let’s see if <code>cron</code> is running. We can get a list of all running programs
with the <code>ps</code> command while using the <code>-A</code> flag:</p>
<pre class="sourceCode bash"><code class="sourceCode bash"><span class="fu">ps</span> -A</code></pre>
<pre><code>## PID TTY TIME CMD
## 1 ? 00:00:13 systemd
## 2 ? 00:00:00 kthreadd
## 3 ? 00:00:03 ksoftirqd/0
## 5 ? 00:00:00 kworker/0:0H
## 7 ? 00:00:11 rcu_sched
## 8 ? 00:00:00 rcu_bh
## 9 ? 00:00:00 migration/0
## ...</code></pre>
<p>You probably have a huge list of programs in your terminal now! Instead of
sifting through this listing line-by-line, let’s pipe the output of this command
to <code>grep</code> and we’ll look for <code>cron</code>:</p>
<pre class="sourceCode bash"><code class="sourceCode bash"><span class="fu">ps</span> -A <span class="kw">|</span> <span class="fu">grep</span> <span class="st">"cron"</span></code></pre>
<pre><code>## 1273 ? 00:00:01 cron</code></pre>
<p>Looks like the <code>cron</code> daemon is running! In order to assign programs to be
executed with <code>cron</code> we need to edit a special text file called the <code>cron</code>
table. Before we edit the <code>cron</code> table we need to select the default text
editor. If you like using <code>nano</code> (the text editor we’ve been using throughout
this book) then enter <code>select-editor</code> into the console, type in the number
that corresponds to <code>nano</code> (usually <code>2</code>) and then press enter:</p>
<pre class="sourceCode bash"><code class="sourceCode bash"><span class="ex">select-editor</span></code></pre>
<pre><code>## Select an editor. To change later, run 'select-editor'.
## 1. /bin/ed
## 2. /bin/nano <---- easiest
## 3. /usr/bin/vim.basic
## 4. /usr/bin/vim.tiny
##
## Choose 1-4 [2]:</code></pre>
<p>Now that we’ve chosen a text editor we can edit the <code>cron</code> table using the
command <code>crontab -e</code> (<strong><code>cron</code></strong> <strong>tab</strong>le <strong>e</strong>dit) which will automatically
open <code>nano</code> with the appropriate file.</p>
<pre class="sourceCode bash"><code class="sourceCode bash"><span class="ex">crontab</span> -e</code></pre>
<pre><code># Edit this file to introduce tasks to be run by cron.
#
# m h dom mon dow command
</code></pre>
<p>Let’s go over the layout of the <code>cron</code> table. First you should notice that any
text after a pound sign (<code>#</code>) is a comment, so it’s not seen by <code>cron</code> (just
like bash comments). The <code>cron</code> table has six columns:</p>
<ol style="list-style-type: decimal">
<li>Minute (<code>m</code>)</li>
<li>Hour (<code>h</code>)</li>
<li>Day of Month (<code>dom</code>)</li>
<li>Month (<code>mon</code>)</li>
<li>Day of Week (<code>dow</code>)</li>
<li>Command to be run (<code>command</code>)</li>
</ol>
<p>Each column is separated by a single space in the table. The first five columns
allow you to specify when you want a particular command to be run. Only certain
values are valid in each column:</p>
<ol style="list-style-type: decimal">
<li>Minute: <code>00 - 59</code> (A particular minute in an hour)</li>
<li>Hour: <code>00 - 23</code> (0 is the midnight hour)</li>
<li>Day of Month: <code>01 - 31</code> (1 is the first day of the month)</li>
<li>Month: <code>01 - 12</code> (1 is January)</li>
<li>Day of Week <code>0 - 6</code> (0 is Sunday)</li>
</ol>
<p>There are also a few other characters that are valid in the <code>cron</code> table. The
most commonly used character is a star (<code>*</code>) which represents <em>all</em> of the
possible values in a column. So a star in the Minute column means “run every
minute,” and a star in the Hour column means “run during every hour.”
Knowing this let’s make our first entry in the <code>cron</code> table. If we want a
command to be executed every minute, during every hour, on every day of the
month, during every month, on every day of the week, then we can put stars in
all of the first five
columns, followed by the command that we want to run. In this case the command
that <code>cron</code> will run every minute will be <code>date >> ~/date-file.txt</code>, which will
append the date and time when the command is executed to a file in our home
directory called <code>date-file.txt</code>. This is what your <code>cron</code> table should look
like before you save and exit from <code>nano</code>:</p>
<pre><code># Edit this file to introduce tasks to be run by cron.
#
# m h dom mon dow command
* * * * * date >> ~/date-file.txt</code></pre>
<p>Save and exit <code>nano</code> just like you would for a regular text file and then wait
a little bit! After a minute has gone by use <code>cat</code> to look at <code>~/date-file.txt</code>:</p>
<pre class="sourceCode bash"><code class="sourceCode bash"><span class="bu">cd</span>
<span class="fu">cat</span> date-file.txt</code></pre>
<pre><code>## Thu Jun 8 18:50:01 UTC 2017</code></pre>
<p>Look like our entry in the <code>cron</code> table is working! Wait another minute and then
look at the file again:</p>
<pre class="sourceCode bash"><code class="sourceCode bash"><span class="fu">cat</span> date-file.txt</code></pre>
<pre><code>## Thu Jun 8 18:50:01 UTC 2017
## Thu Jun 8 18:51:01 UTC 2017</code></pre>
<p>Unless we delete the line that we entered in the <code>cron</code> table, the output from
<code>date</code> will be appended to <code>date-file.txt</code> every minute.</p>
<p>The single line of bash <code>date >> ~/date-file.txt</code> is a much simpler program than
we would probably use in a <code>cron</code> table, though it’s good for illustrating how
a <code>cron</code> table works. If you want to do more complex tasks with <code>cron</code> it’s
better for <code>cron</code> to execute a bash script that you’ve written in advance. That
way you can just specify <code>bash /path/to/script.sh</code> in the last column of the
table.</p>
<p>Using stars in all columns is the simplest line of a <code>cron</code> table, so let’s
look at some examples of more complex table entries:</p>
<pre><code># m h dom mon dow command
00 * * * * bash /path/to/script.sh # Runs every hour at the start of the hour
00 12 * * * bash /path/to/script.sh # Runs every day at noon
* 12 * * * bash /path/to/script.sh # Runs every minute between 12pm and 12:59pm
00 00 05 * * bash /path/to/script.sh # Runs the 5th day of every month at midnight
00 00 * 07 * bash /path/to/script.sh # Runs every day in the month of July at midnight
00 00 * * 2 bash /path/to/script.sh # Runs every Tuesday at midnight</code></pre>
<p>Besides numbers and the star there are a few other characters that you can use
in <code>cron</code> table columns including a hyphen (<code>-</code>) for specifying ranges and a
comma (<code>,</code>) for specifying lists of items. For example <code>00-29</code> in the Minutes
column would specify the first thirty minutes of an hour, while <code>1,5</code> in the
Day of Week column would specify Monday and Friday.</p>
<p>Let’s take a look at another example of a <code>cron</code> table that uses hyphens and
ranges so you can get a sense of how each character works.</p>
<pre><code># m h dom mon dow command
00-04 * * * * bash /path/to/script.sh # Runs every minute for the first five minutes of every hour
00 00 * * 0,6 bash /path/to/script.sh # Runs at midnight every Saturday and Sunday
00 03 01-15 * * bash /path/to/script.sh # Runs at 3am for the first fifteen days of every month
00,30 * * * * bash /path/to/script.sh # Runs at the start and middle of every hour
00 00,12 * * * bash /path/to/script.sh # Runs every day at midnight and noon
00 * 01-07 01,06 * bash /path/to/script.sh # Runs at the start of every hour for the first seven days of the months of January and June</code></pre>
<p>A program that is being run by <code>cron</code> is only as powerful as your imagination
can stretch! If you’re familiar with the social network
<a href="https://twitter.com/">Twitter</a> then you might have come across some Twitter
accounts which create posts automatically like
<a href="https://twitter.com/emojiaquarium">Emoji Aquarium</a>,
<a href="https://twitter.com/pomological">Old Fruit Pictures</a>, or
<a href="https://twitter.com/infinite_scream">Endless Screaming</a>. Many of these “bot”
accounts are powered by <code>cron</code>, which uses Twitter’s HTTP API to post tweets
regularly.</p>
</div>
<div id="summary-25" class="section level3">
<h3><span class="header-section-number">7.4.4</span> Summary</h3>
<ul>
<li><code>scp</code> copies files between a cloud computer and your personal computer. Use
the <code>-r</code> flag in order to copy directories.</li>
<li><code>curl</code> allows you to send HTTP requests to other servers. Use the <code>-O</code> flag to
download files with <code>curl</code>.</li>
<li><code>ps -A</code> lists all of the programs running in the background of your computer.</li>
<li><code>cron</code> allows you to schedule when programs are run. Use <code>crontab -e</code> in order
to edit the <code>cron</code> table.</li>
</ul>
</div>
<div id="exercises-21" class="section level3">
<h3><span class="header-section-number">7.4.5</span> Exercises</h3>
<ol style="list-style-type: decimal">
<li>Write a bash script that takes a file path as an argument and copies that
file to a designated folder on your server.</li>
<li>Find a file online that changes periodically, then write a program to
download that file every time it changes.</li>
<li>Try creating your own Twitter or GitHub bot with the
<a href="https://dev.twitter.com/rest/public">Twitter API</a> or the
<a href="https://developer.github.com/v3/">GitHub API</a>.</li>
</ol>
</div>
</div>
<div id="shutting-down-a-server" class="section level2">
<h2><span class="header-section-number">7.5</span> Shutting Down a Server</h2>
<p>In order to avoid using more DigitalOcean credits than we have to or
being charged for using the service make sure to <strong>destroy</strong> any DigitalOcean
droplets that you started. When you destroy a droplet all files
on the droplet are gone forever, so please be sure that you don’t have any
important information on a droplet before you destroy it. If there are files
you want to save on your droplet use <code>scp</code> in order to copy them to your local
machine. Let’s walk through destroying a droplet. Go back to
<a href="https://m.do.co/c/530d6cfa2b37">DigitalOcean</a> and log in. You should then see
a listing of all of your droplets:</p>
<p><img src="images/do-droplets-list.png" /></p>
<p>Click on <strong>More</strong> on the right side of the screen and a menu should appear.
Click <strong>Destroy</strong> at the bottom of this menu. Then this screen should appear:</p>
<p><img src="images/do-destroy-button.png" /></p>
<p>Click the <strong>Destroy</strong> button and then click <strong>Confirm</strong> in the menu that pops
up. Your droplet should now be destroyed. If you have no active droplets then
the main droplets page should look like this:</p>
<p><img src="images/do-fresh.png" /></p>
<p>Congratulations on your new cloud computing skills!</p>
</div>
</div>
</section>
</div>
</div>
</div>
<a href="git-and-github.html" class="navigation navigation-prev " aria-label="Previous page"><i class="fa fa-angle-left"></i></a>
<a href="start-building.html" class="navigation navigation-next " aria-label="Next page"><i class="fa fa-angle-right"></i></a>
</div>
</div>
<script src="libs/gitbook-2.6.7/js/app.min.js"></script>
<script src="libs/gitbook-2.6.7/js/lunr.js"></script>
<script src="libs/gitbook-2.6.7/js/plugin-search.js"></script>
<script src="libs/gitbook-2.6.7/js/plugin-sharing.js"></script>
<script src="libs/gitbook-2.6.7/js/plugin-fontsettings.js"></script>
<script src="libs/gitbook-2.6.7/js/plugin-bookdown.js"></script>
<script src="libs/gitbook-2.6.7/js/jquery.highlight.js"></script>
<script>
gitbook.require(["gitbook"], function(gitbook) {
gitbook.start({
"sharing": {
"github": false,
"facebook": true,
"twitter": true,
"google": false,
"linkedin": false,
"weibo": false,
"instapaper": false,
"vk": false,
"all": ["facebook", "google", "twitter", "linkedin", "weibo", "instapaper"]
},
"fontsettings": {
"theme": "white",
"family": "sans",
"size": 2
},
"edit": {
"link": null,
"text": null
},
"history": {
"link": null,
"text": null
},
"download": null,
"toc": {
"collapse": "subsection"
}
});
});
</script>
</body>
</html>