Linux cheat sheet

SSH

ssh command consists of 3 different parts:

• ssh command instructs the system to establish an encrypted secure connection with the host machine.
• user_name represents the account that is being accessed on the host.
• host refers to the machine which can be a computer or a router that is being accessed. It can be an IP address (e.g. 192.168.1.24) or domain name(e.g. www.domainname.com).

ssh user_name@host(IP/Domain_name)

ssh-keygen

cat

cat [OPTION] [FILE]...

• display contents of file
• view contents of multiple files

 cat file1 file2

• create file

cat >filename 

• to limit preview of content

cat file | more

cat file | less 

• display line number in file

cat -n filename   

• display $ (end of line) in file

cat -e filename 

• display tab separated lines in file

cat -T filename 

• display multiple files

cat filename0; cat filename1; cat filename2

• standard output with redirection

Standard output of a file can be added into a new file or existing file with ‘>‘ (greater than) symbol. Careful, existing contents of test1 will be overwritten by contents of test file.

cat test0 > test1

similarly contents of multiple can be added into single file (new or existing)

cat test0 test1 test2 > testN 

• Appending standard output with redirection

The contents of test file will be appended at the end of test1 file.

cat test0 >> test1

• Sorting Contents of Multiple Files in a Single File

This will create a file test4 and output of cat command is piped to sort and result will be redirected in a newly created file.

cat test0 test1 test2 test3 | sort > test4

Copying, moving, removing files

• to make a copy of file1 in the current working directory and call it file2

cp file 1 file2

• to move or rename file1 to file2

mv file 1 file2

• to remove file use the following

 rm [OPTION]... FILE...

• to remove empty directories from the filesystem

 rmdir [-p] [-v | –verbose] [–ignore-fail-on-non-empty] directories …..

Special characters

• -

if filename is - (dash), you simply need to give can some indication that you want a litteral file of that name, not the internal alias it has. You can do this easiest by specifying a path to the file

cat ./-

• # Pound/hash

Line beginning with # will not be executed. Used as a comment.

• ; Semicolon

Command separator [semicolon]. Permits putting two or more commands on the same line.

• . Dot

When working with filenames, a leading dot is the prefix of a "hidden" file, a file that an ls will not normally show.

When considering directory names, a . (single dot) represents the current working directory, and .. (two dots) denote the parent directory.

When matching characters, as part of a regular expression, a "." matches a single character.

• \ Backslash

A quoting mechanism for single characters.

• Space (in a filename)

cat "file name with space"

or

cat file\ name\ with\ space

Useful commands

• pwd

print path of the working directory

• ls

It is used to list information about files and directories within the file system.

ls [OPTIONS] [FILES]

option	description
ls -a	list all files including hidden file starting with '.'
ls --color	colored list [=always/never/auto]
ls -d	list directories - with ' */'
ls -F	add one char of */=>@
ls -i	list file's inode index number
ls -l	list with long format - show permissions
ls -la	list long format including hidden files
ls -lh	list long format with readable file size
ls -ls	list with long format with file size
ls -r	list in reverse order
ls -R	list recursively directory tree
ls -s	list file size
ls -t	sort by time & date
ls -X	sort by extension name

List root directory:

ls /

List parent directory:

ls ..

List user's home directory (e.g: /home/user):

ls ~

List all subdirectories:

ls *

• find

find [where to start searching from]
[expression determines what to find] [-options] [what to find]

• grep

 grep [options] pattern [files]

option	description
-c	This prints only a count of the lines that match a pattern
-h	Display the matched lines, but do not display the filenames.
-i	Ignores, case for matching
-l	Displays list of a filenames only.
-n	Display the matched lines and their line numbers.
-v	This prints out all the lines that do not matches the pattern
-e exp	Specifies expression with this option. Can use multiple times.
-f file	Takes patterns from file, one per line
-E	Treats pattern as an extended regular expression (ERE)
-w	Match whole word
-o	Print only the matched parts of a matching line, with each such part on a separate output line.

• chmod

changes the permissions of the file.

chmod [options] mode[,mode] file1 [file2 ...]

option	description
u	user
g	group
o	other
a	all
r	read
w	write (and delete)
x	executable
+	add permission
-	take away permission

• ps

will show the processes status as snapshot, information about your processes, with their associated PID and status, type

PID: Process ID, shows the process identification number.

TTY: Identifies the terminal from which the process was executed.

TIME: Shows the processor’s time occupied by the program.

CMD: Shows the command used to launch the process.

The & at the end runs the job in the background and returns the prompt straight away, allowing you do run other programs while waiting for that one to finish.

• jobs

list current jobs