chapter4.txt

# FILE HANDLING

**CAVEAT:** `bash` does not handle binary data properly in versions `< 4.4`.

## Read a file to a string

Alternative to the `cat` command.

```shell
file_data="$(<"file")"
```

## Read a file to an array (*by line*)

Alternative to the `cat` command.

```shell
# Bash <4
IFS=$'\n' read -d "" -ra file_data < "file"

# Bash 4+
mapfile -t file_data < "file"
```

## Get the first N lines of a file

Alternative to the `head` command.

**CAVEAT:** Requires `bash` 4+

**Example Function:**

```sh
head() {
    # Usage: head "n" "file"
    mapfile -tn "$1" line < "$2"
    printf '%s\n' "${line[@]}"
}
```

**Example Usage:**

```shell
$ head 2 ~/.bashrc
# Prompt
PS1='➜ '

$ head 1 ~/.bashrc
# Prompt
```

## Get the last N lines of a file

Alternative to the `tail` command.

**CAVEAT:** Requires `bash` 4+

**Example Function:**

```sh
tail() {
    # Usage: tail "n" "file"
    mapfile -tn 0 line < "$2"
    printf '%s\n' "${line[@]: -$1}"
}
```

**Example Usage:**

```shell
$ tail 2 ~/.bashrc
# Enable tmux.
# [[ -z "$TMUX"  ]] && exec tmux

$ tail 1 ~/.bashrc
# [[ -z "$TMUX"  ]] && exec tmux
```

## Get the number of lines in a file

Alternative to `wc -l`.

**Example Function (bash 4):**

```sh
lines() {
    # Usage: lines "file"
    mapfile -tn 0 lines < "$1"
    printf '%s\n' "${#lines[@]}"
}
```

**Example Function (bash 3):**

This method uses less memory than the `mapfile` method and works in `bash` 3 but it is slower for bigger files.

```sh
lines_loop() {
    # Usage: lines_loop "file"
    count=0
    while IFS= read -r _; do
        ((count++))
    done < "$1"
    printf '%s\n' "$count"
}
```

**Example Usage:**

```shell
$ lines ~/.bashrc
48

$ lines_loop ~/.bashrc
48
```

## Count files or directories in directory

This works by passing the output of the glob to the function and then counting the number of arguments.

**Example Function:**

```sh
count() {
    # Usage: count /path/to/dir/*
    #        count /path/to/dir/*/
    printf '%s\n' "$#"
}
```

**Example Usage:**

```shell
# Count all files in dir.
$ count ~/Downloads/*
232

# Count all dirs in dir.
$ count ~/Downloads/*/
45

# Count all jpg files in dir.
$ count ~/Pictures/*.jpg
64
```

## Create an empty file

Alternative to `touch`.

```shell
# Shortest.
>file

# Longer alternatives:
:>file
echo -n >file
printf '' >file
```

## Extract lines between two markers

**Example Function:**

```sh
extract() {
    # Usage: extract file "opening marker" "closing marker"
    while IFS=$'\n' read -r line; do
        [[ "$extract" && "$line" != "$3" ]] && \
            printf '%s\n' "$line"

        [[ "$line" == "$2" ]] && extract=1
        [[ "$line" == "$3" ]] && extract=
    done < "$1"
}
```

**Example Usage:**

```shell
# Extract code blocks from MarkDown file.
$ extract ~/projects/pure-bash/README.md '```sh' '```'
# Output here...
```

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