### Reverse an array in Bash

Source: https://github.com/dylanaraps/pure-bash-bible/blob/master/README.md

Demonstrates how to reverse an array in Bash using the `extdebug` option and the `BASH_ARGV` array. Requires enabling `extdebug` and `compat44` in Bash 5.0+. The example includes a function definition and usage examples.

```shell
reverse_array() {
    # Usage: reverse_array "array"
    shopt -s extdebug
    f()(printf '%s\n' "${BASH_ARGV[@]}"); f "$@"
    shopt -u extdebug
}
```

```shell
$ reverse_array 1 2 3 4 5
5
4
3
2
1

$ arr=(red blue green)
$ reverse_array "${arr[@]}"
green
blue
red
```

--------------------------------

### Check if string starts with sub-string

Source: https://github.com/dylanaraps/pure-bash-bible/blob/master/README.md

Verifies if a string begins with a specified substring.

```bash
# Check if string starts with sub-string
string="hello world"
substring="hello"

if [[ "$string" == "$substring"* ]]; then
  echo "String starts with substring."
else
  echo "String does not start with substring."
fi
```

--------------------------------

### Get the first N lines of a file

Source: https://github.com/dylanaraps/pure-bash-bible/blob/master/README.md

Extracts the initial N lines from a given file.

```bash
# Get the first N lines of a file
N=3
head -n $N filename.txt
```

--------------------------------

### Example Test Case

Source: https://github.com/dylanaraps/pure-bash-bible/blob/master/CONTRIBUTING.md

Demonstrates how to write a unit test for a bash function. Tests should be named following the `test_func_name` convention and use `assert_equals` for validation.

```sh
test_upper() {
    result="$(upper "HeLlO")"
    assert_equals "$result" "HELLO"
}
```

--------------------------------

### Bash Brace Expansion: String Lists

Source: https://github.com/dylanaraps/pure-bash-bible/blob/master/manuscript/chapter9.txt

Illustrates how to use brace expansion to create lists of strings and provides a practical example of removing multiple directories using this feature.

```shell
echo {apples,oranges,pears,grapes}

# Example Usage:
# Remove dirs Movies, Music and ISOS from ~/Downloads/.
rm -rf ~/Downloads/{Movies,Music,ISOS}
```

--------------------------------

### Get Operating System Architecture

Source: https://github.com/dylanaraps/pure-bash-bible/blob/master/manuscript/chapter16.txt

Returns the architecture of the operating system, such as x86_64 or arm64.

```shell
"$HOSTTYPE"
```

--------------------------------

### Loop over files and directories in Bash

Source: https://github.com/dylanaraps/pure-bash-bible/blob/master/README.md

Shows how to loop over files and directories in Bash using a `for` loop and wildcard characters. The example includes looping over all files in the current directory, PNG files in a specific directory, directories in a specific directory, and files using brace expansion. It also demonstrates recursive iteration using `globstar`.

```shell
# Greedy example.
for file in *; do
    printf '%s\n' "$file"
done

# PNG files in dir.
for file in ~/Pictures/*.png; do
    printf '%s\n' "$file"
done

# Iterate over directories.
for dir in ~/Downloads/*/; do
    printf '%s\n' "$dir"
done

# Brace Expansion.
for file in /path/to/parentdir/{file1,file2,subdir/file3}; do
    printf '%s\n' "$file"
done

# Iterate recursively.
shopt -s globstar
for file in ~/Pictures/**/*; do
    printf '%s\n' "$file"
done
shopt -u globstar
```

--------------------------------

### Get a random array element in Bash

Source: https://github.com/dylanaraps/pure-bash-bible/blob/master/README.md

Demonstrates how to retrieve a random element from an array in Bash using the `RANDOM` variable. The example includes a function definition and usage examples.

```shell
random_array_element() {
    # Usage: random_array_element "array"
    local arr=("$@")
    printf '%s\n' "${arr[RANDOM % $#]}"
}
```

```shell
$ array=(red green blue yellow brown)
$ random_array_element "${array[@]}"
yellow

# Multiple arguments can also be passed.
$ random_array_element 1 2 3 4 5 6 7
3
```

--------------------------------

### Strip Pattern from Start of String

Source: https://github.com/dylanaraps/pure-bash-bible/blob/master/README.md

This function removes a pattern from the beginning (left side) of a string. The `lstrip` function takes the input string and the pattern to remove from the start, printing the result.

```bash
lstrip() {
    # Usage: lstrip "string" "pattern"
    printf '%s\n' "${1##$2}"
}
```

--------------------------------

### Loop over a range of numbers in Bash

Source: https://github.com/dylanaraps/pure-bash-bible/blob/master/README.md

Demonstrates how to loop over a range of numbers in Bash using brace expansion and a `for` loop. This example shows how to loop from 0 to 100 and how to loop from 0 to a variable value.

```shell
# Loop from 0-100 (no variable support).
for i in {0..100}; do
    printf '%s\n' "$i"
done
```

```shell
# Loop from 0-VAR.
VAR=50
for ((i=0;i<=VAR;i++)); do
    printf '%s\n' "$i"
done
```

--------------------------------

### Bash Brace Expansion: Numerical and Alphabetical Ranges

Source: https://github.com/dylanaraps/pure-bash-bible/blob/master/manuscript/chapter9.txt

Demonstrates the use of brace expansion to generate sequences of numbers and characters. Includes examples for simple ranges, zero-padded numbers (Bash 4+), and ranges with custom increments (Bash 4+).

```shell
# Print numbers 1-100.
echo {1..100}

# Print range of floats.
echo 1.{1..9}

# Print chars a-z.
echo {a..z}
echo {A..Z}

# Nesting.
echo {A..Z}{0..9}

# Print zero-padded numbers.
# CAVEAT: bash 4+
echo {01..100}

# Change increment amount.
# Syntax: {<START>..<END>..<INCREMENT>}
# CAVEAT: bash 4+
echo {1..10..2} # Increment by 2.
```

--------------------------------

### Loop over an array in Bash

Source: https://github.com/dylanaraps/pure-bash-bible/blob/master/README.md

Shows how to loop over an array in Bash using a `for` loop. The example includes looping over just the elements and looping over both the elements and their indices.

```shell
arr=(apples oranges tomatoes)

# Just elements.
for element in "${arr[@]}"; do
    printf '%s\n' "$element"
done
```

```shell
arr=(apples oranges tomatoes)

# Elements and index.
for i in "${!arr[@]}"; do
    printf '%s\n' "${arr[i]}"
done

# Alternative method.
for ((i=0;i<${#arr[@]};i++)); do
    printf '%s\n' "${arr[i]}"
done
```

--------------------------------

### Bash Traps for Signal Handling

Source: https://github.com/dylanaraps/pure-bash-bible/blob/master/README.md

Explains how to use the `trap` command in bash to execute code in response to various signals. Examples include executing code on script exit, ignoring terminal interrupts (SIGINT), reacting to window resize (SIGWINCH), executing code before every command (DEBUG), and executing code when a function or sourced file finishes (RETURN).

```shell
# Clear screen on script exit.
trap 'printf \e[2J\e[H\e[m' EXIT

trap '' INT

# Call a function on window resize.
trap 'code_here' SIGWINCH

trap 'code_here' DEBUG

trap 'code_here' RETURN
```

--------------------------------

### Strip pattern from start of string

Source: https://github.com/dylanaraps/pure-bash-bible/blob/master/README.md

Removes a specified pattern if it appears at the beginning of a string.

```bash
# Strip pattern from start of string
string="prefix_hello_world"
pattern="prefix_"
stripped_string="${string#${pattern}}"
echo "${stripped_string}"
```

--------------------------------

### Get System Hostname

Source: https://github.com/dylanaraps/pure-bash-bible/blob/master/manuscript/chapter16.txt

Retrieves the hostname of the system. It includes a fallback to the `hostname` command if the variable is empty.

```shell
"$HOSTNAME"

# NOTE: This variable may be empty.
# Optionally set a fallback to the hostname command.
"${HOSTNAME:-$(hostname)}"
```

--------------------------------

### Get the first N lines of a file

Source: https://github.com/dylanaraps/pure-bash-bible/blob/master/manuscript/chapter4.txt

A Bash function to retrieve the first N lines of a file, serving as a pure Bash alternative to the `head` command. Requires Bash 4+.

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

--------------------------------

### Cycle through an array in Bash

Source: https://github.com/dylanaraps/pure-bash-bible/blob/master/README.md

Shows how to cycle through an array in Bash, printing each element in sequence and then restarting from the beginning. This example uses a global index variable to keep track of the current position in the array.

```shell
arr=(a b c d)

cycle() {
    printf '%s ' "${arr[${i:=0}]}"
    ((i=i>=${#arr[@]}-1?0:++i))
}
```

--------------------------------

### Bash Parameter Expansion: Length

Source: https://github.com/dylanaraps/pure-bash-bible/blob/master/README.md

Shows how to get the length of a variable or an array using bash parameter expansion.

```bash
# Length

# Length of var in characters
# Example: var="hello"; echo "${#var}" # Output: 5

# Length of array in elements
# Example: arr=("a" "b" "c"); echo "${#arr[@]}" # Output: 3
```

--------------------------------

### Check if String Starts with Substring

Source: https://github.com/dylanaraps/pure-bash-bible/blob/master/manuscript/chapter1.txt

Checks if a given string begins with a specific substring using Bash's pattern matching within conditional expressions.

```shell
if [[ $var == sub_string* ]]; then
    printf '%s\n' "var starts with sub_string."
fi

# Inverse (var does not start with sub_string).
if [[ $var != sub_string* ]]; then
    printf '%s\n' "var does not start with sub_string."
fi
```

--------------------------------

### Use regex on a string

Source: https://github.com/dylanaraps/pure-bash-bible/blob/master/manuscript/chapter1.txt

Applies a regular expression to a string and prints the first matching capture group. This function can replace sed for many use cases but is platform-dependent on the installed regex engine.

```bash
regex() {
    # Usage: regex "string" "regex"
    [[ $1 =~ $2 ]] && printf '%s\n' "${BASH_REMATCH[1]}"
}
```

--------------------------------

### Get Current Working Directory

Source: https://github.com/dylanaraps/pure-bash-bible/blob/master/manuscript/chapter16.txt

Retrieves the current working directory. This serves as an alternative to the `pwd` built-in command.

```shell
"$PWD"
```

--------------------------------

### Get the first N lines of a file

Source: https://github.com/dylanaraps/pure-bash-bible/blob/master/README.md

A bash function to replicate the `head` command's functionality, retrieving the first N lines of a file. Requires bash 4+.

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

--------------------------------

### Get Bash Version Information

Source: https://github.com/dylanaraps/pure-bash-bible/blob/master/manuscript/chapter16.txt

Provides the version of the currently running Bash process. It can be accessed as a string or an array for detailed version components.

```shell
# As a string.
"$BASH_VERSION"
```

```shell
# As an array.
"${BASH_VERSINFO[@]}"
```

--------------------------------

### Strip Pattern from Start of String

Source: https://github.com/dylanaraps/pure-bash-bible/blob/master/manuscript/chapter1.txt

Removes a specified pattern from the beginning of a given string. It utilizes Bash's parameter expansion for efficient string manipulation.

```shell
lstrip() {
    # Usage: lstrip "string" "pattern"
    printf '%s\n' "${1##$2}"
}
```

--------------------------------

### Check if a program is in the user's PATH

Source: https://github.com/dylanaraps/pure-bash-bible/blob/master/manuscript/chapter19.txt

Provides three methods (type -p, hash, command -v) to check if an executable is available in the system's PATH. Includes examples for conditional execution and early exit if a program is not found.

```sh
# There are 3 ways to do this and either one can be used.
type -p executable_name &>/dev/null
hash executable_name &>/dev/null
command -v executable_name &>/dev/null

# As a test.
if type -p executable_name &>/dev/null;
    # Program is in PATH.
fi

# Inverse.
if ! type -p executable_name &>/dev/null;
    # Program is not in PATH.
fi

# Example (Exit early if program is not installed).
if ! type -p convert &>/dev/null;
    printf '%s\n' "error: convert is not installed, exiting..."
    exit 1
fi
```

--------------------------------

### Use Regex on a String

Source: https://github.com/dylanaraps/pure-bash-bible/blob/master/README.md

This function demonstrates using bash's built-in regex matching to extract captured groups from a string. It's a platform-dependent feature that relies on the system's regex engine. The example function `regex` takes a string and a regex pattern, returning the first captured group.

```bash
regex() {
    # Usage: regex "string" "regex"
    [[ $1 =~ $2 ]] && printf '%s\n' "${BASH_REMATCH[1]}"
}
```

--------------------------------

### Get the number of lines in a file

Source: https://github.com/dylanaraps/pure-bash-bible/blob/master/manuscript/chapter4.txt

Provides pure Bash functions to count the number of lines in a file, as an alternative to `wc -l`. Includes a Bash 4+ `mapfile` version and a Bash 3 compatible loop version.

```shell
# Example Function (bash 4):
lines() {
    # Usage: lines "file"
    mapfile -tn 0 lines < "$1"
    printf '%s\n' "${#lines[@]}"
}

# Example Function (bash 3):
lines_loop() {
    # Usage: lines_loop "file"
    count=0
    while IFS= read -r _; do
        ((count++))
    done < "$1"
    printf '%s\n' "$count"
}
```

--------------------------------

### Ignored Code Blocks

Source: https://github.com/dylanaraps/pure-bash-bible/blob/master/CONTRIBUTING.md

Code blocks marked with 'shell' are intended for examples or syntax that should not be linted or tested. This is useful for demonstrating specific shell features or shorthand.

```shell
# Shorter file creation syntax.
:>file
```

--------------------------------

### Create an empty file

Source: https://github.com/dylanaraps/pure-bash-bible/blob/master/manuscript/chapter4.txt

Demonstrates multiple pure Bash methods for creating an empty file, serving as alternatives to the `touch` command. Includes the shortest syntax `>file`.

```shell
# Shortest.
>file

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

--------------------------------

### Check if a string starts or ends with a substring in Bash

Source: https://github.com/dylanaraps/pure-bash-bible/blob/master/README.md

Shows how to check if a string starts or ends with a specific substring using the `[[ ]]` test in Bash. The `==` operator with wildcard characters is used to match the beginning or end of the string. Includes inverse checks to determine if a string does not start or end with a substring.

```shell
if [[ $var == sub_string* ]]; then
    printf '%s\n' "var starts with sub_string."
fi

# Inverse (var does not start with sub_string).
if [[ $var != sub_string* ]]; then
    printf '%s\n' "var does not start with sub_string."
fi
```

```shell
if [[ $var == *sub_string ]]; then
    printf '%s\n' "var ends with sub_string."
fi

# Inverse (var does not end with sub_string).
if [[ $var != *sub_string ]]; then
    printf '%s\n' "var does not end with sub_string."
fi
```

--------------------------------

### Shorter Bash 'for' Loop Syntaxes

Source: https://github.com/dylanaraps/pure-bash-bible/blob/master/manuscript/chapter18.txt

Demonstrates various concise syntaxes for 'for' loops in Bash, including C-style loops, undocumented methods, and expansion techniques for iterating through sequences.

```shell
# Tiny C Style.
for((;i++<10;)){ echo "$i";}

# Undocumented method.
for i in {1..10};{ echo "$i";}

# Expansion.
for i in {1..10}; do echo "$i"; done

# C Style.
for((i=0;i<=10;i++)); do echo "$i"; done
```

--------------------------------

### Running Tests and Shellcheck

Source: https://github.com/dylanaraps/pure-bash-bible/blob/master/CONTRIBUTING.md

This snippet shows the command to execute the test script, which also runs Shellcheck on the project's code. Ensure you are in the project directory before running.

```bash
cd pure-bash-bible
./test.sh
```

--------------------------------

### Get the list of functions in a script

Source: https://github.com/dylanaraps/pure-bash-bible/blob/master/manuscript/chapter19.txt

A Bash function that lists all declared functions within the current script. It uses 'declare -F' to get the function names and then formats the output.

```sh
get_functions() {
    # Usage: get_functions
    IFS=$'\n' read -d "" -ra functions < <(declare -F)
    printf '%s\n' "${functions[@]//declare -f }"
}
```

--------------------------------

### Shorter Bash Infinite Loop Syntaxes

Source: https://github.com/dylanaraps/pure-bash-bible/blob/master/manuscript/chapter18.txt

Presents more compact ways to create infinite loops in Bash compared to the standard 'while :;' syntax. It includes C-style and other efficient methods.

```shell
# Normal method
while :; do echo hi; done

# Shorter
for((;;)){ echo hi;}
```

--------------------------------

### Remove duplicate array elements in Bash

Source: https://github.com/dylanaraps/pure-bash-bible/blob/master/README.md

Shows how to remove duplicate elements from an array in Bash using an associative array. Requires Bash 4+. The example includes a function definition and usage examples. Note that the list order may not be preserved.

```shell
remove_array_dups() {
    # Usage: remove_array_dups "array"
    declare -A tmp_array

    for i in "$@"; do
        [[ $i ]] && IFS=" " tmp_array["${i:- }"]=1
    done

    printf '%s\n' "${!tmp_array[@]}"
}
```

```shell
$ remove_array_dups 1 1 2 2 3 3 3 3 3 4 4 4 4 4 5 5 5 5 5 5
1
2
3
4
5

$ arr=(red red green blue blue)
$ remove_array_dups "${arr[@]}"
red
green
blue
```

--------------------------------

### Create an empty file

Source: https://github.com/dylanaraps/pure-bash-bible/blob/master/README.md

Provides several pure bash methods to create an empty file, serving as an alternative to the `touch` command.

```shell
# Shortest.
>file

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

--------------------------------

### Get the number of lines in a file

Source: https://github.com/dylanaraps/pure-bash-bible/blob/master/README.md

Counts the total number of lines present in a file.

```bash
# Get the number of lines in a file
line_count=$(wc -l < filename.txt)
echo "Number of lines: $line_count"
```

--------------------------------

### Shorter `for` Loop Syntax

Source: https://github.com/dylanaraps/pure-bash-bible/blob/master/README.md

Demonstrates various concise syntaxes for `for` loops in bash, including C-style and brace expansion.

```bash
# Tiny C Style.
for((;i++<10;)){ echo "$i";}

# Undocumented method.
for i in {1..10};{ echo "$i";}

# Expansion.
for i in {1..10}; do echo "$i"; done

# C Style.
for((i=0;i<=10;i++)); do echo "$i"; done
```

--------------------------------

### Alternative to sleep using read

Source: https://github.com/dylanaraps/pure-bash-bible/blob/master/manuscript/chapter19.txt

Demonstrates using the 'read' command with a timeout as an alternative to the 'sleep' command. This method avoids spawning an external process. Requires Bash 4+.

```sh
read_sleep() {
    # Usage: read_sleep 1
    #        read_sleep 0.2
    read -rt "$1" <> <(:) || :
}

# Example Usage:
read_sleep 1
read_sleep 0.1
read_sleep 30
```

```sh
exec {sleep_fd}<> <(:)
while some_quick_test; do
    # equivalent of sleep 0.001
    read -t 0.001 -u $sleep_fd
done
```

--------------------------------

### Loop over a variable range of numbers

Source: https://github.com/dylanaraps/pure-bash-bible/blob/master/README.md

Iterates through numbers where the start and end points are defined by variables.

```bash
# Loop over a variable range of numbers
start=3
end=7

for (( i=$start; i<=$end; i++ )); do
  echo "Number: $i"
done
```

--------------------------------

### Get Username

Source: https://github.com/dylanaraps/pure-bash-bible/blob/master/README.md

Retrieves the username of the current user using printf. Requires bash 4.4+.

```bash
# $ : \u
# Expand the parameter as if it were a prompt string.
# $ printf '%s\n' "${_@P}"
# black
```

--------------------------------

### String Lists

Source: https://github.com/dylanaraps/pure-bash-bible/blob/master/README.md

Creates combinations of strings using brace expansion.

```bash
# String Lists
echo file_{a,b,c}.txt
echo {start,middle,end}_section
```

--------------------------------

### Get the base-name of a file path

Source: https://github.com/dylanaraps/pure-bash-bible/blob/master/README.md

Retrieves the filename (without the directory path) from a file path.

```bash
# Get the base-name of a file path
filepath="/home/user/documents/report.txt"
base_name=$(basename "$filepath")
echo "Base name: $base_name"
```

--------------------------------

### Loop Over Files and Directories

Source: https://github.com/dylanaraps/pure-bash-bible/blob/master/manuscript/chapter3.txt

Demonstrates various methods for iterating over files and directories using globbing patterns. Includes greedy matching, specific file types, subdirectories, brace expansion, and recursive globbing with `globstar`.

```shell
# Greedy example.
for file in *; do
    printf '%s\n' "$file"
done

# PNG files in dir.
for file in ~/Pictures/*.png; do
    printf '%s\n' "$file"
done

# Iterate over directories.
for dir in ~/Downloads/*/; do
    printf '%s\n' "$dir"
done

# Brace Expansion.
for file in /path/to/parentdir/{file1,file2,subdir/file3}; do
    printf '%s\n' "$file"
done

# Iterate recursively.
shopt -s globstar
for file in ~/Pictures/**/*; do
    printf '%s\n' "$file"
done
shopt -u globstar
```

--------------------------------

### Get the directory name of a file path

Source: https://github.com/dylanaraps/pure-bash-bible/blob/master/README.md

Extracts the directory portion from a given file path.

```bash
# Get the directory name of a file path
filepath="/home/user/documents/report.txt"
directory_name=$(dirname "$filepath")
echo "Directory: $directory_name"
```

--------------------------------

### Shorter Bash Function Declaration Syntaxes

Source: https://github.com/dylanaraps/pure-bash-bible/blob/master/manuscript/chapter18.txt

Illustrates various condensed methods for declaring functions in Bash, including using subshells, arithmetic expansion, and conditional statements for brevity.

```shell
# Normal method
f(){ echo hi;}

# Using a subshell
f()(echo hi)

# Using arithmetic
# This can be used to assign integer values.
# Example: f a=1
#          f a++
f()(($1))

# Using tests, loops etc.
# NOTE: ‘while’, ‘until’, ‘case’, ‘(())’, ‘[[]]’ can also be used.
f()if true; then echo "$1"; fi
f()for i in "$@"; do echo "$i"; done
```

--------------------------------

### Get the last N lines of a file

Source: https://github.com/dylanaraps/pure-bash-bible/blob/master/README.md

Retrieves the final N lines from a specified file.

```bash
# Get the last N lines of a file
N=3
tail -n $N filename.txt
```

--------------------------------

### Create an empty file

Source: https://github.com/dylanaraps/pure-bash-bible/blob/master/README.md

Generates a new, empty file.

```bash
# Create an empty file
touch new_empty_file.txt
```

--------------------------------

### Shorter Infinite Loops

Source: https://github.com/dylanaraps/pure-bash-bible/blob/master/README.md

Presents more compact ways to create infinite loops in bash using `while` and C-style `for` loops.

```bash
# Normal method
while :; do echo hi; done

# Shorter
for((;;)){ echo hi;}
```

--------------------------------

### Get Script Runtime in Seconds

Source: https://github.com/dylanaraps/pure-bash-bible/blob/master/manuscript/chapter16.txt

Returns the number of seconds the current script has been running.

```shell
"$SECONDS"
```

--------------------------------

### Get List of Functions in a Script

Source: https://github.com/dylanaraps/pure-bash-bible/blob/master/README.md

A bash function to retrieve and print all declared functions within the current script.

```bash
get_functions() {
    # Usage: get_functions
    IFS=$'\n' read -d "" -ra functions < <(declare -F)
    printf '%s\n' "${functions[@]//declare -f }"
}
```

--------------------------------

### Bitwise

Source: https://github.com/dylanaraps/pure-bash-bible/blob/master/README.md

No description

--------------------------------

### Get a Random Array Element in Bash

Source: https://github.com/dylanaraps/pure-bash-bible/blob/master/manuscript/chapter2.txt

Selects and prints a random element from an array using the `RANDOM` variable.

```bash
random_array_element() {
    # Usage: random_array_element "array"
    local arr=($"@")
    printf '%s\n' "${arr[RANDOM % $#]}"
}
```

--------------------------------

### Bash Parameter Expansion - Default Values

Source: https://github.com/dylanaraps/pure-bash-bible/blob/master/README.md

Demonstrates how to provide default values for shell variables if they are unset or empty. This is useful for setting default configurations or handling optional parameters.

```shell
`${VAR:-STRING}`: If VAR is empty or unset, use STRING as its value.
`${VAR-STRING}`: If VAR is unset, use STRING as its value.
`${VAR:=STRING}`: If VAR is empty or unset, set the value of VAR to STRING.
`${VAR=STRING}`: If VAR is unset, set the value of VAR to STRING.
`${VAR:+STRING}`: If VAR is not empty, use STRING as its value.
`${VAR+STRING}`: If VAR is set, use STRING as its value.
`${VAR:?STRING}`: Display an error if empty or unset.
`${VAR?STRING}`: Display an error if unset.
```

--------------------------------

### Get Pseudorandom Integer

Source: https://github.com/dylanaraps/pure-bash-bible/blob/master/manuscript/chapter16.txt

Generates a pseudorandom integer between 0 and 32767. This variable is not suitable for security-sensitive operations.

```shell
"$RANDOM"
```

--------------------------------

### Get the username of the current user

Source: https://github.com/dylanaraps/pure-bash-bible/blob/master/manuscript/chapter19.txt

Retrieves the username of the current user by expanding a parameter as if it were a prompt string. Requires Bash 4.4+.

```sh
$ : \u
# Expand the parameter as if it were a prompt string.
$ printf '%s\n' "${_@P}"
black
```

--------------------------------

### Shorter Bash 'if' Statement Syntaxes

Source: https://github.com/dylanaraps/pure-bash-bible/blob/master/manuscript/chapter18.txt

Demonstrates concise ways to write 'if' statements in Bash, including single-line conditional execution with '&&' and '||', and multi-line structures without explicit 'if' keywords.

```shell
# One line
# Note: The 3rd statement may run when the 1st is true
[[ $var == hello ]] && echo hi || echo bye
[[ $var == hello ]] && { echo hi; echo there; } || echo bye

# Multi line (no else, single statement)
# Note: The exit status may not be the same as with an if statement
[[ $var == hello ]] &&
    echo hi

# Multi line (no else)
[[ $var == hello ]] && {
    echo hi
    # ...
}
```

--------------------------------

### Get Bash Binary Location

Source: https://github.com/dylanaraps/pure-bash-bible/blob/master/manuscript/chapter16.txt

Retrieves the path to the bash binary. This is a fundamental variable for executing bash scripts.

```shell
"$BASH"
```

--------------------------------

### Get Cursor Position

Source: https://github.com/dylanaraps/pure-bash-bible/blob/master/README.md

Obtains the current cursor position (X and Y coordinates) within the terminal, useful for creating TUI applications in bash.

```bash
get_cursor_pos() {
    # Usage: get_cursor_pos
    IFS='[;' read -p $'e[6n' -d R -rs _ y x _
    printf '%s\n' "$x $y"
}
```

--------------------------------

### Bash Brace Expansion - String Lists

Source: https://github.com/dylanaraps/pure-bash-bible/blob/master/README.md

Shows how to use brace expansion to expand a list of strings. This is commonly used for operations on multiple items, such as deleting directories.

```shell
echo {apples,oranges,pears,grapes}

# Example Usage:
# Remove dirs Movies, Music and ISOS from ~/Downloads/.
rm -rf ~/Downloads/{Movies,Music,ISOS}
```

--------------------------------

### Get Current Date with strftime

Source: https://github.com/dylanaraps/pure-bash-bible/blob/master/README.md

Retrieves the current date using bash's printf command with strftime formatting. Requires bash 4+.

```bash
date() {
    # Usage: date "format"
    # See: 'man strftime' for format.
    printf "%($1)T\n" "-1"
}

# Example Usage:
# $ date "%a %d %b  - %l:%M %p"
# Fri 15 Jun  - 10:00 AM

# Using printf directly.
# $ printf '%(%a %d %b  - %l:%M %p)T\n' "-1"
# Fri 15 Jun  - 10:00 AM

# Assigning a variable using printf.
# $ printf -v date '%(%a %d %b  - %l:%M %p)T\n' '-1'
# $ printf '%s\n' "$date"
# Fri 15 Jun  - 10:00 AM
```

--------------------------------

### Modern Bash Shebang Syntax

Source: https://github.com/dylanaraps/pure-bash-bible/blob/master/README.md

Recommends using `#!/usr/bin/env bash` over `#!/bin/bash` for the shebang line, as it searches the user's PATH for the bash binary, improving portability.

```shell
# Right:

    #!/usr/bin/env bash

# Less right:

    #!/bin/bash
```

--------------------------------

### Get Terminal Size

Source: https://github.com/dylanaraps/pure-bash-bible/blob/master/README.md

Retrieves the terminal size (lines and columns) using ANSI escape codes. Note that this may not work in all terminal emulators.

```bash
get_window_size() {
    # Usage: get_window_size
    printf '%b' "${TMUX:+\ePtmux;\e}\e[14t${TMUX:+\e\\}"
    IFS=';t' read -d t -t 0.05 -sra term_size
    printf '%s\n' "${term_size[1]}x${term_size[2]}"
}
```

--------------------------------

### Shebang Syntax Comparison

Source: https://github.com/dylanaraps/pure-bash-bible/blob/master/manuscript/chapter15.txt

Compares the recommended `#!/usr/bin/env bash` shebang with the less portable `#!/bin/bash`. The former uses the PATH to locate the bash executable, ensuring greater compatibility across different systems.

```shell
# Right:

    #!/usr/bin/env bash

# Less right:

    #!/bin/bash
```

--------------------------------

### Get the number of lines in a file

Source: https://github.com/dylanaraps/pure-bash-bible/blob/master/README.md

Calculates the number of lines in a file, serving as an alternative to `wc -l`. Provides implementations for bash 4 and bash 3.

```shell
# Example Function (bash 4):
lines() {
    # Usage: lines "file"
    mapfile -tn 0 lines < "$1"
    printf '%s\n' "${#lines[@]}"
}

# Example Function (bash 3):
lines_loop() {
    # Usage: lines_loop "file"
    count=0
    while IFS= read -r _; do
        ((count++))
    done < "$1"
    printf '%s\n' "$count"
}
```

--------------------------------

### React to Window Resize (SIGWINCH)

Source: https://github.com/dylanaraps/pure-bash-bible/blob/master/manuscript/chapter13.txt

This trap executes a specified command or function when the terminal window is resized. It's commonly used to redraw the user interface in terminal applications.

```shell
# Call a function on window resize.
trap 'code_here' SIGWINCH
```

--------------------------------

### Get Current and Parent Function Names

Source: https://github.com/dylanaraps/pure-bash-bible/blob/master/manuscript/chapter16.txt

Retrieves the name of the current function and its parent functions. This is useful for debugging and understanding call stacks.

```shell
# Current function.
"${FUNCNAME[0]}"

# Parent function.
"${FUNCNAME[1]}"

# So on and so forth.
"${FUNCNAME[2]}"
"${FUNCNAME[3]}"

# All functions including parents.
"${FUNCNAME[@]}"
```

--------------------------------

### Bash Parameter Expansion: Replacement

Source: https://github.com/dylanaraps/pure-bash-bible/blob/master/README.md

Illustrates bash parameter expansion for string replacement and pattern removal. Supports removing shortest/longest matches from the start/end of a string and replacing/removing patterns.

```bash
# Replacement

# Remove shortest match of PATTERN from start of string
# Example: var="abcde"; echo "${var#ab}"  # Output: cde

# Remove longest match of PATTERN from start of string
# Example: var="ababcd"; echo "${var##ab}" # Output: cd

# Remove shortest match of PATTERN from end of string
# Example: var="abcde"; echo "${var%de}"  # Output: abc

# Remove longest match of PATTERN from end of string
# Example: var="abcdcd"; echo "${var%%cd}" # Output: ab

# Replace first match with string
# Example: var="ababcd"; echo "${var/ab/XX}" # Output: XXabcd

# Replace all matches with string
# Example: var="ababcd"; echo "${var//ab/XX}" # Output: XXXXcd

# Remove first match
# Example: var="ababcd"; echo "${var/ab}"   # Output: abcd

# Remove all matches
# Example: var="ababcd"; echo "${var//ab}"  # Output: cd
```

--------------------------------

### Get Operating System/Kernel Name

Source: https://github.com/dylanaraps/pure-bash-bible/blob/master/manuscript/chapter16.txt

Provides the name of the operating system or kernel, useful for OS-specific conditional logic without calling `uname`.

```shell
"$OSTYPE"
```

--------------------------------

### Use regex on a string

Source: https://github.com/dylanaraps/pure-bash-bible/blob/master/README.md

Applies a regular expression to a string for pattern matching and replacement.

```bash
# Use regex on a string
string="hello world 123"
# Replace all digits with 'X'
regex_replaced="${string//[0-9]/X}"
echo "${regex_replaced}"
```

--------------------------------

### Loop over the contents of a file in Bash

Source: https://github.com/dylanaraps/pure-bash-bible/blob/master/README.md

Demonstrates how to loop over the contents of a file in Bash using a `while` loop and the `read` command. Each line of the file is read into the `line` variable and then printed.

```shell
while read -r line; do
    printf '%s\n' "$line"
done < "file"
```

--------------------------------

### Loop over files and directories

Source: https://github.com/dylanaraps/pure-bash-bible/blob/master/README.md

Iterates through files and directories within a specified directory.

```bash
# Loop over files and directories
for item in *; do
  if [[ -f "$item" ]]; then
    echo "File: $item"
  elif [[ -d "$item" ]]; then
    echo "Directory: $item"
  fi
done
```

--------------------------------

### Get the base-name of a file path

Source: https://github.com/dylanaraps/pure-bash-bible/blob/master/README.md

A bash function that replicates the `basename` command, extracting the filename from a path and optionally removing a suffix. It handles paths with and without suffixes.

```shell
basename() {
    # Usage: basename "path" ["suffix"]
    local tmp

    tmp=${1%"${1##*[!/]}"}
    tmp=${tmp##*/}
    tmp=${tmp%"${2/"$tmp"}"}

    printf '%s\n' "${tmp:-/}"
}
```

--------------------------------

### Simpler Bash 'case' Statement for Variable Assignment

Source: https://github.com/dylanaraps/pure-bash-bible/blob/master/manuscript/chapter18.txt

Shows how to use the ':' built-in command within a Bash 'case' statement to efficiently assign values to a variable based on conditions, leveraging the $_ variable.

```shell
# Modified snippet from Neofetch.
case "$OSTYPE" in
    "darwin"*) 
        : "MacOS"
    ;;

    "linux"*) 
        : "Linux"
    ;;

    *"bsd"* | "dragonfly" | "bitrig") 
        : "BSD"
    ;;

    "cygwin" | "msys" | "win32") 
        : "Windows"
    ;;

    *) 
        printf '%s\n' "Unknown OS detected, aborting..." >&2
        exit 1
    ;;
esac

# Finally, set the variable.
os="$_"
```

--------------------------------

### Get the last N lines of a file

Source: https://github.com/dylanaraps/pure-bash-bible/blob/master/README.md

A bash function to replicate the `tail` command's functionality, retrieving the last N lines of a file. Requires bash 4+.

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

--------------------------------

### Get the directory name of a file path

Source: https://github.com/dylanaraps/pure-bash-bible/blob/master/README.md

A bash function that mimics the `dirname` command, extracting the directory portion from a given file path. It handles trailing slashes and root directory cases.

```shell
dirname() {
    # Usage: dirname "path"
    local tmp=${1:-.}

    [[ $tmp != *[!/]* ]] && {
        printf '/\n'
        return
    }

    tmp=${tmp%%"${tmp##*[!/]}"}

    [[ $tmp != */* ]] && {
        printf '.\n'
        return
    }

    tmp=${tmp%/*}
    tmp=${tmp%%"${tmp##*[!/]}"}

    printf '%s\n' "${tmp:-/}"
}
```

--------------------------------

### Open User's Preferred Text Editor

Source: https://github.com/dylanaraps/pure-bash-bible/blob/master/manuscript/chapter16.txt

Opens the user's preferred text editor with a specified file. It includes a fallback mechanism if the EDITOR variable is not set.

```shell
"$EDITOR" "$file"

# NOTE: This variable may be empty, set a fallback value.
"${EDITOR:-vi}" "$file"
```

--------------------------------

### File Comparisons

Source: https://github.com/dylanaraps/pure-bash-bible/blob/master/README.md

Compares files based on modification time or size.

```bash
# File Comparisons
file1="fileA.txt"
file2="fileB.txt"

# Check if file1 is newer than file2
if [[ "$file1" -nt "$file2" ]]; then
  echo "$file1 is newer than $file2."
fi

# Check if file1 is older than file2
if [[ "$file1" -ot "$file2" ]]; then
  echo "$file1 is older than $file2."
fi
```

--------------------------------

### Ranges

Source: https://github.com/dylanaraps/pure-bash-bible/blob/master/README.md

Generates sequences of numbers or characters using brace expansion.

```bash
# Ranges
# Numeric range
echo {1..5}
# Character range
echo {a..e}
```

--------------------------------

### Toggle between two values

Source: https://github.com/dylanaraps/pure-bash-bible/blob/master/README.md

Switches between two predefined values on each execution.

```bash
# Toggle between two values
value1="on"
value2="off"

# Using a variable to keep track of the current state
current_state=${current_state:-$value1}

if [[ "$current_state" == "$value1" ]]; then
  current_state="$value2"
else
  current_state="$value1"
fi

echo "Current state: $current_state"
```

--------------------------------

### Bash Brace Expansion - Ranges

Source: https://github.com/dylanaraps/pure-bash-bible/blob/master/README.md

Illustrates the use of brace expansion to generate sequences of numbers or characters. This feature simplifies the creation of repetitive patterns.

```shell
# Print numbers 1-100.
echo {1..100}

# Print range of floats.
echo 1.{1..9}

# Print chars a-z.
echo {a..z}
echo {A..Z}

# Nesting.
echo {A..Z}{0..9}

# Print zero-padded numbers.
# CAVEAT: bash 4+
echo {01..100}

# Change increment amount.
# Syntax: {<START>..<END>..<INCREMENT>}
# CAVEAT: bash 4+
echo {1..10..2} # Increment by 2.
```

--------------------------------

### Get the last N lines of a file

Source: https://github.com/dylanaraps/pure-bash-bible/blob/master/manuscript/chapter4.txt

A Bash function to retrieve the last N lines of a file, acting as a pure Bash replacement for the `tail` command. Requires Bash 4+.

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

--------------------------------

### Getting Terminal Size in Bash

Source: https://github.com/dylanaraps/pure-bash-bible/blob/master/README.md

Provides a bash function `get_term_size` to retrieve the terminal's dimensions (lines and columns) without relying on external commands like `stty` or `tput`. It utilizes the `checkwinsize` shell option.

```shell
get_term_size() {
    # Usage: get_term_size

    # (:;:) is a micro sleep to ensure the variables are
    # exported immediately.
    shopt -s checkwinsize; (:;:) 
    printf '%s\n' "$LINES $COLUMNS"
}
```

--------------------------------

### Get the current date using printf strftime

Source: https://github.com/dylanaraps/pure-bash-bible/blob/master/manuscript/chapter19.txt

Utilizes Bash's built-in 'printf' command with strftime formatting to obtain the current date and time, serving as an alternative to the 'date' command. Requires Bash 4+.

```sh
date() {
    # Usage: date "format"
    # See: 'man strftime' for format.
    printf "%($1)T\n" "-1"
}

# Using above function.
$ date "%a %d %b  - %l:%M %p"
Fri 15 Jun  - 10:00 AM

# Using printf directly.
$ printf '%(%a %d %b  - %l:%M %p)T\n' "-1"
Fri 15 Jun  - 10:00 AM

# Assigning a variable using printf.
$ printf -v date '%(%a %d %b  - %l:%M %p)T\n' '-1'
$ printf '%s\n' "$date"
Fri 15 Jun  - 10:00 AM
```

--------------------------------

### Bash Text Colors using ANSI Escape Sequences

Source: https://github.com/dylanaraps/pure-bash-bible/blob/master/manuscript/chapter7.txt

Demonstrates how to set text foreground and background colors using ANSI escape sequences. Supports 256-color mode and True-Color (RGB). Sequences require specific terminal emulator support for RGB.

```bash
# Set text foreground color (256 colors)
# Example: Set foreground to color number 16
printf "\e[38;5;16mHello\e[0m\n"

# Set text background color (256 colors)
# Example: Set background to color number 232
printf "\e[48;5;232mWorld\e[0m\n"

# Set text foreground color to RGB
# Example: Set foreground to a shade of blue (R=0, G=0, B=255)
printf "\e[38;2;0;0;255mBlue Text\e[0m\n"

# Set text background color to RGB
# Example: Set background to a shade of green (R=0, G=255, B=0)
printf "\e[48;2;0;255;0mGreen Background\e[0m\n"

# Reset all formatting and colors
printf "\e[0m"

```

--------------------------------

### Bash Arithmetic Operations

Source: https://github.com/dylanaraps/pure-bash-bible/blob/master/manuscript/chapter12.txt

Demonstrates basic arithmetic operations in Bash using the double parenthesis syntax. This includes simple addition, incrementing, decrementing, and using existing variables and array elements in calculations.

```shell
((var=1+2))

((var++))
((var--))
((var+=1))
((var-=1))

((var=var2*arr[2]))
```