Bash Programing

Bash Programing¶

Scripting in bash
Learn to make bash scripts
Check your Bash Scripts
Pure Bash Bible

Script:

#!/usr/bin/env bash

#Set Exit on Error
set -o errexit
#Set Error on unset Variables
set -o nounset
#Set Error on a Piped Command notworking
set -o pipefail


#Set Debugging when executed like `TRACE=1 ./script.sh`
if [[ "${TRACE-0}" == "1" ]]; then set -o xtrace; fi


#Use [[ ]] for if and while
if [[ "${1-}" =~ ^-*h(elp)?$ ]]; then
    echo 'Usage: ./script.sh arg-one arg-two

This is an awesome bash script to make your life better.

'
    exit
fi

cd "$(dirname "$0")"

main() {
    echo do awesome stuff
}

main "$@"

Strings¶

Trim leading and trailing white-space from string¶

This is an alternative to sed, awk, perl and other tools. The function below works by finding all leading and trailing white-space and removing it from the start and end of the string. The : built-in is used in place of a temporary variable.

Example Function:

trim_string() {
    # Usage: trim_string "   example   string    "
    : "${1#"${1%%[![:space:]]*}"}"
    : "${_%"${_##*[![:space:]]}"}"
    printf '%s\n' "$_"
}

Example Usage:

$ trim_string "    Hello,  World    "
Hello,  World

$ name="   John Black  "
$ trim_string "$name"
John Black

Trim all white-space from string and truncate spaces¶

This is an alternative to sed, awk, perl and other tools. The function below works by abusing word splitting to create a new string without leading/trailing white-space and with truncated spaces.

Example Function:

# shellcheck disable=SC2086,SC2048
trim_all() {
    # Usage: trim_all "   example   string    "
    set -f
    set -- $*
    printf '%s\n' "$*"
    set +f
}

Example Usage:

$ trim_all "    Hello,    World    "
Hello, World

$ name="   John   Black  is     my    name.    "
$ trim_all "$name"
John Black is my name.

Use regex on a string¶

The result of bash's regex matching can be used to replace sed for a large number of use-cases.

CAVEAT: This example only prints the first matching group. When using multiple capture groups some modification is needed.

Example Function:

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

Example Usage:

$ # Trim leading white-space.
$ regex '    hello' '^\s*(.*)'
hello

$ # Validate a hex color.
$ regex "#FFFFFF" '^(#?([a-fA-F0-9]{6}|[a-fA-F0-9]{3}))$'
#FFFFFF

$ # Validate a hex color (invalid).
$ regex "red" '^(#?([a-fA-F0-9]{6}|[a-fA-F0-9]{3}))$'
# no output (invalid)

Example Usage in script:

is_hex_color() {
    if [[ $1 =~ ^(#?([a-fA-F0-9]{6}|[a-fA-F0-9]{3}))$ ]]; then
        printf '%s\n' "${BASH_REMATCH[1]}"
    else
        printf '%s\n' "error: $1 is an invalid color."
        return 1
    fi
}

read -r color
is_hex_color "$color" || color="#FFFFFF"

# Do stuff.

Split a string on a delimiter¶

This is an alternative to cut, awk and other tools.

Example Function:

split() {
   # Usage: split "string" "delimiter"
   IFS=$'\n' read -d "" -ra arr <<< "${1//$2/$'\n'}"
   printf '%s\n' "${arr[@]}"
}

Example Usage:

$ split "apples,oranges,pears,grapes" ","
apples
oranges
pears
grapes

$ split "1, 2, 3, 4, 5" ", "
1
2
3
4
5

# Multi char delimiters work too!
$ split "hello---world---my---name---is---john" "---"
hello
world
my
name
is
john

Change a string to lowercase¶

Example Function:

lower() {
    # Usage: lower "string"
    printf '%s\n' "${1,,}"
}

Example Usage:

$ lower "HELLO"
hello

$ lower "HeLlO"
hello

$ lower "hello"
hello

Change a string to uppercase¶

Example Function:

upper() {
    # Usage: upper "string"
    printf '%s\n' "${1^^}"
}

Example Usage:

$ upper "hello"
HELLO

$ upper "HeLlO"
HELLO

$ upper "HELLO"
HELLO

Reverse a string case¶

Example Function:

reverse_case() {
    # Usage: reverse_case "string"
    printf '%s\n' "${1~~}"
}

Example Usage:

$ reverse_case "hello"
HELLO

$ reverse_case "HeLlO"
hElLo

$ reverse_case "HELLO"
hello

Trim quotes from a string¶

Example Function:

trim_quotes() {
    # Usage: trim_quotes "string"
    : "${1//\'}"
    printf '%s\n' "${_//\"}"
}

Example Usage:

$ var="'Hello', \"World\""
$ trim_quotes "$var"
Hello, World

Strip all instances of pattern from string¶

Example Function:

strip_all() {
    # Usage: strip_all "string" "pattern"
    printf '%s\n' "${1//$2}"
}

Example Usage:

$ strip_all "The Quick Brown Fox" "[aeiou]"
Th Qck Brwn Fx

$ strip_all "The Quick Brown Fox" "[[:space:]]"
TheQuickBrownFox

$ strip_all "The Quick Brown Fox" "Quick "
The Brown Fox

Strip first occurrence of pattern from string¶

Example Function:

strip() {
    # Usage: strip "string" "pattern"
    printf '%s\n' "${1/$2}"
}

Example Usage:

$ strip "The Quick Brown Fox" "[aeiou]"
Th Quick Brown Fox

$ strip "The Quick Brown Fox" "[[:space:]]"
TheQuick Brown Fox

Strip pattern from start of string¶

Example Function:

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

Example Usage:

$ lstrip "The Quick Brown Fox" "The "
Quick Brown Fox

Strip pattern from end of string¶

Example Function:

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

Example Usage:

$ rstrip "The Quick Brown Fox" " Fox"
The Quick Brown

Percent-encode a string¶

Example Function:

urlencode() {
    # Usage: urlencode "string"
    local LC_ALL=C
    for (( i = 0; i < ${#1}; i++ )); do
        : "${1:i:1}"
        case "$_" in
            [a-zA-Z0-9.~_-])
                printf '%s' "$_"
            ;;

            *)
                printf '%%%02X' "'$_"
            ;;
        esac
    done
    printf '\n'
}

Example Usage:

$ urlencode "https://github.com/dylanaraps/pure-bash-bible"
https%3A%2F%2Fgithub.com%2Fdylanaraps%2Fpure-bash-bible

Decode a percent-encoded string¶

Example Function:

urldecode() {
    # Usage: urldecode "string"
    : "${1//+/ }"
    printf '%b\n' "${_//%/\\x}"
}

Example Usage:

$ urldecode "https%3A%2F%2Fgithub.com%2Fdylanaraps%2Fpure-bash-bible"
https://github.com/dylanaraps/pure-bash-bible

Check if string contains a sub-string¶

Using a test:

if [[ $var == *sub_string* ]]; then
    printf '%s\n' "sub_string is in var."
fi

# Inverse (substring not in string).
if [[ $var != *sub_string* ]]; then
    printf '%s\n' "sub_string is not in var."
fi

# This works for arrays too!
if [[ ${arr[*]} == *sub_string* ]]; then
    printf '%s\n' "sub_string is in array."
fi

Using a case statement:

case "$var" in
    *sub_string*)
        # Do stuff
    ;;

    *sub_string2*)
        # Do more stuff
    ;;

    *)
        # Else
    ;;
esac

Check if string starts with sub-string¶

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

Check if string ends with sub-string¶

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

Length of string¶

var="some string"
echo ${#var}
# 11

Arrays¶

Reverse an array¶

Enabling extdebug allows access to the BASH_ARGV array which stores the current function’s arguments in reverse.

Example Function:

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

Example Usage:

$ reverse_array 1 2 3 4 5
5
4
3
2
1

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

Remove duplicate array elements¶

Create a temporary associative array. When setting associative array values and a duplicate assignment occurs, bash overwrites the key. This allows us to effectively remove array duplicates.

Example Function:

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[@]}"
}

Example Usage:

$ 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

Random array element¶

Example Function:

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

Example Usage:

$ 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

Cycle through an array¶

Each time the printf is called, the next array element is printed. When the print hits the last array element it starts from the first element again.

arr=(a b c d)

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

Toggle between two values¶

This works the same as above, this is just a different use case.

arr=(true false)

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

Loops¶

Loop over a range of numbers¶

Alternative to seq.

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

Loop over a variable range of numbers¶

Alternative to seq.

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

Loop over an array¶

arr=(apples oranges tomatoes)

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

Loop over an array with an index¶

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

Loop over the contents of a file¶

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

Loop over files and directories¶

Don’t use ls.

# 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

File Handling¶

Read a file to a string¶

Alternative to the cat command.

file_data="$(<"file")"

Read a file to an array (by line)¶

Alternative to the cat command.

# 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.

Example Function:

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

Example Usage:

$ head 2 ~/.bashrc
# Prompt
PS1='➜ '

$ head 1 ~/.bashrc
# Prompt

Get the last N lines of a file¶

Alternative to the tail command.

Example Function:

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

Example Usage:

$ 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.x):

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

Example Function (bash 3.x):

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

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

Example Usage:

$ 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:

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

Example Usage:

# 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.

# Shortest.
>file

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

Extract lines between two markers¶

Example Function:

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:

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

File Paths¶

Get the directory name of a file path¶

Alternative to the dirname command.

Example Function:

dirname() {
    # Usage: dirname "path"
    printf '%s\n' "${1%/*}/"
}

Example Usage:

$ dirname ~/Pictures/Wallpapers/1.jpg
/home/black/Pictures/Wallpapers/

$ dirname ~/Pictures/Downloads/
/home/black/Pictures/

Get the base-name of a file path¶

Alternative to the basename command.

Example Function:

basename() {
    # Usage: basename "path"
    : "${1%/}"
    printf '%s\n' "${_##*/}"
}

Example Usage:

$ basename ~/Pictures/Wallpapers/1.jpg
1.jpg

$ basename ~/Pictures/Downloads/
Downloads

Variables¶

Assign and access a variable using a variable¶

$ hello_world="value"

# Create the variable name.
$ var="world"
$ ref="hello_$var"

# Print the value of the variable name stored in 'hello_$var'.
$ printf '%s\n' "${!ref}"
value

Alternatively, on bash 4.3+:

$ hello_world="value"
$ var="world"

# Declare a nameref.
$ declare -n ref=hello_$var

$ printf '%s\n' "$ref"
value

Name a variable based on another variable¶

$ var="world"
$ declare "hello_$var=value"
$ printf '%s\n' "$hello_world"
value

Escape Sequences¶

Text Colors¶

Note

Sequences requiring RGB values only work in True-Color Terminal Emulators.

Sequence	What does it do?	Value
`\e[38;5;<NUM>m`	Set text foreground color.	`0-255`
`\e[48;5;<NUM>m`	Set text background color.	`0-255`
`\e[38;2;<R>;<G>;<B>m`	Set text foreground color to RGB color.	`R`, `G`, `B`
`\e[48;2;<R>;<G>;<B>m`	Set text background color to RGB color.	`R`, `G`, `B`

Text Attributes¶

Sequence	What does it do?
`\e[m`	Reset text formatting and colors.
`\e[1m`	Bold text.
`\e[2m`	Faint text.
`\e[3m`	Italic text.
`\e[4m`	Underline text.
`\e[5m`	Slow blink.
`\e[7m`	Swap foreground and background colors.

Cursor Movement¶

Sequence	What does it do?	Value
`\e[<LINE>;<COLUMN>H`	Move cursor to absolute position.	`line`, `column`
`\e[H`	Move cursor to home position (`0,0`).
`\e[<NUM>A`	Move cursor up N lines.	`num`
`\e[<NUM>B`	Move cursor down N lines.	`num`
`\e[<NUM>C`	Move cursor right N columns.	`num`
`\e[<NUM>D`	Move cursor left N columns.	`num`
`\e[s`	Save cursor position.
`\e[u`	Restore cursor position.

Erasing Text¶

Sequence	What does it do?
`\e[K`	Erase from cursor position to end of line.
`\e[1K`	Erase from cursor position to start of line.
`\e[2K`	Erase the entire current line.
`\e[J`	Erase from the current line to the bottom of the screen.
`\e[1J`	Erase from the current line to the top of the screen.
`\e[2J`	Clear the screen.
`\e[2J\e[H`	Clear the screen and move cursor to `0,0`.

Parameter Extension¶

Indirection¶

Parameter	What does it do?
`${!VAR}`	Access a variable based on the value of `VAR`
`${!VAR*}`	Expand to `IFS` separated list of variable names starting with `VAR`
`${!VAR@}`	Expand to `IFS` separated list of variable names starting with `VAR`. If double-quoted, each variable name expands to a separate word.

Replacement¶

Parameter	What does it do?
`${VAR#PATTERN}`	Remove shortest match of pattern from start of string.
`${VAR##PATTERN}`	Remove longest match of pattern from start of string.
`${VAR%PATTERN}`	Remove shortest match of pattern from end of string.
`${VAR%%PATTERN}`	Remove longest match of pattern from end of string.
`${VAR/PATTERN/REPLACE}`	Replace first match with string.
`${VAR//PATTERN/REPLACE}`	Replace all matches with string.
`${VAR/PATTERN}`	Remove first match.
`${VAR//PATTERN}`	Remove all matches.

Length¶

Parameter	What does it do?
`${#VAR}`	Length of var in characters.
`${#ARR[@]}`	Length of array in elements.

Expansion¶

Parameter	What does it do?
`${VAR:OFFSET}`	Remove first `N` chars from variable.
`${VAR:OFFSET:LENGTH}`	Get substring from `N` character to `N` character. <br> (`${VAR:10:10}`: Get sub-string from char `10` to char `20`)
`${VAR:: OFFSET}`	Get first `N` chars from variable.
`${VAR:: -OFFSET}`	Remove last `N` chars from variable.
`${VAR: -OFFSET}`	Get last `N` chars from variable.
`${VAR:OFFSET:-OFFSET}`	Cut first `N` chars and last `N` chars.

Case Modification¶

Parameter	What does it do?
`${VAR^}`	Uppercase first character.
`${VAR^^}`	Uppercase all characters.
`${VAR,}`	Lowercase first character.
`${VAR,,}`	Lowercase all characters.
`${VAR~}`	Reverse case of first character.
`${VAR~~}`	Reverse case of all characters.

Default Value¶

Parameter	What does it do?
`${VAR:-STRING}`	If `VAR` is empty or unset, use `STRING` as its value.
`${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.

String Lists¶

echo {apples,oranges,pears,grapes}

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

Conditional Expressions¶

File Conditionals¶

Expression	Value	What does it do?
`-a`	`file`	If file exists.
`-b`	`file`	If file exists and is a block special file.
`-c`	`file`	If file exists and is a character special file.
`-d`	`file`	If file exists and is a directory.
`-e`	`file`	If file exists.
`-f`	`file`	If file exists and is a regular file.
`-g`	`file`	If file exists and its set-group-id bit is set.
`-h`	`file`	If file exists and is a symbolic link.
`-k`	`file`	If file exists and its sticky-bit is set
`-p`	`file`	If file exists and is a named pipe (FIFO).
`-r`	`file`	If file exists and is readable.
`-s`	`file`	If file exists and its size is greater than zero.
`-t`	`fd`	If file descriptor is open and refers to a terminal.
`-u`	`file`	If file exists and its set-user-id bit is set.
`-w`	`file`	If file exists and is writable.
`-x`	`file`	If file exists and is executable.
`-G`	`file`	If file exists and is owned by the effective group ID.
`-L`	`file`	If file exists and is a symbolic link.
`-N`	`file`	If file exists and has been modified since last read.
`-O`	`file`	If file exists and is owned by the effective user ID.
`-S`	`file`	If file exists and is a socket.

File Comparisons¶

Expression	What does it do?
`file -ef file2`	If both files refer to the same inode and device numbers.
`file -nt file2`	If `file` is newer than `file2` (uses modification time) or `file` exists and `file2` does not.
`file -ot file2`	If `file` is older than `file2` (uses modification time) or `file2` exists and `file` does not.

Variable Conditionals¶

Expression	Value	What does it do?
`-o`	`opt`	If shell option is enabled.
`-v`	`var`	If variable has a value assigned.
`-R`	`var`	If variable is a name reference.
`-z`	`var`	If the length of string is zero.
`-n`	`var`	If the length of string is non-zero.

Variable Comparisons¶

Expression	What does it do?
`var = var2`	Equal to.
`var == var2`	Equal to (synonym for `=`).
`var != var2`	Not equal to.
`var < var2`	Less than (in ASCII alphabetical order.)
`var > var2`	Greater than (in ASCII alphabetical order.)

Arithmetic Operators¶

Assignment¶

Operators	What does it do?
`=`	Initialize or change the value of a variable.

Arithmetic¶

Operators	What does it do?
`+`	Addition
`-`	Subtraction
`*`	Multiplication
`/`	Division
`**`	Exponentiation
`%`	Modulo
`+=`	Plus-Equal (Increment a variable.)
`-=`	Minus-Equal (Decrement a variable.)
`*=`	Times-Equal (Multiply a variable.)
`/=`	Slash-Equal (Divide a variable.)
`%=`	Mod-Equal (Remainder of dividing a variable.)

Bitwise¶

Operators	What does it do?
`<<`	Bitwise Left Shift
`<<=`	Left-Shift-Equal
`>>`	Bitwise Right Shift
`>>=`	Right-Shift-Equal
`&`	Bitwise AND
`&=`	Bitwise AND-Equal
`\	`	Bitwise OR
`\	=`	Bitwise OR-Equal
`~`	Bitwise NOT
`^`	Bitwise XOR
`^=`	Bitwise XOR-Equal

Logical¶

Operators	What does it do?
`!`	NOT
`&&`	AND
`\	\	`	OR

Miscellaneous¶

Operators	What does it do?	Example
`,`	Comma Separator	`((a=1,b=2,c=3))`

Arithmetic¶

Simpler syntax to set variables¶

# Simple math
((var=1+2))

# Decrement/Increment variable
((var++))
((var--))
((var+=1))
((var-=1))

# Using variables
((var=var2*arr[2]))

Ternary Tests¶

# Set the value of var to var2 if var2 is greater than var.
# var: variable to set.
# var2>var: Condition to test.
# ?var2: If the test succeeds.
# :var: If the test fails.
((var=var2>var?var2:var))

Traps¶

Traps allow a script to execute code on various signals. In pxltrm (a pixel art editor written in bash) traps are used to redraw the user interface on window resize. Another use case is cleaning up temporary files on script exit.

Traps should be added near the start of scripts so any early errors are also caught.

Note

For a full list of signals, see trap -l.

Do something on script exit¶

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

Ignore terminal interrupt (CTRL+C, SIGINT)¶

trap '' INT

React to window resize¶

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

Do something before every command¶

trap 'code_here' DEBUG

Do something when a shell function or a sourced file finishes executing¶

trap 'code_here' RETURN

Performance¶

Disable Unicode¶

If unicode is not required, it can be disabled for a performance increase. Results may vary however there have been noticeable improvements in neofetch and other programs.

# Disable unicode.
LC_ALL=C
LANG=C

Obsolete Syntax¶

Shebang¶

Use #!/usr/bin/env bash instead of #!/bin/bash.

The former searches the user's PATH to find the bash binary.
The latter assumes it is always installed to /bin/ which can cause issues.

Command Substitution¶

Use $() instead of ` `.

# Right.
var="$(command)"

# Wrong.
var=`command`

# $() can easily be nested whereas `` cannot.
var="$(command "$(command)")"

Function Declaration¶

Do not use the function keyword, it reduces compatibility with older versions of bash.

# Right.
do_something() {
    # ...
}

# Wrong.
function do_something() {
    # ...
}

Internal Varables¶

Get the location to the `bash` binary¶

"$BASH"

Get the version of the current running `bash` process¶

# As a string.
"$BASH_VERSION"

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

Open the user's preferred text editor¶

"$EDITOR" "$file"

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

Get the name of the current function¶

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

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

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

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

Get the host-name of the system¶

"$HOSTNAME"

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

Get the architecture of the Operating System¶

"$HOSTTYPE"

Get the name of the Operating System / Kernel¶

This can be used to add conditional support for different Operating Systems without needing to call uname.

"$OSTYPE"

Get the current working directory¶

This is an alternative to the pwd built-in.

"$PWD"

Get the number of seconds the script has been running¶

"$SECONDS"

Get a pseudorandom integer¶

Each time $RANDOM is used, a different integer between 0 and 32767 is returned. This variable should not be used for anything related to security (this includes encryption keys etc).

"$RANDOM"

Information about the terminal¶

Get the terminal size in lines and columns (from a script)¶

This is handy when writing scripts in pure bash and stty/tput can’t be
called.

Example Function:

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"
}

Example Usage:

# Output: LINES COLUMNS
$ get_term_size
15 55

Get the terminal size in pixels¶

CAVEAT: This does not work in some terminal emulators.

Example Function:

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]}"
}

Example Usage:

# Output: WIDTHxHEIGHT
$ get_window_size
1200x800

# Output (fail):
$ get_window_size
x

Get the current cursor position¶

This is useful when creating a TUI in pure bash.

Example Function:

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

Example Usage:

# Output: X Y
$ get_cursor_pos
1 8

Conversion¶

Convert a hex color to RGB¶

Example Function:

hex_to_rgb() {
    # Usage: hex_to_rgb "#FFFFFF"
    #        hex_to_rgb "000000"
    : "${1/\#}"
    ((r=16#${_:0:2},g=16#${_:2:2},b=16#${_:4:2}))
    printf '%s\n' "$r $g $b"
}

Example Usage:

$ hex_to_rgb "#FFFFFF"
255 255 255

Convert an RGB color to hex¶

Example Function:

rgb_to_hex() {
    # Usage: rgb_to_hex "r" "g" "b"
    printf '#%02x%02x%02x\n' "$1" "$2" "$3"
}

Example Usage:

$ rgb_to_hex "255" "255" "255"
#FFFFFF

Code Golf¶

Shorter `for` loop syntax¶

# 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

Shorter infinite loops¶

# Normal method
while :; do echo hi; done

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

Shorter function declaration¶

# 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

Shorter `if` syntax¶

# 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
    # ...
}

Simpler `case` statement to set variable¶

The : built-in can be used to avoid repeating variable= in a case statement. The $_ variable stores the last argument of the last command. : always succeeds so it can be used to store the variable value.

# 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="$_"

Others¶

Use `read` as an alternative to the `sleep` command¶

Surprisingly, sleep is an external command and not a bash built-in.

CAVEAT: Requires bash 4+

Example Function:

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

Example Usage:

read_sleep 1
read_sleep 0.1
read_sleep 30

For performance-critical situations, where it is not economic to open and close an excessive number of file descriptors, the allocation of a file descriptor may be done only once for all invocations of read:

(See the generic original implementation at https://blog.dhampir.no/content/sleeping-without-a-subprocess-in-bash-and-how-to-sleep-forever)

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

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

# 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; then
    # Program is in PATH.
fi

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

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

Get the current date using `strftime`¶

Bash’s printf has a built-in method of getting the date which can be used in place of the date command.

CAVEAT: Requires bash 4+

Example Function:

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

Example Usage:

# 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

Get the username of the current user¶

CAVEAT: Requires bash 4.4+

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

Generate a UUID V4¶

CAVEAT: The generated value is not cryptographically secure.

Example Function:

uuid() {
    # Usage: uuid
    C="89ab"

    for ((N=0;N<16;++N)); do
        B="$((RANDOM%256))"

        case "$N" in
            6)  printf '4%x' "$((B%16))" ;;
            8)  printf '%c%x' "${C:$RANDOM%${#C}:1}" "$((B%16))" ;;

            3|5|7|9)
                printf '%02x-' "$B"
            ;;

            *)
                printf '%02x' "$B"
            ;;
        esac
    done

    printf '\n'
}

Example Usage:

$ uuid
d5b6c731-1310-4c24-9fe3-55d556d44374

Progress bars¶

This is a simple way of drawing progress bars without needing a for loop
in the function itself.

Example Function:

bar() {
    # Usage: bar 1 10
    #            ^----- Elapsed Percentage (0-100).
    #               ^-- Total length in chars.
    ((elapsed=$1*$2/100))

    # Create the bar with spaces.
    printf -v prog  "%${elapsed}s"
    printf -v total "%$(($2-elapsed))s"

    printf '%s\r' "[${prog// /-}${total}]"
}

Example Usage:

for ((i=0;i<=100;i++)); do
    # Pure bash micro sleeps (for the example).
    (:;:) && (:;:) && (:;:) && (:;:) && (:;:)

    # Print the bar.
    bar "$i" "10"
done

printf '\n'

Get the list of functions in a script¶

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

Bypass shell aliases¶

# alias
ls

# command
# shellcheck disable=SC1001
\ls

Bypass shell functions¶

# function
ls

# command
command ls

Run a command in the background¶

This will run the given command and keep it running, even after the terminal or SSH connection is terminated. All output is ignored.

bkr() {
    (nohup "$@" &>/dev/null &)
}

bkr ./some_script.sh # some_script.sh is now running in the background

Temp cd in to folder¶

pushd / popd

for d1 in $(ls -d */)
do
  # Store original working directory.
  original_wd="$(pwd)"
  cd "$d1"
  for d2 in $(ls -d */)
  do
    pushd "$d2"
    # Do something
    popd
  done
  # Return to original working directory
  cd "${original_wd}"
done

Arguments¶

Environment Variables¶

$0   :name of shell or shell script.
$1, $2, $3, ... :positional parameters.
$#   :number of positional parameters.
$?   :most recent foreground pipeline exit status.
$-   :current options set for the shell.
$$   :pid of the current shell (not subshell).
$!   :is the PID of the most recent background command.

$DESKTOP_SESSION     current display manager
$EDITOR   preferred text editor.
$LANG   current language.
$PATH   list of directories to search for executable files (i.e. ready-to-run programs)
$PWD    current directory
$SHELL  current shell
$USER   current username
$HOSTNAME   current hostname

Setting Default Arguments in Bash¶

#!/bin/bash
FIRST_ARG="${1:-no_first_arg}"
SECOND_ARG="${2:-no_second_arg}"
THIRD_ARG="${3:-no_third_arg}"
echo ${FIRST_ARG}
echo ${SECOND_ARG}
echo ${THIRD_ARG}

Timeout in scripts

#!/bin/bash
TMOUT=5
echo You have 5 seconds to respond...
read
echo ${REPLY:-noreply}

Bash File Testing¶

Check if file exists:

#!/bin/bash
file="./file"
if [ -f $file ]; then
  echo "File exists"
else 
  echo "File does not exists"
fi

Check for line in file before adding¶

if ! grep -qF "/mnt/dev" /etc/fstab; then
  echo "/dev/sda1 /mnt/dev ext4 defaults 0 0" | sudo tee -a /etc/fstab
fi

Mount filesystem if does not already exist¶

if ! mountpoint -q "$DATA_DIR"; then
  mount -o discard,defaults,noatime "$VOLUME_NAME" "$DATA_DIR"
fi

Set Strict mode to exit on failure¶

http://redsymbol.net/articles/unofficial-bash-strict-mode/