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README.md

pure bash bible

A [WIP] collection of pure bash alternatives to external processes.


The goal of this repository is to document known and unknown methods of doing various tasks using only built-in bash features. Using the snippets from this guide can help to remove unneeded dependencies from your scripts and in most cases make them that little bit faster. I came across these tips and discovered a few while developing neofetch, pxltrm and some other smaller projects.

This repository is open to contribution. If you see something that is incorrectly described, buggy or outright wrong, open an issue or send a pull request. If you know a handy snippet that is not included in this list, send a pull request!

NOTE: Error handling (checking if a file exists, etc) is not included. These are meant to be snippets you can incorporate into your scripts and not full blown utilities.


Table of Contents

Strings

Trim white-space from string.

NOTE: This also truncates multiple spaces inside the string.

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

Split a string on a delimiter.

# To multiple variables.
string="1,2,3"
IFS=, read -r var1 var2 var3 <<< "$string"

# To an array.
IFS=, read -ra vars <<< "$string"

Change a string to lowercase.

NOTE: Requires bash 4+

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

Change a string to uppercase.

NOTE: Requires bash 4+

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

Trim quotes from a string.

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

Strip characters from start of string.

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

Strip characters from end of string.

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

Variables

Assign and access a variable using a variable.

# First Example.

var1="world"
var2="hello_${var1}"
declare "${var2}=test_string"
printf '%s\n' "${!var2}"


# Second Example.

# Assign to a variable named after the
# value stored in '$var'.
var="test"
read -rp "input text: " "${var?}"

# Access the variable indirectly.
printf '%s\n' "set var \$$var to '${!var}'"

# Access the variable directly.
printf '%s\n' "set var \$$var to '$test'"

Arrays

Reverse an array.

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

reverse_array() {
    # Usage: reverse_array "array"
    #        reverse_array 1 2 3 4 5 6
    shopt -s extdebug
    f()(printf '%s ' "${BASH_ARGV[@]}"); f "$@"
    shopt -u extdebug

    printf '\n'
}

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.

NOTE: Requires bash 4+

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[@]}"
}

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))
}

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.

NOTE: Requires bash 4+

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

Get the last N lines of a file.

Alternative to the tail command.

NOTE: Requires bash 4+

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

Get the number of lines in a file.

Alternative to wc -l.

NOTE: Requires bash 4+

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

Iterate over files.

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

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

Count files or directories in directory.

This works by passing the output of the glob as function arguments. We then count the arguments and print the number.

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

Create an empty file.

Alternative to touch.

:> file

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

File Paths

Get the directory name of a file path.

Alternative to the dirname command.

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

Get the base-name of a file path.

Alternative to the basename command.

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

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))

Colors

Convert a hex color to RGB.

hex_to_rgb() {
    # Usage: hex_to_rgb "#FFFFFF"
    ((r=16#${1:1:2}))
    ((g=16#${1:3:2}))
    ((b=16#${1:5:6}))

    printf '%s\n' "$r $g $b"
}

Convert an RGB color to hex.

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

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.

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 terminal size in pixels.

NOTE: This does not work in some terminal emulators.

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]}"
}

Get the current cursor position.

This is useful when creating a TUI in pure bash.

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

Code Golf

Shorter for loop syntax.

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

# Undocumented method.
# Note: This is commented to make shellcheck play nice.
# 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 code; done

# Shorter
for((;;)){ code;}

Shorter function declaration.

# Normal method
f(){ echo hi;}

# Using a subshell
f()(echo hi)

# Using arithmetic
# You can use this to assign integer values.
# Example: f a=1
#          f a++
f()(($1))

# Using tests, loops etc.
# Note: You can also use ‘while’, ‘until’, ‘case’, ‘(())’, ‘[[]]’.
# Note: These are commented to make shellcheck play nice.
# f()if true; then echo "$1"; fi
# f()for i in "$@"; do echo "$i"; done

Shorter if syntax.

# One line
[[ "$var" == hello ]] && echo hi || echo bye
[[ "$var" == hello ]] && { echo hi; echo there; } || echo bye

# Multi line (no else, single statement)
[[ "$var" == hello ]] && \
    echo hi

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

Simpler case statement to set variable.

We can use the : builtin to avoid repeating variable= in a case statement. The $_ variable stores the last argument of the last successful command. : always succeeds so we can abuse it to store the variable value.

# Example snippet from Neofetch.
case "$(uname)" in
    "SunOS"):    "Solaris" ;;
    "Haiku"):    "Haiku" ;;
    "MINIX"):    "MINIX" ;;
    "AIX"):      "AIX" ;;
    "IRIX"*):    "IRIX" ;;
    "FreeMiNT"): "FreeMiNT" ;;

    "Linux" | "GNU"*)
        : "Linux"
    ;;

    *"BSD" | "DragonFly" | "Bitrig")
        : "BSD"
    ;;

    "CYGWIN"* | "MSYS"* | "MINGW"*)
        : "Windows"
    ;;

    *)
        printf '%s\n' "Unknown OS detected: '$kernel_name', aborting..." >&2
        printf '%s\n' "Open an issue on GitHub to add support for your OS." >&2
        exit 1
    ;;
esac

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

Internal Variables

NOTE: This list does not include every internal variable (You can help by adding a missing entry!).

For a complete list, see: http://tldp.org/LDP/abs/html/internalvariables.html

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"

Other

Get the current date using strftime.

Bash’s printf has a built-in method of getting the date which we can use in place of the date command in a lot of cases.

NOTE: Requires bash 4+

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

# Examples:

# Using date.
date "+%a %d %b  - %l:%M %p"

# Using printf.
printf '%(%a %d %b  - %l:%M %p)T\n' '-1'

# Assigning a variable.
printf -v date '%(%a %d %b  - %l:%M %p)T\n' '-1'

Bypass shell aliases.

# alias
ls

# command
# shellcheck disable=SC1001
\ls

Bypass shell functions.

# function
ls

# command
command ls