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](https://github.com/dylanaraps/neofetch),
[pxltrm](https://github.com/dylanaraps/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](#strings)
* [Trim leading and trailing white-space from string.](#trim-leading-and-trailing-white-space-from-string)
* [Trim all white-space from string and truncate spaces.](#trim-all-white-space-from-string-and-truncate-spaces)
* [Use REGEX on a string.](#use-regex-on-a-string)
* [Split a string on a delimiter.](#split-a-string-on-a-delimiter)
* [Change a string to lowercase.](#change-a-string-to-lowercase)
* [Change a string to uppercase.](#change-a-string-to-uppercase)
* [Trim quotes from a string.](#trim-quotes-from-a-string)
* [Strip all instances of pattern from string.](#strip-all-instances-of-pattern-from-string)
* [Strip first occurrence of pattern from string.](#strip-first-occurrence-of-pattern-from-string)
* [Strip pattern from start of string.](#strip-pattern-from-start-of-string)
* [Strip pattern from end of string.](#strip-pattern-from-end-of-string)
* [Variables](#variables)
* [Assign and access a variable using a variable.](#assign-and-access-a-variable-using-a-variable)
* [Arrays](#arrays)
* [Reverse an array.](#reverse-an-array)
* [Remove duplicate array elements.](#remove-duplicate-array-elements)
* [Cycle through an array.](#cycle-through-an-array)
* [Toggle between two values.](#toggle-between-two-values)
* [File handling](#file-handling)
* [Read a file to a string.](#read-a-file-to-a-string)
* [Read a file to an array (*by line*).](#read-a-file-to-an-array-by-line)
* [Get the first N lines of a file.](#get-the-first-n-lines-of-a-file)
* [Get the last N lines of a file.](#get-the-last-n-lines-of-a-file)
* [Get the number of lines in a file.](#get-the-number-of-lines-in-a-file)
* [Iterate over files.](#iterate-over-files)
* [Count files or directories in directory.](#count-files-or-directories-in-directory)
* [Create an empty file.](#create-an-empty-file)
* [File Paths](#file-paths)
* [Get the directory name of a file path.](#get-the-directory-name-of-a-file-path)
* [Get the base-name of a file path.](#get-the-base-name-of-a-file-path)
* [Arithmetic](#arithmetic)
* [Simpler syntax to set variables.](#simpler-syntax-to-set-variables)
* [Ternary tests.](#ternary-tests)
* [Colors](#colors)
* [Convert a hex color to RGB.](#convert-a-hex-color-to-rgb)
* [Convert an RGB color to hex.](#convert-an-rgb-color-to-hex)
* [Information about the terminal](#information-about-the-terminal)
* [Get the terminal size in lines and columns (*from a script*).](#get-the-terminal-size-in-lines-and-columns-from-a-script)
* [Get the terminal size in pixels.](#get-the-terminal-size-in-pixels)
* [Get the current cursor position.](#get-the-current-cursor-position)
* [Code Golf](#code-golf)
* [Shorter `for` loop syntax.](#shorter-for-loop-syntax)
* [Shorter infinite loops.](#shorter-infinite-loops)
* [Shorter function declaration.](#shorter-function-declaration)
* [Shorter `if` syntax.](#shorter-if-syntax)
* [Simpler `case` statement to set variable.](#simpler-case-statement-to-set-variable)
* [Internal Variables](#internal-variables)
* [Get the location to the `bash` binary.](#get-the-location-to-the-bash-binary)
* [Get the version of the current running `bash` process.](#get-the-version-of-the-current-running-bash-process)
* [Open the user's preferred text editor.](#open-the-users-preferred-text-editor)
* [Get the name of the current function.](#get-the-name-of-the-current-function)
* [Get the host-name of the system.](#get-the-host-name-of-the-system)
* [Get the architecture of the Operating System.](#get-the-architecture-of-the-operating-system)
* [Get the name of the Operating System / Kernel.](#get-the-name-of-the-operating-system--kernel)
* [Get the current working directory.](#get-the-current-working-directory)
* [Get the number of seconds the script has been running.](#get-the-number-of-seconds-the-script-has-been-running)
* [Other](#other)
* [Get the current date using `strftime`.](#get-the-current-date-using-strftime)
* [Bypass shell aliases.](#bypass-shell-aliases)
* [Bypass shell functions.](#bypass-shell-functions)
## Strings
### Trim leading and trailing white-space from string.
**Example Function:**
```sh
trim_string() {
# Usage: trim_string " example string "
: "${1#"${1%%[![:space:]]*}"}"
: "${_%"${_##*[![:space:]]}"}"
printf '%s\n' "$_"
}
```
**Example Usage:**
```shell
$ trim_string " Hello, World "
Hello, World
$ name=" John Black "
$ trim_string "$name"
John Black
```
### Trim all white-space from string and truncate spaces.
**Example Function:**
```sh
# shellcheck disable=SC2086,SC2048
trim_all() {
# Usage: trim_all " example string "
set -f
set -- $*
printf '%s\n' "$*"
set +f
}
```
**Example Usage:**
```shell
$ 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.
We can use the result of `bash`'s regex matching to create a simple `sed`
replacement.
**NOTE**: This is one of the few platform dependant `bash` features.
`bash` will use whatever regex engine is installed on the user's system.
Stick to POSIX regex features if aiming for compatibility.
**NOTE**: This example only prints the first matching group. When using
multiple capture groups some modification will be needed.
**Example Function:**
```sh
regex() {
# Usage: regex "string" "regex"
[[ $1 =~ $2 ]] && printf '%s\n' "${BASH_REMATCH[1]}"
}
```
**Example Usage:**
```shell
$ # 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:**
```shell
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.
```shell
string="1,2,3"
# To multiple variables.
IFS=, read -r var1 var2 var3 <<< "$string"
# To an array.
IFS=, read -ra vars <<< "$string"
```
### Change a string to lowercase.
**NOTE:** Requires `bash` 4+
**Example Function:**
```sh
lower() {
# Usage: lower "string"
printf '%s\n' "${1,,}"
}
```
**Example Usage:**
```shell
$ lower "HELLO"
hello
$ lower "HeLlO"
hello
$ lower "hello"
hello
```
### Change a string to uppercase.
**NOTE:** Requires `bash` 4+
**Example Function:**
```sh
upper() {
# Usage: upper "string"
printf '%s\n' "${1^^}"
}
```
**Example Usage:**
```shell
$ upper "hello"
HELLO
$ upper "HeLlO"
HELLO
$ upper "HELLO"
HELLO
```
### Trim quotes from a string.
**Example Function:**
```sh
trim_quotes() {
# Usage: trim_quotes "string"
: "${1//\'}"
printf "%s\\n" "${_//\"}"
}
```
**Example Usage:**
```shell
$ var="'Hello', \"World\""
$ trim_quotes "$var"
Hello, World
```
### Strip all instances of pattern from string.
**Example Function:**
```sh
strip_all() {
# Usage: strip_all "string" "pattern"
printf '%s\n' "${1//$2}"
}
```
**Example Usage:**
```shell
$ 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:**
```sh
strip() {
# Usage: strip "string" "pattern"
printf '%s\n' "${1/$2}"
}
```
**Example Usage:**
```shell
$ 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:**
```sh
lstrip() {
# Usage: lstrip "string" "pattern"
printf '%s\n' "${1##$2}"
}
```
**Example Usage:**
```shell
$ lstrip "The Quick Brown Fox" "The "
Quick Brown Fox
```
### Strip pattern from end of string.
**Example Function:**
```sh
rstrip() {
# Usage: rstrip "string" "pattern"
printf '%s\n' "${1%%$2}"
}
```
**Example Usage:**
```shell
$ rstrip "The Quick Brown Fox" " Fox"
The Quick Brown
```
## Variables
### Assign and access a variable using a variable.
```shell
hello_world="test"
# Create the variable name.
var1="world"
var2="hello_${var1}"
# Print the value of the variable name stored in 'hello_$var1'.
printf '%s\n' "${!var2}"
```
## Arrays
### Reverse an array.
Enabling `extdebug` allows access to the `BASH_ARGV` array which stores
the current function’s arguments in reverse.
**Example Function:**
```sh
reverse_array() {
# Usage: reverse_array "array"
shopt -s extdebug
f()(printf '%s\n' "${BASH_ARGV[@]}"); f "$@"
shopt -u extdebug
}
```
**Example Usage:**
```shell
$ reverse_array 1 2 3 4 5
5
4
3
2
1
$ arr=(red blue green)
$ reverse_array "${arr[@]}"
green
blue
red
```
### Remove duplicate array elements.
Create a temporary associative array. When setting associative array
values and a duplicate assignment occurs, bash overwrites the key. This
allows us to effectively remove array duplicates.
**NOTE:** Requires `bash` 4+
**Example Function:**
```sh
remove_array_dups() {
# Usage: remove_array_dups "array"
declare -A tmp_array
for i in "$@"; 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 "${arr[@]}"
red
green
blue
```
### Cycle through an array.
Each time the `printf` is called, the next array element is printed. When
the print hits the last array element it starts from the first element
again.
```sh
arr=(a b c d)
cycle() {
printf '%s ' "${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.
```sh
arr=(true false)
cycle() {
printf '%s ' "${arr[${i:=0}]}"
((i=i>=${#arr[@]}-1?0:++i))
}
```
## File handling
### Read a file to a string.
Alternative to the `cat` command.
```shell
file_data="$(<"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.
**NOTE:** Requires `bash` 4+
```sh
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+
```sh
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+
```sh
lines() {
# Usage lines "file"
mapfile -tn 0 lines < "$1"
printf '%s\n' "${#lines[@]}"
}
```
### Iterate over files.
Don’t use `ls`.
```sh
_() {
# 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.
```sh
count() {
# Usage: count /path/to/dir/*
# count /path/to/dir/*/
printf '%s\n' "$#"
}
```
### Create an empty file.
Alternative to `touch`.
```sh
_() {
# Shortest.
:> file
# Longer alternatives:
echo -n > file
printf '' > file
}
```
## File Paths
### Get the directory name of a file path.
Alternative to the `dirname` command.
```sh
dirname() {
# Usage: dirname "path"
printf '%s\n' "${1%/*}/"
}
```
### Get the base-name of a file path.
Alternative to the `basename` command.
```sh
basename() {
# Usage: basename "path"
: "${1%/}"
printf '%s\n' "${_##*/}"
}
```
## Arithmetic
### Simpler syntax to set variables.
```sh
_() {
# Simple math
((var=1+2))
# Decrement/Increment variable
((var++))
((var--))
((var+=1))
((var-=1))
# Using variables
((var=var2*arr[2]))
}
```
### Ternary tests.
```sh
_() {
# 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.
```sh
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.
```sh
rgb_to_hex() {
# Usage: rgb_to_hex "r" "g" "b"
printf '#%02x%02x%02x\n' "$1" "$2" "$3"
}
```
## Information about the terminal
### Get the terminal size in lines and columns (*from a script*).
This is handy when writing scripts in pure bash and `stty`/`tput` can’t be
called.
```sh
get_term_size() {
# Usage: get_term_size
# (:;:) is a micro sleep to ensure the variables are
# exported immediately.
shopt -s checkwinsize; (:;:)
printf '%s\n' "$LINES $COLUMNS"
}
```
### Get the terminal size in pixels.
**NOTE**: This does not work in some terminal emulators.
```sh
get_window_size() {
# Usage: get_window_size
printf '%b' "${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.
```sh
get_cursor_pos() {
# Usage: get_cursor_pos
IFS='[;' read -p $'\e[6n' -d R -rs _ y x _
printf '%s\n' "$x $y"
}
```
## Code Golf
### Shorter `for` loop syntax.
```sh
_() {
# Tiny C Style.
for((;i++<10;)){ echo "$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.
```sh
_() {
# Normal method
while :; do echo hi; done
# Shorter
for((;;)){ echo hi;}
}
```
### Shorter function declaration.
```sh
_() {
# 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 "$i"; done
}
```
### Shorter `if` syntax.
```sh
_() {
# One line
[[ "$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.
```sh
_() {
# Example snippet from Neofetch.
case "$(uname)" in
"Linux" | "GNU"*)
: "Linux"
;;
*"BSD" | "DragonFly" | "Bitrig")
: "BSD"
;;
"CYGWIN"* | "MSYS"* | "MINGW"*)
: "Windows"
;;
*)
printf '%s\n' "Unknown OS detected, aborting..." >&2
exit 1
;;
esac
# Finally, set the variable.
os="$_"
}
```
## 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.
```sh
: "$BASH"
```
### Get the version of the current running `bash` process.
```sh
# As a string.
: "$BASH_VERSION"
# As an array.
: "${BASH_VERSINFO[@]}"
```
### Open the user's preferred text editor.
```sh
: "$EDITOR" "$file"
# NOTE: This variable may be empty, set a fallback value.
: "${EDITOR:-vi}" "$file"
```
### Get the name of the current function.
```sh
# Current function.
: "${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.
```sh
: "$HOSTNAME"
# NOTE: This variable may be empty.
# Optionally set a fallback to the hostname command.
: "${HOSTNAME:-$(hostname)}"
```
### Get the architecture of the Operating System.
```sh
: "$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`.
```sh
: "$OSTYPE"
```
### Get the current working directory.
This is an alternative to the `pwd` built-in.
```sh
: "$PWD"
```
### Get the number of seconds the script has been running.
```sh
: "$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+
```sh
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.
```sh
# alias
: ls
# command
# shellcheck disable=SC1001
: \ls
```
### Bypass shell functions.
```sh
# function
: ls
# command
: command ls
```