SHELL-TEST-THEME
NEW: pure sh bible (📖 A collection of pure POSIX sh alternatives to external processes).
pure bash bible
A collection of pure bash alternatives to external processes.
The goal of this book is to document commonly-known and lesser-known methods of doing various tasks using only built-in bash features. Using the snippets from this bible can help remove unneeded dependencies from scripts and in most cases make them faster. I came across these tips and discovered a few while developing neofetch, pxltrm and other smaller projects.
The snippets below are linted using shellcheck and tests have been written where applicable. Want to contribute? Read the CONTRIBUTING.md. It outlines how the unit tests work and what is required when adding snippets to the bible.
See something incorrectly described, buggy or outright wrong? Open an issue or send a pull request. If the bible is missing something, open an issue and a solution will be found.
This book is also available to purchase on leanpub. https://leanpub.com/bash
Or you can buy me a coffee.
Table of Contents
- FOREWORD
- STRINGS
- Trim leading and trailing white-space from string
- Trim all white-space from string and truncate spaces
- Use regex on a string
- Split a string on a delimiter
- Change a string to lowercase
- Change a string to uppercase
- Reverse a string case
- Trim quotes from a string
- Strip all instances of pattern from string
- Strip first occurrence of pattern from string
- Strip pattern from start of string
- Strip pattern from end of string
- Percent-encode a string
- Decode a percent-encoded string
- Check if string contains a sub-string
- Check if string starts with sub-string
- Check if string ends with sub-string
- ARRAYS
- LOOPS
- FILE HANDLING
- FILE PATHS
- VARIABLES
- ESCAPE SEQUENCES
- PARAMETER EXPANSION
- BRACE EXPANSION
- CONDITIONAL EXPRESSIONS
- ARITHMETIC OPERATORS
- ARITHMETIC
- TRAPS
- PERFORMANCE
- OBSOLETE SYNTAX
- INTERNAL VARIABLES
- Get the location to the
bashbinary - Get the version of the current running
bashprocess - Open the user's preferred text editor
- Get the name of the current function
- Get the host-name of the system
- Get the architecture of the Operating System
- Get the name of the Operating System / Kernel
- Get the current working directory
- Get the number of seconds the script has been running
- Get a pseudorandom integer
- Get the location to the
- INFORMATION ABOUT THE TERMINAL
- CONVERSION
- CODE GOLF
- OTHER
- Use
readas an alternative to thesleepcommand - Check if a program is in the user's PATH
- Get the current date using
strftime - Get the username of the current user
- Generate a UUID V4
- Progress bars
- Get the list of functions in a script
- Bypass shell aliases
- Bypass shell functions
- Run a command in the background
- Use
- AFTERWORD
FOREWORD
A collection of pure bash alternatives to external processes and programs. The bash scripting language is more powerful than people realise and most tasks can be accomplished without depending on external programs.
Calling an external process in bash is expensive and excessive use will cause a noticeable slowdown. Scripts and programs written using built-in methods (where applicable) will be faster, require fewer dependencies and afford a better understanding of the language itself.
The contents of this book provide a reference for solving problems encountered when writing programs and scripts in bash. Examples are in function formats showcasing how to incorporate these solutions into code.
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 is one of the few platform dependent bash features.
bash will use whatever regex engine is installed on the user's system.
Stick to POSIX regex features if aiming for compatibility.
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
CAVEAT: Requires bash 4+
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
CAVEAT: Requires bash 4+
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
CAVEAT: Requires bash 4+
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
CAVEAT: Requires bash 4+
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 [[ $var != *sub_string ]]; then
printf '%s\n' "var does not end with sub_string."
fi
ARRAYS
Reverse an array
Enabling extdebug allows access to the BASH_ARGV array which stores
the current function’s arguments in reverse.
CAVEAT: Requires shopt -s compat44 in bash 5.0+.
Example Function:
reverse_array() {
# Usage: reverse_array "array"
shopt -s extdebug
f()(printf '%s\n' "${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 "${arr[@]}"
green
blue
red
Remove duplicate array elements
Create a temporary associative array. When setting associative array
values and a duplicate assignment occurs, bash overwrites the key. This
allows us to effectively remove array duplicates.
CAVEAT: Requires bash 4+
CAVEAT: List order may not stay the same.
Example Function:
remove_array_dups() {
# Usage: remove_array_dups "array"
declare -A tmp_array
for i in "$@"; 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
CAVEAT: bash does not handle binary data properly in versions < 4.4.
Read a file to a string
Alternative to the cat command.
file_data="$(<"file")"
Read a file to an array (by line)
Alternative to the cat command.
# Bash <4 (discarding empty lines).
IFS=$'\n' read -d "" -ra file_data < "file"
# Bash <4 (preserving empty lines).
while read -r line; do
file_data+=("$line")
done < "file"
# Bash 4+
mapfile -t file_data < "file"
Get the first N lines of a file
Alternative to the head command.
CAVEAT: Requires bash 4+
Example Function:
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.
CAVEAT: Requires bash 4+
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):
lines() {
# Usage: lines "file"
mapfile -tn 0 lines < "$1"
printf '%s\n' "${#lines[@]}"
}
Example Function (bash 3):
This method uses less memory than the mapfile method and works in bash 3 but it is slower for bigger files.
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"
local tmp=${1:-.}
[[ $tmp != *[!/]* ]] && {
printf '/\n'
return
}
tmp=${tmp%%"${tmp##*[!/]}"}
[[ $tmp != */* ]] && {
printf '.\n'
return
}
tmp=${tmp%/*}
tmp=${tmp%%"${tmp##*[!/]}"}
printf '%s\n' "${tmp:-/}"
}
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" ["suffix"]
local tmp
tmp=${1%"${1##*[!/]}"}
tmp=${tmp##*/}
tmp=${tmp%"${2/"$tmp"}"}
printf '%s\n' "${tmp:-/}"
}
Example Usage:
$ basename ~/Pictures/Wallpapers/1.jpg
1.jpg
$ basename ~/Pictures/Wallpapers/1.jpg .jpg
1
$ 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
Contrary to popular belief, there is no issue in utilizing raw escape sequences. Using tput abstracts the same ANSI sequences as if printed manually. Worse still, tput is not actually portable. There are a number of tput variants each with different commands and syntaxes (try tput setaf 3 on a FreeBSD system). Raw sequences are fine.
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
NOTE: Prepend 2 to any code below to turn it's effect off
(examples: 21=bold text off, 22=faint text off, 23=italic text off).
| 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 |
Blinking text. |
\e[7m |
Highlighted text. |
\e[8m |
Hidden text. |
\e[9m |
Strike-through text. |
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 EXPANSION
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. ( ${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? | CAVEAT |
|---|---|---|
${VAR^} |
Uppercase first character. | bash 4+ |
${VAR^^} |
Uppercase all characters. | bash 4+ |
${VAR,} |
Lowercase first character. | bash 4+ |
${VAR,,} |
Lowercase all characters. | bash 4+ |
${VAR~} |
Reverse case of first character. | bash 4+ |
${VAR~~} |
Reverse case of all characters. | bash 4+ |
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. |
BRACE EXPANSION
Ranges
# Syntax: {<START>..<END>}
# 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.
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
PATHto find thebashbinary. - The latter assumes it is always installed to
/bin/which can cause issues.
NOTE: There are times when one may have a good reason for using #!/bin/bash or another direct path to the binary.
# Right:
#!/usr/bin/env bash
# Less right:
#!/bin/bash
Command Substitution
Use $() instead of ` `.
# Right.
var="$(command)"
# Wrong.
var=`command`
# $() can easily be nested whereas `` cannot.
var="$(command "$(command)")"
Function Declaration
Do not use the function keyword, it reduces compatibility with older versions of bash.
# Right.
do_something() {
# ...
}
# Wrong.
function do_something() {
# ...
}
INTERNAL VARIABLES
Get the location to the bash binary
"$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="$_"
OTHER
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: read_sleep 1
# read_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
AFTERWORD
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