๐Ÿ“– A collection of pure bash alternatives to external processes.

Overview

NEW: pure sh bible ( ๐Ÿ“– 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

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 functionโ€™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 "[email protected]"
    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 "[email protected]"; 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=("[email protected]")
    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[@]}-1?0:++i)) }">
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[@]}-1?0:++i)) }">
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

Donโ€™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='โžœ '

$ 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:

< "$1" }">
extract() {
    # Usage: extract file "opening marker" "closing marker"
    while IFS=$'\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; m Set text foreground color. 0-255
\e[48;5; m Set text background color. 0-255
\e[38;2; ; ; m Set text foreground color to RGB color. R, G, B
\e[48;2; ; ; 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[ ; H Move cursor to absolute position. line, column
\e[H Move cursor to home position (0,0).
\e[ A Move cursor up N lines. num
\e[ B Move cursor down N lines. num
\e[ C Move cursor right N columns. num
\e[ 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.
${[email protected]} 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: {
   
    ..
    
     }
    
   

# 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: {
   
    ..
    
     ..
     
      }
     
    
   
# 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 PATH to find the bash binary.
  • 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 canโ€™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: โ€˜whileโ€™, โ€˜untilโ€™, โ€˜caseโ€™, โ€˜(())โ€™, โ€˜[[]]โ€™ can also be used.
f()if true; then echo "$1"; fi
f()for i in "[email protected]"; do echo "$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.

&2 exit 1 ;; esac # Finally, set the variable. os="$_"">
# Modified snippet from Neofetch.
case "$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

Bashโ€™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.

/dev/null &) } bkr ./some_script.sh # some_script.sh is now running in the background">
bkr() {
    (nohup "[email protected]" &>/dev/null &)
}

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

Capture the return value of a function without command substitution

CAVEAT: Requires bash 4+

This uses local namerefs to avoid using var=$(some_func) style command substitution for function output capture.

to_upper() {
  local -n ptr=${1}

  ptr=${ptr^^}
}

foo="bar"
to_upper foo
printf "%s\n" "${foo}" # BAR

AFTERWORD

Thanks for reading! If this bible helped you in any way and you'd like to give back, consider donating. Donations give me the time to make this the best resource possible. Can't donate? That's OK, star the repo and share it with your friends!

Rock on. ๐Ÿค˜

Comments
  • more rationale for not using #!/bin/bash?

    more rationale for not using #!/bin/bash?

    Hey @dylanaraps, I think is super useful, thanks for making it! I was wondering if you could add a bit on your rationale for using #!/usr/l/bin/env bash instead. This is generally my thoughts: https://stackoverflow.com/a/55927235, but I'm not a bash expert.

    discussion 
    opened by mavenraven 31
  • basename is not equivalent to the utility

    basename is not equivalent to the utility

    This shouldn't be as hard as dirname; trailing slashes are the crux in the first counter-examples, then there is lack of support for the second optional argument, which nobody knows about anyway:

    ~ $ basename_pbb() {
        # Usage: basename "path"
        : "${1%/}"
        printf '%s\n' "${_##*/}"
    }
    ~ $ basename_pbb something//
    
    ~ $ basename something//
    something
    ~ $ basename_pbb /
    
    ~ $ basename /
    /
    ~ $ basename_pbb dir/something thing
    something
    ~$ basename dir/something thing
    some
    

    The POSIX standard helps, https://pubs.opengroup.org/onlinepubs/9699919799/

    According to this, some compliant implementation could go like this:

    • If string consists entirely of / characters, the result is a single / character.
    • Otherwise:
      • If there are any trailing / characters in string, they shall be removed.
      • If there are any / characters remaining in string, the prefix of string up to and including the last / character in string shall be removed.
    • If the suffix operand is present, is not identical to the characters remaining in string, and is identical to a suffix of the characters remaining in string, the suffix suffix shall be removed from string. Otherwise, string is not modified by this step. It shall not be considered an error if suffix is not found in string.

    Meaning that this implementation is spec-conformant:

    basename_pbb() {
        # Usage: basename "path" ["suffix"]
        local tmp
        if [[ $1 =~ ^/+$ ]]; then
            tmp=/
        else
            tmp=${1%"${1##*[!/]}"} # $1 without trailing slashes
            tmp=${tmp##*/} # $tmp without its longest prefix ending in a slash
        fi
        [[ $tmp != "$2" ]] && tmp=${tmp%"$2"}
        printf '%s\n' "$tmp"
    }
    

    If you want to have fun codegolfing it, be my guest! But don't remove the quotes around $2, because the asterisk and question mark characters are legal in filenames.

    enhancement 
    opened by calimeroteknik 12
  • Caveat: $_ does not work well with a trap on DEBUG

    Caveat: $_ does not work well with a trap on DEBUG

    When a trap on DEBUG is set, $_ will always be the last argument of the last trap command, making it unusable. This could be noted somewhere in the README :slightly_smiling_face:

    bug 
    opened by pawamoy 9
  • Bash 5.0 breaks reverse_array in scripts

    Bash 5.0 breaks reverse_array in scripts

    In Bash 5.0 scripts, BASH_ARGV also gets the current function's arguments when extdebug is enabled for the first time, so reverse_array's results are printed twice. Relevant section of the changelog:

    The shell doesn't automatically set BASH_ARGC and BASH_ARGV at startup unless it's in debugging mode, as the documentation has always said, but will dynamically create them if a script references them at the top level without having enabled debugging mode.

    Compatibility with the old version is available with the compat44 shell option.

    bug 
    opened by Crestwave 4
  • Added background commands

    Added background commands

    The syntax looks ugly, but this is the optimal way to do it. It doesn't have any extraneous output due to the parenthesis surrounding the command, and also sends error output to /dev/null.

    opened by CoolOppo 4
  • Random array element function

    Random array element function

    Hi,

    Just noticed this repo and thought I would share this function that I came up with a few days ago:

    function random_array_element {
        declare -n array="$1"  # namerefs are cool!  And it even works with the array being a local variable in another function!
    
        local length=${#array[@]}
        local index=$(( RANDOM % length))
        local choice=${array[$index]}
    
        echo "$choice"
    }
    

    Used like:

    function choose_icon {
        # Return random icon.
        local icons=(
            /usr/share/icons/oxygen/22x22/emotes/face-embarrassed.png
            /usr/share/icons/oxygen/22x22/emotes/face-foot-in-mouth.png
            /usr/share/icons/oxygen/22x22/emotes/face-clown.png
            /usr/share/icons/oxygen/22x22/emotes/face-confused.png
            /usr/share/icons/oxygen/22x22/emotes/face-crying.png
            /usr/share/icons/oxygen/22x22/emotes/face-laugh.png
            /usr/share/icons/oxygen/22x22/emotes/face-laughing.png
            /usr/share/icons/oxygen/22x22/emotes/face-quiet.png
            /usr/share/icons/oxygen/22x22/emotes/face-raspberry.png
            /usr/share/icons/oxygen/22x22/emotes/face-sad.png
            /usr/share/icons/oxygen/22x22/emotes/face-sleeping.png
            /usr/share/icons/oxygen/22x22/emotes/face-smirk.png
            /usr/share/icons/oxygen/22x22/emotes/face-surprise.png
            /usr/share/icons/oxygen/22x22/emotes/face-uncertain.png
            /usr/share/icons/oxygen/22x22/emotes/face-wink.png
            /usr/share/icons/oxygen/22x22/emotes/face-worried.png
            /usr/share/icons/oxygen/22x22/emotes/face-yawn.png
            /usr/share/icons/oxygen/22x22/emotes/food-cake.png
            /usr/share/icons/oxygen/22x22/emotes/food-pizza.png
        )
        echo $(random_array_element icons)
    }
    
    choose_icon  # => "/usr/share/icons/oxygen/22x22/emotes/food-cake.png"
    
    opened by alphapapa 4
  • [Enhancement] Sample of obsolete code

    [Enhancement] Sample of obsolete code

    Obsolete code

    There are a ton of obsolete code samples in nearly every programming language.

    Bash samples

    Use $()

    There is no main reason. $() is new `` is old.

    `` vs $()
    
    # Sample
    var=`something` vs var=$()
    

    Header use #!/usr/bin/env

    Use always #!/usr/bin/env because #!/usr/bin/env searches the path of bash. Not on all linux permutations the default bash location is /bin/bash so always use that.

    #!/usr/bin/env vs #!/bin/bash
    
    #Sample
    #!/usr/bin/env
    

    Functions

    The function key word is not required any more. Don't use that because it reduced the compatibility of different bash versions.

    function do_something () {
    }
    
    # vs
    
    do_something () {
    }
    

    These are only few examples I have a lot more if you want I could submit a PR.

    Regards

    opened by deltaxflux 4
  • Improved lines() function

    Improved lines() function

    The existing implementation of the lines() function may be impractical in some scenarios. Its use of mapfile requires reading the entire file into memory before counting the number of lines. While this will be fine with smaller files, it could be a problem when used on larger files on systems with limited memory.

    I have replaced it with an implementation that uses a while loop so that only a single line is stored in memory at a time.

    Relevant link: How can I read a file (data stream, variable) line-by-line (and/or field-by-field)?

    opened by sector-f 4
  • Implement sleep without reading from stdin of the caller

    Implement sleep without reading from stdin of the caller

    Before you merge it, is it worth adding a tip for performance-critical situations? The idea is to open the fd only once for all invocations, i.e.:

    while some_quick_test; do
        read -rt 0.001 -u $snore_fd
    done {snore_fd}<> <(:)
    

    I do mention the blog entry in the commit message, which includes the performance optimisation, but I doubt anyone will see it there. Maybe worth adding a link to it and its archived version in the text itself. What do you think?

    opened by amiryal 3
  • License usage

    License usage

    I want to copy this line to my script:

    file_data="$(<"file")"
    

    Should I now add LICENSE.md from the root of this repository to my snippet?

    opened by techtonik 3
  • Bypass functions trick does not work

    Bypass functions trick does not work

    In the "Bypass shell aliases and functions", the trick of using a backslash does not work for functions \ (though it does work for aliases).

    This is on my Mac running

    GNU bash, version 3.2.57(1)-release (x86_64-apple-darwin15)
    Copyright (C) 2007 Free Software Foundation, Inc.
    

    e.g.

    $ logname
    sal
    $ function logname() { echo 'There is no logname, only Zuul.'; }
    $ logname
    There is no logname, only Zuul.
    $ \logname
    There is no logname, only Zuul.
    

    Instead, you can use

    $ $(which logname)
    sal
    

    or possibly with -f added

    see https://salferrarello.com/command-line-which-wrong-result/#skipping-aliases-and-functions for more information.

    opened by salcode 3
  • read, store, write a file including nulls

    read, store, write a file including nulls

    Read and write a file including nulls, pure bash, no externals, no sub-shell forks. not limited to late versions. Slow and ram-hungry. Every byte is read one byte at a time, and stored as a hex pair in an array.

    https://gist.github.com/bkw777/c1413d0e3de6c54524ddae890fe8d705

    Variations on this could store the data more compactly and read the file faster, by removing "-n 1" and reading all contiguous non-null bytes together and storing normally in a variable rather than an array, and only the nulls would be stored in some encoded form, (and a further simple enhancement would store contiguous strings of nulls with a single code that means N nulls instead of null) and the loop would only tick over on every null instead of on every byte. But the resulting data wouldn't be as convenient to work with, depedning on why you wanted to read the file and what you wanted to do with the data. Reading binary data and wanting to operate on the binary values, read them as numbers, count bytes, edit specifically positioned bytes in-place etc, an array of ints or hex pairs was more convenient for what I was working on. But if you were merely storing and reproducing the data without needing to parse it or edit it, this other method would be more efficient.

    But in either the simple or fancy cases, the point and the essential trick is the same:

    • combination of LANG, IFS, and read option flags to arrange that null is the only delimiter and no other bytes have any special meaning.
    • on each read, be it a byte or a chunk, consult the return value from read to determine the difference between "got nothing because eof" and "got nothing because delimiter"

    htof() could run a lot faster if willing to abuse the commandline to hold the entire file. It could be a single printf with a singe brace-expansion with a global replace instead of a loop that does a printf for each byte. x=" ${h[*]}" ;printf '%b' "${x// /\\x}"

    opened by bkw777 0
  • basename with one argument fails with set -u

    basename with one argument fails with set -u

    As the second argument can be unbound in basename, the following script will fail:

    #!/usr/bin/env bash
    
    set -u
    
    basename() {
        # Usage: basename "path" ["suffix"]
        local tmp
    
        tmp=${1%"${1##*[!/]}"}
        tmp=${tmp##*/}
        tmp=${tmp%"${2/"$tmp"}"}
    
        printf '%s\n' "${tmp:-/}"
    }
    
    basename ~/Pictures/Wallpapers/1.jpg
    

    with

    ./basename.sh: line 11: 2: unbound variable
    
    opened by lunik1 0
  • shift at least...

    shift at least...

    This has bitten me multiple times, not sure if there is a good place for it in the book?

    When a function accepts optional parameters for arg1, and arg2, and unlimited additional parameters, this may look okay, but it does not work, use arg1, arg2, shift 2, then read remaining args in from [email protected]

    local a="$1" b="$2"
    shift 2
    local c="[email protected]"
    

    The desired effect is not achieved, because shift 2 doesn't shift any, and returns signal 1 (usually without detection), If only one or zero args are provided to the function. It's a mess to debug.

    This sets a and b to arg1 and arg2, and c to any remaining data, without any error signals, even if there are no args:

    local a="$1" b="$2"
    shift 2 || shift || true
    local c="[email protected]"
    

    I was wondering if anyone has a less messy way to achieve that desired effect?

    opened by georgalis 0
  • Add warning about shorter `if` and `&&`

    Add warning about shorter `if` and `&&`

    Definitely nice to use && as a shorter if, but there are caveats, when using && in a function.

    This is probably the single largest error I used to make. It took me far too long to remember this.

    opened by e40 4
  • Pedantic use of ':'

    Pedantic use of ':' "in place of a temporary variable."

    Hi. I'm new to your project, and am starting at the beginning (no guarantee I'll continue)...

    In your very first snippet, labeled "Trim leading and trailing white-space from string", you write: "The : built-in is used in place of a temporary variable."

    Is that really what's happening? AFACIT the ; is a no-op built-in command, having nothing to to do with any variable. Your snippet seems to be using the $_ shell variable, which according to the man page "expands to the last argument the previous simple command executed in the foreground, after expansion."

    If I'm understanding the snippet correctly, the explanation should read something like "The $_ shell variable is used in place of a temporary variable. It is updated as an undocumented side-effect of the : null built-in.

    Also, since the side-effect that the snippets exploits seems at odds with the documentation for the : command ("null command: no effect; the command DOES NOTHING"), is there documentation supporting this use? What I mean is, are the bash developers committed to the continued existence of this behavior, or it just an undocumented serendipitous occurrence for an officially 'undefined' result that could change at any time without notice?

    opened by Boruch-Baum 1
Owner
dylan
23, self taught, creator of @kisslinux
dylan
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