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    README.md

    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

    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 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 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

    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. 🤘

    项目简介

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    源项目地址

    https://github.com/dylanaraps/pure-bash-bible

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