dylanaraps/pure-sh-bible

Also see: pure bash bible (📖 A collection of pure bash alternatives to external processes).

pure sh bible

A collection of pure POSIX sh 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 POSIX sh 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 KISS Linux and other smaller projects.

The snippets below are all linted using shellcheck.

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.

• Leanpub book: (coming soon)
• Buy me a coffee:

Table of Contents

• STRINGS
  • Strip pattern from start of string
  • Strip pattern from end of string
  • Trim leading and trailing white-space from string
  • Trim all white-space from string and truncate spaces
  • Check if string contains a sub-string
  • Check if string starts with sub-string
  • Check if string ends with sub-string
  • Split a string on a delimiter
  • Trim quotes from a string
• FILES
  • Parsing a key=val file.
  • Get the first N lines of a file
  • Get the number of lines in a file
  • Count files or directories in directory
  • Create an empty file
• FILE PATHS
  • Get the directory name of a file path
  • Get the base-name of a file path
• LOOPS
  • Loop over a (small) range of numbers
  • Loop over a variable range of numbers
  • Loop over the contents of a file
  • Loop over files and directories
• VARIABLES
  • Name a variable based on another variable
• ESCAPE SEQUENCES
  • Text Colors
  • Text Attributes
  • Cursor Movement
  • Erasing Text
• PARAMETER EXPANSION
  • Prefix and Suffix Deletion
  • Length
  • Default Value
• CONDITIONAL EXPRESSIONS
  • File Conditionals
  • Variable Conditionals
  • Variable Comparisons
• ARITHMETIC OPERATORS
  • Assignment
  • Arithmetic
  • Bitwise
  • Logical
  • Miscellaneous
• ARITHMETIC
  • Ternary Tests
  • Check if a number is a float
  • Check if a number is an integer
• TRAPS
  • Do something on script exit
  • Ignore terminal interrupt (CTRL+C, SIGINT)
• OBSOLETE SYNTAX
  • Command Substitution
• INTERNAL AND ENVIRONMENT VARIABLES
  • Open the user's preferred text editor
  • Get the current working directory
  • Get the PID of the current shell
  • Get the current shell options
• AFTERWORD

STRINGS

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

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.

Example Function:

trim_string() {
    # Usage: trim_string "   example   string    "

    # Remove all leading white-space.
    # '${1%%[![:space:]]*}': Strip everything but leading white-space.
    # '${1#${XXX}}': Remove the white-space from the start of the string.
    trim=${1#${1%%[![:space:]]*}}

    # Remove all trailing white-space.
    # '${trim##*[![:space:]]}': Strip everything but trailing white-space.
    # '${trim%${XXX}}': Remove the white-space from the end of the string.
    trim=${trim%${trim##*[![:space:]]}}

    printf '%s\n' "$trim"
}

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    "

    # Disable globbing to make the word-splitting below safe.
    set -f

    # Set the argument list to the word-splitted string.
    # This removes all leading/trailing white-space and reduces
    # all instances of multiple spaces to a single ("  " -> " ").
    set -- $*

    # Print the argument list as a string.
    printf '%s\n' "$*"

    # Re-enable globbing.
    set +f
}

Example Usage:

$ trim_all "    Hello,    World    "
Hello, World

$ name="   John   Black  is     my    name.    "
$ trim_all "$name"
John Black is my name.

Check if string contains a sub-string

Using a case statement:

case $var in
    *sub_string1*)
        # Do stuff
    ;;

    *sub_string2*)
        # Do other stuff
    ;;

    *)
        # Else
    ;;
esac

Check if string starts with sub-string

Using a case statement:

case $var in
    sub_string1*)
        # Do stuff
    ;;

    sub_string2*)
        # Do other stuff
    ;;

    *)
        # Else
    ;;
esac

Check if string ends with sub-string

Using a case statement:

case $var in
    *sub_string1)
        # Do stuff
    ;;

    *sub_string2)
        # Do other stuff
    ;;

    *)
        # Else
    ;;
esac

Split a string on a delimiter

This is an alternative to cut, awk and other tools.

Example Function:

split() {
    # Disable globbing.
    # This ensures that the word-splitting is safe.
    set -f

    # Store the current value of 'IFS' so we
    # can restore it later.
    old_ifs=$IFS

    # Change the field separator to what we're
    # splitting on.
    IFS=$2

    # Create an argument list splitting at each
    # occurance of '$2'.
    #
    # This is safe to disable as it just warns against
    # word-splitting which is the behavior we expect.
    # shellcheck disable=2086
    set -- $1

    # Print each list value on its own line.
    printf '%s\n' "$@"

    # Restore the value of 'IFS'.
    IFS=$old_ifs

    # Re-enable globbing.
    set +f
}

Example Usage:

$ split "apples,oranges,pears,grapes" ","
apples
oranges
pears
grapes

$ split "1, 2, 3, 4, 5" ", "
1
2
3
4
5

Trim quotes from a string

Example Function:

trim_quotes() {
    # Usage: trim_quotes "string"

    # Disable globbing.
    # This makes the word-splitting below safe.
    set -f

    # Store the current value of 'IFS' so we
    # can restore it later.
    old_ifs=$IFS

    # Set 'IFS' to ["'].
    IFS=\"\'

    # Create an argument list, splitting the
    # string at ["'].
    #
    # Disable this shellcheck error as it only
    # warns about word-splitting which we expect.
    # shellcheck disable=2086
    set -- $1

    # Set 'IFS' to blank to remove spaces left
    # by the removal of ["'].
    IFS=

    # Print the quote-less string.
    printf '%s\n' "$*"

    # Restore the value of 'IFS'.
    IFS=$old_ifs

    # Re-enable globbing.
    set +f
}

Example Usage:

$ var="'Hello', \"World\""
$ trim_quotes "$var"
Hello, World

FILES

Parsing a key=val file.

This could be used to parse a simple key=value configuration file.

# Setting 'IFS' tells 'read' where to split the string.
while IFS='=' read -r key val; do
    # Skip over lines containing comments.
    # (Lines starting with '#').
    [ "${key##\#*}" ] || continue

    # '$key' stores the key.
    # '$val' stores the value.
    printf '%s: %s\n' "$key" "$val"

    # Alternatively replacing 'printf' with the following
    # populates variables called '$key' with the value of '$val'.
    #
    # NOTE: I would extend this with a check to ensure 'key' is
    #       a valid variable name.
    # export "$key=$val"
    #
    # Example with error handling:
    # export "$key=$val" 2>/dev/null ||
    #     printf 'warning %s is not a valid variable name\n' "$key"
done < "file"

Get the first N lines of a file

Alternative to the head command.

Example Function:

head() {
    # Usage: head "n" "file"
    while IFS= read -r line; do
        printf '%s\n' "$line"
        i=$((i+1))
        [ "$i" = "$1" ] && return
    done < "$2"

    # 'read' used in a loop will skip over
    # the last line of a file if it does not contain
    # a newline and instead contains EOF.
    #
    # The final line iteration is skipped as 'read'
    # exits with '1' when it hits EOF. 'read' however,
    # still populates the variable.
    #
    # This ensures that the final line is always printed
    # if applicable.
    [ -n "$line" ] && printf %s "$line"
}

Example Usage:

$ head 2 ~/.bashrc
# Prompt
PS1='➜ '

$ head 1 ~/.bashrc
# Prompt

Get the number of lines in a file

Alternative to wc -l.

Example Function:

lines() {
    # Usage: lines "file"

    # '|| [ -n "$line" ]': This ensures that lines
    # ending with EOL instead of a newline are still
    # operated on in the loop.
    #
    # 'read' exits with '1' when it sees EOL and
    # without the added test, the line isn't sent
    # to the loop.
    while IFS= read -r line || [ -n "$line" ]; do
        lines=$((lines+1))
    done < "$1"

    printf '%s\n' "$lines"
}

Example Usage:

$ lines ~/.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/*/
    [ -e "$1" ] \
        && printf '%s\n' "$#" \
        || printf '%s\n' 0
}

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.

:>file

# OR (shellcheck warns for this)
>file

FILE PATHS

Get the directory name of a file path

Alternative to the dirname command.

Example Function:

dirname() {
    # Usage: dirname "path"

    # If '$1' is empty set 'dir' to '.', else '$1'.
    dir=${1:-.}

    # Strip all trailing forward-slashes '/' from
    # the end of the string.
    #
    # "${dir##*[!/]}": Remove all non-forward-slashes
    # from the start of the string, leaving us with only
    # the trailing slashes.
    # "${dir%%"${}"}": Remove the result of the above
    # substitution (a string of forward slashes) from the
    # end of the original string.
    dir=${dir%%"${dir##*[!/]}"}

    # If the variable *does not* contain any forward slashes
    # set its value to '.'.
    [ "${dir##*/*}" ] && dir=.

    # Remove everything *after* the last forward-slash '/'.
    dir=${dir%/*}

    # Again, strip all trailing forward-slashes '/' from
    # the end of the string (see above).
    dir=${dir%%"${dir##*[!/]}"}

    # Print the resulting string and if it is empty,
    # print '/'.
    printf '%s\n' "${dir:-/}"
}

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"]

    # Strip all trailing forward-slashes '/' from
    # the end of the string.
    #
    # "${1##*[!/]}": Remove all non-forward-slashes
    # from the start of the string, leaving us with only
    # the trailing slashes.
    # "${1%%"${}"}:  Remove the result of the above
    # substitution (a string of forward slashes) from the
    # end of the original string.
    dir=${1%${1##*[!/]}}

    # Remove everything before the final forward-slash '/'.
    dir=${dir##*/}

    # If a suffix was passed to the function, remove it from
    # the end of the resulting string.
    dir=${dir%"$2"}

    # Print the resulting string and if it is empty,
    # print '/'.
    printf '%s\n' "${dir:-/}"
}

Example Usage:

$ basename ~/Pictures/Wallpapers/1.jpg
1.jpg

$ basename ~/Pictures/Wallpapers/1.jpg .jpg
1

$ basename ~/Pictures/Downloads/
Downloads

LOOPS

Loop over a (small) range of numbers

Alternative to seq and only suitable for small and static number ranges. The number list can also be replaced with a list of words, variables etc.

# Loop from 0-10.
for i in 0 1 2 3 4 5 6 7 8 9 10; do
    printf '%s\n' "$i"
done

Loop over a variable range of numbers

Alternative to seq.

# Loop from var-var.
start=0
end=50

while [ "$start" -le "$end" ]; do
    printf '%s\n' "$start"
    start=$((start+1))
done

Loop over the contents of a file

while IFS= read -r line || [ -n "$line" ]; do
    printf '%s\n' "$line"
done < "file"

Loop over files and directories

Don’t use ls.

CAVEAT: When the glob does not match anything (empty directory or no matching files) the variable will contain the unexpanded glob. To avoid working on unexpanded globs check the existence of the file contained in the variable using the appropriate file conditional. Be aware that symbolic links are resolved.

# Greedy example.
for file in *; do
    [ -e "$file" ] || [ -L "$file" ] || continue
    printf '%s\n' "$file"
done

# PNG files in dir.
for file in ~/Pictures/*.png; do
    [ -f "$file" ] || continue
    printf '%s\n' "$file"
done

# Iterate over directories.
for dir in ~/Downloads/*/; do
    [ -d "$dir" ] || continue
    printf '%s\n' "$dir"
done

VARIABLES

Name and access a variable based on another variable

$ var="world"
$ eval "hello_$var=value"
$ eval printf '%s\n' "\$hello_$var"
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.

Text Attributes

Cursor Movement

Erasing Text

PARAMETER EXPANSION

Prefix and Suffix Deletion

Length

Default Value

CONDITIONAL EXPRESSIONS

For use in [ ] if [ ]; then and test.

File Conditionals

Variable Conditionals

Variable Comparisons

ARITHMETIC OPERATORS

Assignment

Arithmetic

Bitwise

Logical

Miscellaneous

ARITHMETIC

Ternary Tests

# Set the value of var to var2 if var2 is greater than var.
# 'var2 > var': Condition to test.
# '? var2': If the test succeeds.
# ': var': If the test fails.
var=$((var2 > var ? var2 : var))

Check if a number is a float

Example Function:

is_float() {
    # Usage: is_float "number"

    # The test checks to see that the input contains
    # a '.'. This filters out whole numbers.
    [ -z "${1##*.*}" ] &&
        printf %f "$1" >/dev/null 2>&1
}

Example Usage:

$ is_float 1 && echo true
$

$ is_float 1.1 && echo true
$ true

Check if a number is an integer

Example Function:

is_int() {
    # usage: is_int "number"
    printf %d "$1" >/dev/null 2>&1
}

Example Usage:

$ is_int 1 && echo true
$ true

$ is_int 1.1 && echo true
$

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.

Do something on script exit

# Clear screen on script exit.
trap 'printf \\033[2J\\033[H\\033[m' EXIT

# Run a function on script exit.
# 'clean_up' is the name of a function.
trap clean_up EXIT

Ignore terminal interrupt (CTRL+C, SIGINT)

trap '' INT

OBSOLETE SYNTAX

Command Substitution

Use $() instead of ` `.

# Right.
var="$(command)"

# Wrong.
var=`command`

# $() can easily be nested whereas `` cannot.
var="$(command "$(command)")"

INTERNAL AND ENVIRONMENT VARIABLES

Open the user's preferred text editor

"$EDITOR" "$file"

# NOTE: This variable may be empty, set a fallback value.
"${EDITOR:-vi}" "$file"

Get the current working directory

This is an alternative to the pwd built-in.

"$PWD"

Get the PID of the current shell

"$$"

Get the current shell options

"$-"

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. 🤘