Yusuf Aktepe
Table of contents

POSIX: ‘Here string’ alternative 

# Bash
read -r VAR <<< "$(command)"
# Dash
read -r VAR <<+++
    "$(command)"
+++

This is an alternative to awk '!Line[$0]++', which prints unique lines.

unique() {
    IFS_Old=$IFS
    IFS=$'\n'
    set --

    while IFS= read -r Line; do
        case $IFS$*$IFS in
            *"$IFS$Line$IFS"*) ;; # Seen
            *) set -- "$@" "$Line" ;; # Unique
        esac
    done

    printf '%s\n' "$@"

    IFS=$IFS_Old
}

$ printf '%s\n' foo bar baz foo baz | unique

Bash builtin date 

# GNU date
$ date '+%H:%M · %a, %b %d · %Y'
# Bash
$ printf '%(%H:%M · %a, %b %d · %Y)T\n'

# Output:
# 23:40 · Thu, Feb 22 · 2024

Debug script recursively 

# Follows 'exec's
bash -c "set -x; export SHELLOPTS; ./script"

Pipe two commands, return the exit status of the first 

# Returns 1 if `maim` selection canceled, 0 otherwise
mispipe "maim -usl -b 2 -c 0.35,0.55,0.85,0.25" "xclip -sel c -t image/png"

Reverse an array 

shopt -s extdebug
printf '%s\n' "${ARRAY[@]}"
shopt -u extdebug

printf '%s\n' "${ARRAY[RANDOM % $#]}"

Cycle through an array 

ARRAY=(a b c d)

cycle_array() {
    printf '%s\n' "${ARRAY[${i:=0}]}"
    (( i = i >= ${#ARRAY[@]}-1 ? 0 : ++i ))
}

Get first/last n element of an array 

printf '%s\n' "${ARRAY[@]:0:n}"
printf '%s\n' "${ARRAY[@]: -n}"

# Last n args POSIX
set -- arg1 arg2 arg3
shift "$(($#-n))"
printf '%s\n' "${@}"

head and tail with bash 

head() {
    mapfile -tn "$1" Line < "$2"
    printf '%s\n' "${line[@]}"
}

tail() {
    mapfile -tn 0 Line < "$2"
    printf '%s\n' "${line[@]: -$1}"
}

$ head 2 file
$ tail 2 file

Ternary test 

## Set the value of x to y if y is greater than x.
# x → variable to set
# y > x → condition to test
# ? y → if the test succeeds
# : x → if the test fails
(( x = y > x ? y : x ))

Log script output 

# Start logging
exec 6>&1      # save stdout
exec 7>&2      # save stderr
exec &>"$1"

# Stop logging
exec 1>&6 6>&- # Restore stdout
exec 2>&7 7>&- # Restore stderr

# Redirect error
printf '%s\n' 'Err: This is an error message.' >&2