Bash Notes

basename

Don’t use basename, use:

file=${path##*/}

bash

dirname

Don’t use dirname, use:

dir=${path%/*}

bash

shopt

If you set shopt in a function, reset to its default state with trap:

func() {
  trap "$(shopt -p globstar)" RETURN
  shopt -q -s globstar
}

bash

find, grep, print0, -0, -z

Don’t use find in for loops, because filenames can contain spaces. Try to use globstar and nullglob or null byte terminated strings.

Instead of:

func() {
    for file in $(find /usr/lib* -type f -name 'lib*.a' -print0 ); do
      echo $file
    done
}

bash

use:

func() {
    trap "$(shopt -p nullglob globstar)" RETURN
    shopt -q -s nullglob globstar

    for file in /usr/lib*/**/lib*.a; do
      [[ -f $file ]] || continue
      echo "$file"
    done
}

bash

Or collect the filenames in an array, if you need them more than once:

func() {
    trap "$(shopt -p globstar)" RETURN
    shopt -q -s globstar

    filenames=( /usr/lib*/**/lib*.a )

    for file in "${filenames[@]}"; do
      [[ -f $file ]] || continue
      echo "$file"
    done
}

bash

Or, if you really want to use find, use -print0 and an array:

func() {
    mapfile -t -d '' filenames < <(find /usr/lib* -type f -name 'lib*.a' -print0)
    for file in "${filenames[@]}"; do
      echo "$file"
    done
}

bash

-d '' is the same as -d $'\0' and sets the null byte as the delimiter.

or:

func() {
    find /usr/lib* -type f -name 'lib*.a' -print0 | while read -r -d '' file; do
      echo "$file"
    done
}

bash

func() {
    while read -r -d '' file; do
      echo "$file"
    done < <(find /usr/lib* -type f -name 'lib*.a' -print0)
}

bash

Use the tool options for null terminated strings, like -print0, -0, -z, etc.

prefix or suffix array elements

Instead of:

func() {
  other-cmd $(for k in "$@"; do echo "prefix-$k"; done)
}

bash

func() {
  other-cmd "${@/#/prefix-}"
}

bash

or suffix:

func() {
  other-cmd "${@/%/-suffix}"
}

bash

Join array elements with a separator char

Here we have an associate array _drivers, where we want to print the keys separated by ',':

if [[ ${!_drivers[*]} ]]; then
    echo "rd.driver.pre=$(IFS=, ;echo "${!_drivers[*]}")" > "${initdir}"/etc/cmdline.d/00-watchdog.conf
fi

bash