From 9cb1c4df7b9ce1a9ad1312621b0f2b16a94fba3a Mon Sep 17 00:00:00 2001 From: Daniel Baumann Date: Mon, 3 Jun 2024 15:54:25 +0200 Subject: Adding upstream version 060+5. Signed-off-by: Daniel Baumann --- docs/BASH.md | 213 +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ 1 file changed, 213 insertions(+) create mode 100644 docs/BASH.md (limited to 'docs/BASH.md') diff --git a/docs/BASH.md b/docs/BASH.md new file mode 100644 index 0000000..e72a2f6 --- /dev/null +++ b/docs/BASH.md @@ -0,0 +1,213 @@ +# BASH Notes + +## basename +Don't use `basename`, use: +```shell + file=${path##*/} +``` + +## dirname +Don't use `dirname`, use: +```shell + dir=${path%/*} +``` + +## shopt +If you set `shopt` in a function, reset to its default state with `trap`: +```shell +func() { + trap "$(shopt -p globstar)" RETURN + shopt -q -s globstar +} +``` + +## 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: +```shell +func() { + for file in $(find /usr/lib* -type f -name 'lib*.a' -print0 ); do + echo $file + done +} +``` + +use: +```shell +func() { + trap "$(shopt -p nullglob globstar)" RETURN + shopt -q -s nullglob globstar + + for file in /usr/lib*/**/lib*.a; do + [[ -f $file ]] || continue + echo "$file" + done +} +``` + +Or collect the filenames in an array, if you need them more than once: +```shell +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 +} +``` + +Or, if you really want to use `find`, use `-print0` and an array: +```shell +func() { + mapfile -t -d '' filenames < <(find /usr/lib* -type f -name 'lib*.a' -print0) + for file in "${filenames[@]}"; do + echo "$file" + done +} +``` + +Note: `-d ''` is the same as `-d $'\0'` and sets the null byte as the delimiter. + +or: +```shell +func() { + find /usr/lib* -type f -name 'lib*.a' -print0 | while read -r -d '' file; do + echo "$file" + done +} +``` + +or +```shell +func() { + while read -r -d '' file; do + echo "$file" + done < <(find /usr/lib* -type f -name 'lib*.a' -print0) +} +``` + +Use the tool options for null terminated strings, like `-print0`, `-0`, `-z`, etc. + +## prefix or suffix array elements + +Instead of: +```shell +func() { + other-cmd $(for k in "$@"; do echo "prefix-$k"; done) +} +``` +do +```shell +func() { + other-cmd "${@/#/prefix-}" +} +``` + +or suffix: +```shell +func() { + other-cmd "${@/%/-suffix}" +} +``` + +## Join array elements with a separator char + +Here we have an associate array `_drivers`, where we want to print the keys separated by ',': +```shell + if [[ ${!_drivers[*]} ]]; then + echo "rd.driver.pre=$(IFS=, ;echo "${!_drivers[*]}")" > "${initdir}"/etc/cmdline.d/00-watchdog.conf + fi +``` + +## Optional parameters to commands + +If you want to call a command `cmd` with an option, if a variable is set, rather than doing: + +```shell +func() { + local param="$1" + + if [[ $param ]]; then + param="--this-special-option $param" + fi + + cmd $param +} +``` + +do it like this: + +```shell +func() { + local param="$1" + + cmd ${param:+--this-special-option "$param"} +} + +# cmd --this-special-option 'abc' +func 'abc' + +# cmd +func '' + +# cmd +func +``` + +If you want to specify the option even with an empty string do this: + +```shell +func() { + local -a special_params + + if [[ ${1+_} ]]; then + # only declare `param` if $1 is set (even as null string) + local param="$1" + fi + + # check if `param` is set (even as null string) + if [[ ${param+_} ]]; then + special_params=( --this-special-option "${param}" ) + fi + + cmd ${param+"${special_params[@]}"} +} + +# cmd --this-special-option 'abc' +func 'abc' + +# cmd --this-special-option '' +func '' + +# cmd +func +``` + +Or more simple, if you only have to set an option: +```shell +func() { + if [[ ${1+_} ]]; then + # only declare `param` if $1 is set (even as null string) + local param="$1" + fi + + cmd ${param+--this-special-option} +} + +# cmd --this-special-option +func 'abc' + +# cmd --this-special-option +func '' + +# cmd +func +``` + -- cgit v1.2.3