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|
# 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
```
|
