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doc: more careful $_ advise

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Kay Marquardt (Gnadelwartz) 2021-01-08 15:29:36 +01:00
parent 58e789a079
commit f0a2843f2a
1 changed files with 15 additions and 13 deletions
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doc/7_develop.md
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@ -262,34 +262,36 @@ VAR="$(( 1 + 2 ))" -> (( var=1+2 ))
INDEX="$(( ${INDEX} + 1 ))" -> (( INDEX++ )) INDEX="$(( ${INDEX} + 1 ))" -> (( INDEX++ ))
``` ```
For more examples see [Pure bash bible](https://github.com/dylanaraps/pure-bash-bible)
The special variable `$_` stores the expanded __last__ argument of the previous command. The special variable `$_` stores the expanded __last__ argument of the previous command.
This allows some nice optimisations, especially in combination with the no-op command `:`, This allows a nice optimisation in combination with the no-op command `:`, but be aware of `$_` pitfalls.
but be aware of the pitfalls.
```bash ```bash
# mkdir plus cd to it # $_ example: mkdir plus cd to it
mkdir "tmpdir$$" && cd "$_" # rmpdir1234 (process id) mkdir "somedir-$$" && cd "$_" # somedir-1234 (process id)
# manipulate a variable multiple times without storing intermediate results # manipulate a variable multiple times without storing intermediate results
start="1a23_b__x###" foo="1a23_b__x###"
... ...
: "${start//[0-9]}" # a_b__x### : "${foo//[0-9]}" # a_b__x###
: "${_%%#*}" # a_b__x : "${_%%#*}" # a_b__x
bar="${_/__x/_c}" # a_b_c bar="${_/__x/_c}" # a_b_c
# BE AWARE OF ... # BE AWARE OF ...
# pitfall/magic missing quotes: $_ is LAST arg # pitfall missing quotes: $_ is LAST arg
: ${SOMEVAR} # String in var $_ -> "var" : ${SOMEVAR} # "String in var" $_ -> "var"
: $(<"file") # Content of\n file $_ -> "file" : $(<"file") # "Content of\n file" $_ -> "file"
# pitfall/magic command substitution: globbing and IFS is applied! # pitfall test command
[ -n "$MYVAR" ] && echo "$_" # outputs "]"
# pitfall command substitution: globbing and IFS is applied!
: "$(echo "a* is born")"# $_ -> a* is globbed even quoted! : "$(echo "a* is born")"# $_ -> a* is globbed even quoted!
: "$(echo "a b c")" # $_ -> "a b c" : "$(echo "a b c")"# $_ -> "a b c"
: "$(<"file")" # "Content of\n file" $_ -> "Content of file" : "$(<"file")" # "Content of\n file" $_ -> "Content of file"
``` ```
For more examples see [Pure bash bible](https://github.com/dylanaraps/pure-bash-bible)
#### Prepare a new version #### Prepare a new version
After some development it may time to create a new version for the users. a new version can be in sub version upgrade, e.g. for fixes and smaller additions or After some development it may time to create a new version for the users. a new version can be in sub version upgrade, e.g. for fixes and smaller additions or
@ -384,5 +386,5 @@ fi
#### [Prev Function Reference](6_reference.md) #### [Prev Function Reference](6_reference.md)
#### $$VERSION$$ v1.25-dev-31-g6f8515d #### $$VERSION$$ v1.25-dev-36-g58e789a