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doc: $_ pitfalls

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Kay Marquardt (Gnadelwartz) 2021-01-07 08:58:34 +01:00
parent 6f8515d701
commit d3253fdf76
1 changed files with 22 additions and 11 deletions
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doc/7_develop.md
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@ -261,21 +261,32 @@ ADDME="$ADDME something to add" -> ADDME+=" something to add""
VAR="$(( 1 + 2 ))" -> (( var=1+2 ))
INDEX="$(( ${INDEX} + 1 ))" -> (( INDEX++ ))
```
# manipulate the same variable multiple times without storing intermediate
START="1a23_b__x###"
The special variable stores the expanded __last__ argument of the previous command.
This allows some useful optimisations, especially in combination with the no-op command `:`.
BAR="${START//[0-9]}" # a_b__x###
BAR="${BAR%%#*}" # a_b__x
BAR="${BAR/__x/_c}" # a_b_c
```bash
# mkdir plus cd to it
mkdir "tmpdir$$" && cd "$_" # rmpdir1234 (process id)
: "${START//[0-9]}" # a_b__x###
# manipulate a variable multiple times without storing intermediate results
start="1a23_b__x###"
...
: "${start//[0-9]}" # a_b__x###
: "${_%%#*}" # a_b__x
BAR="${_/__x/_c}" # a_b_c
bar="${_/__x/_c}" # a_b_c
# works also for output of commands
: "$(date "+%A")" # Weekday
UPPERDAY="${_^^}" # WEEKDAY
# BE AWARE OF ...
# pitfall/magic missing quotes: $_ is LAST arg
: ${SOMEVAR} # String in var $_ -> "var"
: $(<"file") # Content of\n file $_ -> "file"
# pitfall/magic command substitution: globbing and IFS is applied!
: "$(echo "a* is born")"# $_ -> a* is globbed even quoted!
: "$(echo "a b c")" # $_ -> "a b c"
: "$(<"file")" # "Content of\n file" $_ -> "Content of file"
```
For more examples see [Pure bash bible](https://github.com/dylanaraps/pure-bash-bible)
@ -372,5 +383,5 @@ fi
#### [Prev Function Reference](6_reference.md)
#### $$VERSION$$ v1.25-dev-30-gefca2e0
#### $$VERSION$$ v1.25-dev-31-g6f8515d