====== Shell ======

===== Block comment =====

With "END" being just an example of any random string that must not appear in the commented section:
<code bash>
: << 'END'
sleep 1
END
</code>

===== Iterate over a list as a string =====

Different options:
<code bash>
# option 1, without variable
for x in a b c; do echo $x; done

# option 2, with string
list="a b c"
for x in $list; do echo $x; done         # only works with sh and bash
for x in ${=list}; do echo $x; done      # only works with zsh
for x in $(echo $list}; do echo $x; done # works with any shell

# option 3, with an array
list=(a b c)
for x in $list; do echo $x; done
</code>

===== Do something recursively on all files =====

Use ''find'', with ''{}'' being the file name:
<code bash>
find . -type f -exec <command> {} \;
find -iname "<pattern>" -exec <command> {} \;
</code>

Manual solution:
<code bash>
#!/bin/sh

function search_dir()
{
    DIR=$1;

    for file in $DIR/*; do
        if [[ -f $file ]]; then
            echo "Do something with $file"
        fi;
    done


    for element in $DIR/* ; do
        if [[ -e $element && -d $element && \
            $(basename "$element") != ".." && \
            $(basename "$element") != "." ]]; then

            search_dir "$element";
        fi;
    done;
}

search_dir "."
</code>

===== Misc =====

  * **Default value for variable**: <code bash>
${<variable>:-<default-value>}
</code>
  * **Get directory of current script** (for instance in order to run a script that is stored in the same directory, or in a known relative path):<code bash>
BASEDIR=$(dirname "$0")
</code>
  * **echo to stderr**, just redirect the output:<code bash>
echo "foo" 1>&2
</code>
  * **Convert dash-time to ISO** (2024-04-21_15-50-30 to 2024-04-21T15:50:30, for instance to use it as input for ''date''): <code bash>
sed -r "s/([0-9]{4})-([0-9]{2})-([0-9]{2})_([0-9]{2})-([0-9]{2})-([0-9]{2})/\1-\2\-3T\4:\5:\6/"
</code>
  * **Get target name of symbolic link**:<code bash>
readlink <symbolic-link>
</code>
  * **Execute command**:<code bash>
var=`cat file`
var=$(cat file)
</code>
  * **Evaluate mathematical expression** (''zsh'' also supports floating-point operations):<code bash>
echo $((2*3))
</code>
  * **time with redirected output** (also works for groups of commands):<code bash>
time (cat file1 > file2)
time (cat file1 > /dev/null; sync)
</code>
  * **Check number of arguments and print usage:**<code bash>
if [[ $# -lt 2 ]]; then
        echo "Usage: foo.sh <arg1> <arg2>" 1>&2
        exit 1
fi
</code>
  * Load command output as a file:<code>
diff <(grep x file1) <(grep y file2)
</code>