() {} # declare function with global scope for internal declarations
() () # declare function with local scope for internal declarations (subprocess)

DEBUG

https://linuxconfig.org/how-to-debug-bash-scripts

bash -x $script

To see current state of all variables

echo $-

trap signals

ARG is a command to be read and executed when the shell receives the signal(s) SIGNAL_SPEC. If ARG is absent (and a single SIGNAL_SPEC is supplied) or `-', each specified signal is reset to its original value. If ARG is the null string each SIGNAL_SPEC is ignored by the shell and by the commands it invokes

trap n [condition...]
trap [action condition...]
  -l # print a list of signal names and their corresponding numbers
  -p # display the trap commands associated with each SIGNAL_SPEC

trap $cmd $signal # launch $cmd when signal $signal is enabled

EXAMPLES

get stdin

function _read() { while IFS= read line; do echo $line; done; }
ls | _read

give answer to command

echo -e "ct\nct"|sudo passwd root
yes ct | sudo passwd root

give the different pids of subprocess in script

file=/tmp/test
paths=$(df -l |grep "^/dev.*120.*" |sed "s|^.* \([^ ]*\)$|\1|" | sed "/save/d")
#paths='/ /var'
echo '#!/bin/bash' > $file
for path in $paths
do
        echo "# $path" >> $file
        echo "(echo > free2zero; pid=\$!; echo \"start $path - \\\$! \$! - \\\$BASHPID \$BASHPID - \\\$PPID \$PPID - \\\$\\\$ \$\$\"; cd $path; sleep 60; rm free2zero; echo \"end $path - \\\$! \$! - \\\$BASHPID \$BASHPID - \\\$PPID \$PPID - \\\$\\\$ \$\$\"; exit) &" >> $file
done
echo "echo \"itself : \\\$! \$! - \\\$BASHPID \$BASHPID - \\\$PPID \$PPID - \\\$\\\$ \$\$\"; sleep 30" >> $file
chmod +x $file
cat $file
sh $file

wipe free space in devices

file=/tmp/test
paths=$(df -l |grep "^/dev.*" |sed "s|^.* \([^ ]*\)$|\1|" |xargs)
paths=${paths/\/save/}
echo '#!/bin/bash' > $file
for path in $paths
do
        echo "# $path" >> $file
        echo "(echo \"start $path \$(date +\"%T - %N\")\"; cd $path; dd if=/dev/zero of=free2zero; rm free2zero; echo \"end $path \$(date +\"%T - %N\")\"; exit) &" >> $file
done
chmod +x $file
sh $file