Bash – How to suppress stderr warning messages from a command inside command substitution

You're right; pkill isn't generating the message, bash is.  You suggest that

$ ./test1.sh 2> /dev/null

is a possible solution.  As UVV points out, the equivalent action from within the script is

exec 2> /dev/null

This redirects the stderr for the script to /dev/null from this statement until it is changed back.  Clumsy ways of changing it back include

exec 2> /dev/tty

which redirects stderr to the terminal.  This is probably (but not necessarily) where it was originally.

Or

exec 2>&1

which sets stderr to be the same as stdout, and is likely to be wrong.

A more reliable way is

exec 3>&2
exec 2> /dev/null
(do stuff where you don't want to see the stderr.)
exec 2>&3

which saves the original stderr in file descriptor 3, and later restores it.

Other ways to suppress just the announcement of the process death include

(sleep 10 & pkill sleep) 2> /dev/null

and

{ sleep 10 & pkill sleep;} 2> /dev/null

which change the stderr for only the grouped commands.

What you are trying to achieve is possible indeed, but only with GNU sed, using the e command. So if you don't mind possible portability issues, you can give it a try.

E.g. your example will look something like this:

>echo aaabbdd | sed -e 's/a*/echo `echo -n & | wc -m`/;e'
3bbdd

if you remove the ;e, you will see an intermediate command (which helps understand why there are two echo'es)

echo `echo -n aaa | wc -m`bbdd

If you want to replace multiple matches, this could be more tricky, but still can be done by chaining substitutions:

>echo aabbaadd | sed -re 's/a+/`echo -n &|wc -m`/g;s/.*/echo &/;e'
2bb2dd

And certainly there are better tools for your task, such as awk, which features a built-in length() function.

References:

• GNU Sed Manual: https://www.gnu.org/software/sed/manual/sed.html#Extended-Commands

P.S.

Also take a note that if your input data contains quotes, you will have to escape them carefully, or temporarily replace them (i.e. with sed y/// command) for this to work.