Bash – How to modify output in bash command pipeline

The right thing to do here is to set up bash to prompt for installation, as explained in SamK's answer. I'll answer strictly from a shell usage perspective.

First, the text you're trying to grab is on the command's standard error, but a pipe redirects the standard output, so you need to redirect stderr to stdout.

htop 2>&1 | tail -1

To use the output of a command as part of a command line, use command substitution.

$(htop 2>&1 | tail -1)

The result of the command substitution is split into words and each word is interpreted as a wildcard pattern. Here this happens to do the right thing: this is a command line with words separated by spaces, and there are no wildcard characters.

To evaluate a string as a shell command, use eval. To treat the result of the command as a string rather than a list of wildcard patterns, put it in double quotes.

eval "$(htop 2>&1 | tail -1)"

Of course, before evaluating a shell command like that, make sure it's really what you want to execute.

If you're using Bash 4 or later, you can use coprocesses:

function f () { a=3; b=4 ; echo "`date`: $a $b"; }
coproc cat
f >&${COPROC[1]}
exec {COPROC[1]}>&-
read c <&${COPROC[0]}
echo a $a
echo b $b
echo c $c

will output

a 3
b 4
c Sun Jun 29 10:08:15 NZST 2014: 3 4

coproc creates a new process running a given command (here, cat). It saves the PID into COPROC_PID and standard output/input file descriptors into an array COPROC (just like pipe(2), or see here or here).

Here we run the function with standard output pointed at our coprocess running cat, and then read from it. Since cat just spits its input back out, we get the output of the function into our variable. exec {COPROC[1]}>&- just closes the file descriptor so that cat doesn't keep waiting forever.

Note that read takes only one line at a time. You can use mapfile to get an array of lines, or just use the file descriptor however you want to use it in a different way.

exec {COPROC[1]}>&- works in current versions of Bash, but earlier 4-series versions require you to save the file descriptor into a simple variable first: fd=${COPROC[1]}; exec {fd}>&-. If your variable is unset it will close standard output.

If you're using a 3-series version of Bash, you can get the same effect with mkfifo, but it's not much better than using an actual file at that point.