Shell Programming and Scripting

I have the following command in a Bash shell script:

who | grep -w $1 | some other commands

If grep fails, an error message is displayed. How do I test if grep fails and still be able to pipe it's output to the rest of the commands?


I have the following solution:

a=`who | grep -w $1`
if [ $? -eq 0 ]
then
display error message
fi
printf "$a" | some other commands


Is this the best solution, or is there another method?
Just in case I have some other set of commands, and the variable is assigned some other output, I read somewhere that its not a good idea to assign variables to a large amount of data, or binary data.

what do you mean "fails" ? $1 is blank??
returns nothing???

why use grep if you still want to pass everything else to "some-other-commands" ?

you could just run things twice.

who isn't all that expensive a utility.

Fails as in, grep doesn't find any results.

Then output an error message such as, "User is not logged on"

Er... why not? Is there another command I can use?

I don't want to run things twice, because there are further error conditions in the subsequent set of commands. The idea is, I have a long sequence of commands, connected with pipes. If the execution fails at grep because user is not logged on, then output an error message. After grep, I retrieve the tty(s) of the user(from the grep'ed results), and if all his ttys are non-writeable, output another error message, and so on and so forth. There are multiple error conditions in the commands, and if it fails at any point, I would like to know where it fails, and output an appropriate error message.

ok. makes more sense.
can't be done.

you must use temp files.