Proper chaining and piping of commands

Shell Programming and Scripting
1

can someone please help me fix the below one liner?

what i want to do is run a command, and if that command is successful, pipe the output of the command to another command. i'd prefer not to use any temp files for this.

who -blahblah ; if [ $? -ne 0 ] ; then echo exit; fi | egrep username
2

I do not know if what you just posted is a "loose" interpretation of what you want but not real or you have actually tried to run that command.

You may do it as:

who | grep username

You can not check if the command is successful and then try to pipe it to another program. That's not how it works.
When you use a PIPE both programs run in parallel and the output of the first becomes the input of the second.

3

You haven't said what operating system or shell you're using.

If you're on a system using a recent version of bash , you might want to look at the bash manage for the definition of the PIPESTATUS array. With the commands:

who | grep name
echo ${PIPESTATUS[@]}

with a recent bash you would get the output from the grep followed by a line showing the exit value of both commands in the pipeline. For example:

bash-3.2$ who | grep dwc
dwc          console  Dec  8 21:19 
dwc          ttys000  Dec  8 21:22 
dwc          ttys001  Dec  8 21:22 
dwc          ttys002  Dec  8 21:22 
dwc          ttys003  Dec  8 21:22 
dwc          ttys004  Dec  8 21:22 
dwc          ttys005  Dec  8 21:22 
dwc          ttys006  Dec  8 21:22 
dwc          ttys007  Dec  8 21:22 
bash-3.2$ echo ${PIPESTATUS[@]}
0 0
bash-3.2$ who | grep unknown
bash-3.2$ echo ${PIPESTATUS[@]}
0 1
bash-3.2$ who -x | grep dwc
who: illegal option -- x
Usage: who [-abdHlmpqrsTtu] [file]
	who am i
bash-3.2$  echo ${PIPESTATUS[@]}
1 1
bash-3.2$