linux cli: conditional command dependent on original program in pipe chain

If I开发者_开发问答 type:

progA | progB && progC

then progC is conditional on progB, and not on progA. Is there a way to both pipe the output of progA to progB, and then, after progA is done (and if I have to wait for progB to be done too, then that's not a problem) have progC be conditional on prog A?

Also, I need to keep the pipe -- as in I can't afford to do progA > file; ....; progB < file and loose the time efficiency.

EDIT: How to use the return code of the first program in a pipe command line is a similar question, but does not have constraint of keeping the pipe, and it also assumes that I have access to the code of progB.

---

Use a named pipe.

With variations on grep as progA and cat as progB:

$ cat input
foo
bar 
baz
$ grep fool < input > fifo & cat fifo & wait %1 && echo good || echo bad
[1] 13876
[2] 13877
[1]-  Exit 1                  grep fool < input > fifo
bad
[2]+  Done                    cat fifo
$ grep foo < input > fifo & cat fifo & wait %1 && echo good || echo bad
[1] 13878
[2] 13879
foo
[1]-  Done                    grep foo < input > fifo
good
[2]+  Done                    cat fifo

Note that wait without a job ID will always return 0 no matter what the exit status of the background job was and that wait %1 in a separate command will not work when the job exits before you start the wait. I wouldn't want to put this into production really. All this was tested on bash btw and might be slightly different for other shells.

---

I'm not familiar with a way to do this on plain command line. But you could do this in a shell script, the following would make sure each previous program exited with success and pass the output along to the next program. You'd use it like this myscript.sh progA progB progC. If you didn't want direct dependency on previous program success you could use a counter and check manually for whatever param order you wanted.

#!/usr/bin/env bash

OUTPUT=""

# loop through each argument
while [ $# -ne 0 ]; do

  # run each argument as a command and store output
  OUTPUT=$($1 $OUTPUT)
  RESULT=$?

  # check for success
  if [ "$RESULT" -ne "0" ]; then
    # do whatever else on error here
    exit $RESULT
  fi

  # next arg
  shift
done