Question

If I 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.

Answer 1

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.