Piping output of a command into grep and then into another command

Question

I am taking output of one script, then piping this putput to grep and piping the output of grep into another script:

./script1 | grep 'expr' | ./script2

However, the second script never gets any input. Again, nothing gets printed when I substitute script2 with cat and script1 with ping:

ping localhost | grep localhost | cat

Same thing happens if I replace grep with awk:

ping localhost|awk '{print $1}'|cat

What is going on?

https://unix.stackexchange.com/a/503197/130767 – FreeSoftwareServers Mar 10 '21 at 02:29 — FreeSoftwareServers, Mar 10 '21 at 02:29

score 10 · Accepted Answer · answered Apr 30 '18 at 09:59

10

grep is buffering (because it determines that its output isn’t a terminal; strictly speaking, this is the C library’s behaviour).

To disable this, run it with unbuffer -p (the -p is necessary for unbuffer to read from its standard input):

ping localhost | unbuffer -p grep localhost | cat

or tell grep to buffer by line (if it supports this):

ping localhost | grep --line-buffered localhost | cat

answered Apr 30 '18 at 09:59

Stephen Kitt

3

TY kindly for `--line-buffered` flag, since I do not have rights to install `unbuffer` on the target machine – Artalus Apr 30 '18 at 10:06
Note: even without modifying buffer behaviour, grep will output all remaining output when script1 exits. – ctrl-alt-delor Apr 30 '18 at 11:42

1 Answers1