How to trap INT signal infinitely many times?

Question

I'm piping output of an interactive command (ghci) through sed-based script to add some colors:

ghci | colorize.sh

where colorize.sh is something like:

#!/bin/bash
trap '' INT
sed '...some pattern...'

Now if I hit Ctrl-C I want only ghci to receive it (it does not terminate), and I want sed to thrive (or perhaps get restarted?) and still process the output of ghci.

This script does not work and I don't know why.

Answer 1

First, let me start out by saying that this doesn't answer your question, but I hope might help clarify what's happening. I suspect that what you think is happening might not really be happening. Consider this simple example:

# The 'writer' reads input from standard input and
# echos it to standard output.  It handles SIGINT by
# printing INT to standard output.
$ cat writer
#!/bin/bash
function foo() {
    echo "INT"
}

trap foo INT

while read x; do echo $x; done

# The 'reader' reads input from standard input and pipes what is
# read to 'sed', which converts it to upper case.  It ignores SIGINT.
# When it receives EOF on standard input, it writes "done".
$ cat reader
#!/bin/bash
trap '' INT
cat | sed -e 's/\(.*\)/\U\1/'
echo "done"

Now, when I run both, piping the output of writer into reader:

$ ./writer | ./reader
hello
HELLO
^CINT
^CINT
^CINT
world
WORLD
^D
done
$

The writer script reads reads from standard input and writes to the standard output – the pipe. The reader scripts reads from standard input — the pipe — and writes to standard output. When I hit Ctrl-C, the writer writes "INT"; the reader ignores the signal (multiple times). Eventually, I enter Ctrl-D (EOF), and the writer terminates. When the reader receives the EOF, it terminates and writes "done".

Note that the reader ignores the SIGINT more than once, and that neither the pipe nor sed is interrupted when the writer handles the SIGINT.