IGNORING SIGNALS IN SUB-COMMANDS
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If you process some command through `awk`, and want to print some information
at the end, like so:

    some_command | awk ' .... END { print "some info" } '


However this may not work as expected when the user press `ctrl-c`, when we
still want awk to print out the processed info so far. This problem can be
solved as below:

    some_command | ( trap '' INT; awk ' .... END { print "some info" }' )


On a high level, this works because `some_command` terminates following SIGINT.
On the receiving end of the pipe, awk ignores SIGINT. Since it receives no more
data from some_command, the `END` block executes and prints the processed
information received so far.

Note that ignoring SIGINT in the sub-shell (`trap '' INT`) causes the
subprocesses of that shell to ignoring that signal too. However this behavior is
not obvious so let me explain.

First, POSIX shell standard says:

> the shell executes the utility in a separate utility environment (see Shell
> Execution Environment) with actions equivalent to calling the execl() function
> as defined in the System Interfaces volume of POSIX.1-2017


Then in POSIX `execl()` document, it says:

> Except for SIGCHLD, signals set to be ignored (SIG_IGN) by the calling process
> image shall be set to be ignored by the new process image. 

Note that only SIG_IGN is inherited by the subprocesses.


And finally, the proper way to ignore a signal in shell, is by trapping with
empty string:

    trap '' INT