How to properly execute shell script from nodejs

Question

I need to execute a bash script from nodejs app. Usually I would do something like this:

var childProcess = require('child_process');
childProcess.execFile('./test.sh', function (error, stdout, stderr) {
  console.log(error, stdout, stderr);
})

But I recently ran into a bash script that never terminated when it was executed like that. This is a minimal example that reproduces the issue:

#!/bin/bash
LC_CTYPE=C cat /dev/urandom | tr -dc "a-zA-Z0-9" | fold -w 8 | head -n 1

LC_CTYPE=C - force the tr to interpret everything as ascii characters
tr -dc "a-zA-Z0-9" < /dev/urandom - read pseudorandom bytes, discard everything except a-zA-Z0-9
fold -w 8 - wrap output into lines 8 characters long
head -n 1 - read the first line (8 characters)

When executed from terminal it works fine and it terminates immediately:

# ./test.sh 
0J5hsGeh

When executed using the above nodejs script it just hangs:

# node test.js

It seems that the head -n 1 already terminated but the cat /dev/urandom is still running.

Question: How to execute the script from nodejs so that it does not hang?

Env:

OS: Ubuntu 16.04
Nodejs version: v6.11.2

Set -x in your shebang or run it via bash -x, we'll have a clearer picture of what's happening — Ulises André Fierro
– Ulises André Fierro, Commented Aug 23, 2017 at 19:23
Guys, thanks for suggestions. However, they do not help. exit at the end of the script does nothing and in this case it is not even executed because the previous line never terminates. The -x does not help either because the script never finishes so the output generated by -x is not printed. Anyways, I am not trying to debug the bash script. I am trying to find out why executing it through nodejs makes a difference. — Prvaak
– Prvaak, Commented Aug 24, 2017 at 13:00

Prvaak · Accepted Answer · 2017-09-18 13:33:21Z

2

This is caused by the fact that NodeJS ignores SIGPIPE signal. When a subprocess is spawned the SIGPIPE signal is ignored by the spawned subprocess as well.

Because of that the cat command does not terminate when the pipe is broken after the head terminated after reading the first line. So the cat reads data from /dev/urandom forever.

It should be fixed in one of the future versions of NodeJS (not sure which though). For now I just use the workaround from that issue.

var childProcess = require('child_process');
process.on('SIGPIPE', noop); // <----
childProcess.execFile('./test.sh', function (error, stdout, stderr) {
  console.log(error, stdout, stderr);
})
process.removeListener('SIGPIPE', noop); // <----

answered Sep 18, 2017 at 13:33

Prvaak

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