Redirect stderr to stdout in C shell

csh, io-redirection

Solution

The `csh` shell has never been known for its extensive ability to manipulate file handles in the redirection process.

You can redirect both standard output and error to a file with:

xxx >& filename

but that's not quite what you were after, redirecting standard error to the current standard output.

However, if your underlying operating system exposes the standard output of a process in the file system (as Linux does with `/dev/stdout`), you can use that method as follows:

xxx >& /dev/stdout

This will force both standard output and standard error to go to the same place as the current standard output, effectively what you have with the `bash` redirection, `2>&1`.

Just keep in mind this isn't a `csh` feature. If you run on an operating system that doesn't expose standard output as a file, you can't use this method.

However, there is another method. You can combine the two streams into one if you send it to a pipeline with `|&`, then all you need to do is find a pipeline component that writes its standard input to its standard output. In case you're unaware of such a thing, that's exactly what `cat` does if you don't give it any arguments. Hence, you can achieve your ends in this specific case with:

xxx |& cat

Of course, there's also nothing stopping you from running `bash` (assuming it's on the system somewhere) within a `csh` script to give you the added capabilities. Then you can use the rich redirections of that shell for the more complex cases where `csh` may struggle.

Let's explore this in more detail. First, create an executable `echo_err` that will write a string to `stderr`:

#include <stdio.h>
int main (int argc, char *argv[]) {
    fprintf (stderr, "stderr (%s)\n", (argc > 1) ? argv[1] : "?");
    return 0;
}

Then a control script `test.csh` which will show it in action:

#!/usr/bin/csh

ps -ef ; echo ; echo $$ ; echo

echo 'stdout (csh)'
./echo_err csh

bash -c "( echo 'stdout (bash)' ; ./echo_err bash ) 2>&1"

The `echo` of the PID and `ps` are simply so you can ensure it's `csh` running this script. When you run this script with:

./test.csh >test.out 2>test.err

(the initial redirection is set up by `bash` before `csh` starts running the script), and examine the `out/err` files, you see:

test.out:
    UID     PID    PPID  TTY        STIME     COMMAND
    pax    5708    5364  cons0      11:31:14  /usr/bin/ps
    pax    5364    7364  cons0      11:31:13  /usr/bin/tcsh
    pax    7364       1  cons0      10:44:30  /usr/bin/bash

    5364

    stdout (csh)
    stdout (bash)
    stderr (bash)

test.err:
    stderr (csh)

You can see there that the `test.csh` process is running in the C shell, and that calling `bash` from within there gives you the full `bash` power of redirection.

The `2>&1` in the `bash` command quite easily lets you redirect standard error to the current standard output (as desired) without prior knowledge of where standard output is currently going.

Problem

Original source