A couple of weeks ago I covered some basic I/O redirection in bash (see I/O redirection in bash). Well, there's actually a lot more to it, so for this TOTW I thought I'd touch on a few more advanced usages.

Redirecting stderr

Sometimes, you may find that part or all of the output of a command isn't properly redirected. As a quick example, navigate to any directory that is not a Git repository, and run git status. You should see something like this:

$ git status
fatal: not a git repository (or any of the parent directories): .git

Yet, if you try to redirect it with a standard redirect, the output is still displayed, and the file you redirect to remains empty.

$ git status > output
fatal: not a git repository (or any of the parent directories): .git
$ cat output
$ 

The reason is quite simple: the output from git status is an error message, which is typically output on standard error (stderr), while I/O redirection operates on standard output (stdout) by default. When redirecting output (or input, for that matter), one can optionally provide a file descriptor specifying which output stream to redirect. On a typical UNIX-like system, stdout is file descriptor 1, and stderr is file descriptor 2. So if we want to catch that stderr output, we just need to prepend a 2 to the redirection operator.

$ git status 2> output
$ cat output
fatal: not a git repository (or any of the parent directories): .git

You can probably guess that if you leave the file descriptor out, it will default to 1. In some cases, you may want to redirect both stderr and stdout to the same file. But many programs output both on stderr and stdout, and we may want to redirect both of them.

Redirecting stderr and stdout

So, we can specify a file descriptor to redirect stdout or stderr (or any other file descriptor, really), but many programs output on both stderr and stdout, and it's often useful to redirect both. Here's a small Python script print.py that outputs on line on stdout and one on stderr.

import sys

print("some standard output", file=sys.stdout)
print("some error output", file=sys.stderr)

Note: That's Python as in Python 3.

If we redirect stdout only, then the stderr line is still printed to the terminal.

$ python3 print.py 1> stdout_output # recall that the 1 can be omitted
some error output
$ cat stdout_output
some standard output

Similarly, redirecting only stderr leaves the stdout output on the terminal.

$ python3 print.py 2> stderr_output
some standard output
$ cat stderr_output
some error output

Quite intuitively, if we want to redirect both stderr and stdout to one file each, we can simply do two redirections following one another.

$ python3 print.py 1> stdout_output 2> stderr_output
$ cat stdout_output
some standard output
$ cat stderr_output
some error output

There's also the possibility to redirect both stdout and stderr to the same file using the special & character in place of a file descriptor.

$ python3 print.py &> output
$ cat output
some standard output
some error output

And with that and the previous article, I've shared pretty much everything I feel is useful with output redirection. In some future Tip of the Week, I'm sure I'll get into input redirecton as well, as it's much the same.