First thing to note is that there's couple of ways depending on your purpose and shell, therefore this requires slight understanding of multiple aspects. Most typical, is via
2> in Bourne-like shells, such as
dash (which is symlinked to
bash; first is the default and POSIX-compliant shell and the other is what most users use for interactive session. They differ in syntax and features, but luckily for us error stream redirection works the same (except the
&> non standard one). In case of csh and its derivatives, the stderr redirection doesn't quite work there.
Let's come back to
2> part. Two key things to notice:
> means redirection operator, where we open a file and
2 integer stands for stderr file descriptor; in fact this is exactly how POSIX standard for shell language defines redirection in section 2.7:
> redirection, the
1 integer is implied for
echo Hello World > /dev/null is just the same as
echo Hello World 1>/dev/null. Note, that the integer or redirection operator cannot be quoted, otherwise shell doesn't recognize them as such, and instead treats as literal string of text. As for spacing, it's important that integer is right next to redirection operator, but file can either be next to redirection operator or not, i.e.
command 2>/dev/null and
command 2> /dev/null will work just fine.
The somewhat simplified syntax for typical command in shell would be
command [arg1] [arg2] 2> /dev/null
The trick here is that redirection can appear anywhere. That is both
2> command [arg1] and
command 2> [arg1] are valid. Note that for
bash shell, there there exists
&> way to redirect both stdout and stderr streams at the same time, but again - it's bash specific and if you're striving for portability of scripts, it may not work. See also Ubuntu Wiki and What is the difference between &> and 2>&1.
> redirection operator truncates a file and overwrites it, if the file exists. The
2>> may be used for appending
stderr to file.
If you may notice,
> is meant for one single command. For scripts, we can redirect stderr stream of the whole script from outside as in
myscript.sh 2> /dev/null or we can make use of exec built-in. The exec built-in has the power to rewire the stream for the whole shell session, so to speak, whether interactively or via script. Something like
exec 2> ./my_log_file.txt
In this example, the log file should show
stat: cannot stat '/etc/non_existing_file': No such file or directory.
Yet another way is via functions. As kopciuszek noted in his answer, we can write function declaration with already attached redirection, that is
} 2> my_log_file.txt