Redirect STDOUT and STDERR in Linux Shell

Unix shells use three standard I/O streams, which are represented by numeric file descriptors:

• 0 - STDIN - The standard input stream
• 1 - STDOUT - The standard output stream
• 2 - STDERR - The standard error stream

By default, both output and error streams are printed straight to the terminal. But sometimes, we might prefer to redirect them to files for persistence.

Basic

To redirect the streams to files, we can use the n> operator, where n is the file descriptor (in this case, either 1 or 2).

command 1> file_path
command 2> file_path

The first line redirects STDOUT, while the second line redirects STDERR.

Of course, you can also redirect both STDOUT and STDERR at the same time by combining the operators.

command 1> STDOUT.log 2> STDERR.log

Redirecting STDOUT and STDERR to the Same File

In the case where you need to redirect both STDOUT and STDERR to the same file, you can use this command.

command > file_path 2>&1

The 2>&1 construct translates to “send the STDERR to the same place you are sending the STDOUT.”

Pay attention to the order of redirection above. First, we redirect STDOUT to file_path, then we redirect STDERR to STDOUT.

If we do it in reversed order, like command 2>&1 > file_path, the command won’t work as intended. STDERR is redirected to STDOUT which is still the terminal screen, then STDOUT is redirected to file_path.

If you’re using Bash, the more concise construct &> which is equal to 2>&1.

command &> file_path

Redirect and Append to File

In the case where you want to append to the end of the file instead of overwriting it, you can use the >> operator instead.

command >> info.log 2>> error.log

Redirect to Another Process

You can also redirect to another process using the pipe (|) operator.

command1 | command2 | command3

In this case, the output from command1 will act as the input for command2, and the output from command2 will also act as the input for command3, and so on.

Redirect to Nowhere

Finally, in the case where you need to redirect to nowhere due to reasons like very huge output, or if you just don’t want to see the output, you can redirect to /dev/null.

command > /dev/null
command 2> /dev/null

This works because dev/null is like the black hole of the Linux file system. Anything you throw there will never see the light again.