How to Redirect stderr to stdout in Bash
Introduction
Before we begin talking about how to redirect stderr to stdout in Bash, let's briefly understand – What is stdout and stderr?
You may observe that the error messages are written on the screen when forwarding the output of a command to a file or piping it to another program.
When a program is performed in Bash and other Linux shells, it uses three standard I/O streams. A number file descriptor is assigned to each stream:
0 - stdin, the standard input stream.
1 - stdout, the standard output stream.
2 - stderr, the standard error stream.
A file descriptor is nothing more than an integer that represents an open file.
The input stream supplies data to the program, which is often done by typing on a keyboard.
The output of the program is sent to the standard input stream, while error messages are sent to the standard error stream. Both the input and error streams are printed on the screen by default.
In this tutorial, you will redirect stderr to stdout in Bash. We will also address a few FAQs on how to redirect stderr to stdout in Bash.
Advantages of stdout and stderr
- Standardized Output: "stdout" provides a consistent way to display program output.
- Scripting: Facilitates automation and scripting by capturing output for further processing.
- Error Reporting: "stderr" allows separate, reliable transmission of error messages and exceptions from program output.
- Diagnostic Information: Key for debugging and identifying problems in a program.
- Enhanced Debugging: Helps in identifying and resolving both standard output issues and error-related problems.
Redirecting Output
Redirection is a method of capturing a program's output and sending it as input to another program or file.
The n> operator, where n is the file descriptor number, can be used to redirect streams.
When n is omitted, it defaults to 1, the normal output stream is used. The following two instructions, for example, are identical, both redirect the command output (stdout) to a file.
command > file
command 1> file
Use the 2> operator to reroute the standard error (stderr):
command 2> file
Both stderr and stdout can be written to two different files:
command 2> error.txt 1> output.txt
Redirect stderr to /dev/null to prevent error messages from being displayed on the screen:
command 2> /dev/null
Redirecting stderr to stdout
When saving a program's output to a file, it's usual practice to redirect stderr to stdout so that everything is contained in one file.
Use the following to redirect stderr to stdout and have error messages written to the same file as standard output:
command > file 2>&1
2>&1 redirects stderr to the current location of stdout, and > file redirects stdout to file.
The order in which the redirections are made is crucial. The following example, for example, redirects solely stdout to a file. This occurs, because stderr is redirected to stdout before stdout is sent to file.
command 2>&1 > file
The &> construct can also be used to route stderr to stdout. &> has the same meaning in Bash as 2>&1:
command &> file
FAQs to Redirect stderr to stdout in Bash
What does it mean to redirect stderr to stdout in Bash?
Redirecting stderr to stdout in Bash means merging the error messages with regular output, allowing them to be displayed together.
Why would I want to redirect stderr to stdout?
Redirecting stderr to stdout helps consolidate all output in one stream, making it easier to capture, view, and process both regular output and error messages.
Can I redirect stderr to a file instead of stdout?
Yes, you can redirect stderr directly to a file by using the command 2> filename. This will send all error messages to the specified file.
How do I redirect both stderr and stdout to separate files?
To redirect stderr to one file and stdout to another, you can use the commands 2> error.txt to redirect stderr and 1> output.txt to redirect stdout.
Is it possible to discard stderr and only capture stdout?
Yes, you can discard stderr by using the command 2> /dev/null. This redirects stderr to the null device, effectively discarding error messages.
What happens if I redirect stderr to stdout and both streams contain data?
When stderr is redirected to stdout, both the error messages and regular output will be combined in the redirected stream, displaying them together.
Can I redirect stderr and stdout separately within a Bash script?
Yes, within a Bash script, you can redirect stderr and stdout independently using different file descriptors and redirection operators for each stream.
Conclusion
When working on the command line, it's critical to understand the concepts of redirections and file descriptors.
Use the 2>&1 or &> constructions to redirect stderr and stdout.
If you have any queries, please leave a comment below and we’ll be happy to respond to them.