Bash script tutorials from https://linuxconfig.org/bash-scripting-tutorial
In Linux, stdin is the standard input stream. This accepts text as its input. Text output from the command to the shell is delivered via stdout (standard out stream). Error messages from the command are sent through the stderr (standard error) stream.
So you can see that there are two output streams, stdout and stderr, and one input stream, stdin.
Streams in Linux—like almost everything else—are treated as though they were files. You can read text from a file, and you can write text into a file. Both of these actions involve a stream of data. So the concept of handling a stream of data as a file isn’t that much of a stretch.
Each file associated with a process is allocated a unique number to identify it. This is known as the file descriptor. Whenever an action is required to be performed on a file, the file descriptor is used to identify the file. These values are always used for stdin, stdout, and stderr:
- 0: stdin
- 1: stdout
- 2: stderr
The > redirection symbol works with stdout by default. You can use one of the numeric file descriptors to indicate which standard output stream you wish to redirect.
To explicitly redirect stdout, use this redirection instruction:
1>
To explicitly redirect stderr, use this redirection instruction:
2>
./error.sh > capture.txt 2>&1Let's break that down
./error.sh: Launches the error.sh script file.> capture.txt: Redirects thestdoutstream to the capture.txt file,>is a shorthand to1>2>&1: This uses the&>redirect instruction. This instruction allows you to tell the shell to make one stream go to the same destination as another stream. In this case, we’re saying “redirect stream 2, stderr, to the same destination that stream 1, stdout, is being redirected to.”