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5.2. Advanced redirection features
5.2.1. Use of file descriptors
There are three types of I/O, which each have their own identifier, called a file descriptor:
standard input: 0
standard output: 1
standard error: 2
In the following descriptions, if the file descriptor number is omitted, and the first character of the redirection operator is <, the redirection refers to the standard input (file descriptor 0). If the first character of the redirection operator is >, the redirection refers to the standard output (file descriptor 1).
Some practical examples will make this more clear:
ls > dirlist 2>&1
will direct both standard output and standard error to the file dirlist, while the command
ls 2>&1 > dirlist
will only direct standard output to dirlist. This can be a useful option for programmers.
Things are getting quite complicated here, don't confuse the use of the ampersand here with the use of it in Section 4.1.2.1, where the ampersand is used to run a process in the background. Here, it merely serves as an indication that the number that follows is not a file name, but rather a location that the data stream is pointed to. Also note that the bigger-than sign should not be separated by spaces from the number of the file descriptor. If it would be separated, we would be pointing the output to a file again. The example below demonstrates this:
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The first command that nancy executes is correct (eventhough no errors are generated and thus the file to which standard error is redirected is empty). The second command expects that 2 is a file name, which does not exist in this case, so an error is displayed.
All these features are explained in detail in the Bash Info pages.
5.2.2. Examples
5.2.2.1. Analyzing errors
If your process generates a lot of errors, this is a way to thoroughly examine them:
command 2>&1 | less
This is often used when creating new software using the make command, such as in:
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5.2.2.2. Separating standard output from standard error
Constructs like these are often used by programmers, so that output is displayed in one terminal window, and errors in another. Find out which pseudo terminal you are using issuing the tty command first:
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5.2.2.3. Writing to output and files simultaneously
You can use the tee command to copy input to standard output and one or more output files in one move. Using the -a option to tee results in appending input to the file(s). This command is useful if you want to both see and save output. The > and >> operators do not allow to perform both actions simultaneously.
This tool is usually called on through a pipe (|), as demonstrated in the example below:
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