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:
[nancy@asus /var/tmp]$ ls 2> tmp
[nancy@asus /var/tmp]$ ls -l tmp
-rw-rw-r-- 1 nancy nancy 0 Sept 7 12:58 tmp
[nancy@asus /var/tmp]$ ls 2 > tmp
ls: 2: No such file or directory
|
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.
