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There are 3 file descriptors, stdin, stdout and stderr (std=standard).
Basically you can:
- redirect stdout to a file
- redirect stderr to a file
- redirect stdout to a stderr
- redirect stderr to a stdout
- redirect stderr and stdout to a file
- redirect stderr and stdout to stdout
- redirect stderr and stdout to stderr
1 'represents' stdout and 2 stderr.
A little note for seeing this things: with the less command you can view both stdout
(which will remain on the buffer) and the stderr that will be printed on the screen, but erased as
you try to 'browse' the buffer.
This will cause the ouput of a program to be written to a file.
ls -l > ls-l.txt
Here, a file called 'ls-l.txt' will be created and it will contain what you would see on the
screen if you type the command 'ls -l' and execute it.
This will cause the stderr ouput of a program to be written to a file.
grep da * 2> grep-errors.txt
Here, a file called 'grep-errors.txt' will be created and it will contain what you would see
the stderr portion of the output of the 'grep da *' command.
This will cause the stderr ouput of a program to be written to the same filedescriptor
than stdout.
grep da * 1>&2
Here, the stdout portion of the command is sent to stderr, you may notice that in differen ways.
This will cause the stderr ouput of a program to be written to the same filedescriptor
than stdout.
grep * 2>&1
Here, the stderr portion of the command is sent to stdout, if you pipe to less, you'll see that
lines that normally 'dissapear' (as they are written to stderr) are being kept now (because
they're on stdout).
This will place every output of a program to a file. This is suitable sometimes
for cron entries, if you want a command to pass in absolute silence.
rm -f $(find / -name core) &> /dev/null
This (thinking on the cron entry) will delete every file called 'core' in any directory. Notice
that you should be pretty sure of what a command is doing if you are going to wipe it's output.
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