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All about redirection
by Mike G mikkey at dynamo.com.ar
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There are 3 file descriptors, stdin, stdout and stderr (std=standard).
Basically you can:
1. redirect stdout to a file
2. redirect stderr to a file
3. redirect stdout to a stderr
4. redirect stderr to a stdout
5. redirect stderr and stdout to a file
6. redirect stderr and stdout to stdout
7. 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.
EXAMPLE: stdout 2 file
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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.
EXAMPLE: stderr 2 file
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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.
EXAMPLE: stdout 2 stderr
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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.
EXAMPLE: stderr 2 stdout
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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).
EXAMPLE: stderr and stdout 2 file
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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. Next Previous Contents
[[added to wiki.robotz.com 5/22/2007]]
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