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Linux Command Line and Shell Scripting Bible - Third Edition
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Sometimes, it's beneficial to send output both to the monitor and to a file for logging. Instead of having to redirect output twice, you can use the special tee command.
The tee command is like a T-connector for pipes. It sends data from STDIN to two destinations at the same time. One destination is STDOUT. The other destination is a filename specified on the tee command line:
tee filename
Because tee redirects data from STDIN, you can use it with the pipe command to redirect output from any command:
$ date | tee testfile
Sun Oct 19 18:56:21 EDT 2014
$ cat testfile
Sun Oct 19 18:56:21 EDT 2014
$
The output appears in STDOUT and is written to the file specified. Be careful: By default, the tee command overwrites the output file on each use:
$ who | tee testfile
rich pts/0 2014-10-17 18:41 (192.168.1.2)
$ cat testfile
rich pts/0 2014-10-17 18:41 (192.168.1.2)
$
If you want to append data to the file, you must use the -a option:
$ date...
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