How to log output in bash and see it in the terminal at the same time?

Question

I have some scripts where I need to see the output and log the result to a file, with the simplest example being:

$ update-client > my.log

I want to be able to see the output of the command while it's running, but also have it logged to the file. I also log stderr, so I would want to be able to log the error stream while seeing it as well.

score 59 · Accepted Answer · answered Jul 09 '10 at 19:07

59

update-client 2>&1 | tee my.log

2>&1 redirects standard error to standard output, and tee sends its standard input to standard output and the file.

answered Jul 09 '10 at 19:07

Matthew Flaschen

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You can also append the log file, as explained here, and do other stuffs with tee: http://linux.101hacks.com/unix/tee-command-examples/ – thegeek Jul 12 '10 at 08:18

score 7 · Answer 2 · answered Feb 18 '16 at 07:54

You can use the tee command for that:

command | tee /path/to/logfile

The equivelent without writing to the shell would be:

command > /path/to/logfile

If you want to append (>>) and show the output in the shell, use the -a option:

command | tee -a /path/to/logfile

Please note that the pipe will catch stdout only, errors to stderr are not processed by the pipe with tee. If you want to log errors (from stderr), use:

command 2>&1 | tee /path/to/logfile

This means: run command and redirect the stderr stream (2) to stdout (1). That will be passed to the pipe with the tee application.

Learn about this at askubuntu site

Can we tell tee to ignore `clear` and other control charater? Using any of these answer works great except if you do `cat /path/to/logfile` the clear command is reproduced. — Louis Loudog Trottier, Nov 07 '19 at 02:25

score 6 · Answer 3 · answered Jul 09 '10 at 19:09

6

Just use tail to watch the file as it's updated. Background your original process by adding & after your above command After you execute the command above just use

$ tail -f my.log

It will continuously update. (note it won't tell you when the file has finished running so you can output something to the log to tell you it finished. Ctrl-c to exit tail)

answered Jul 09 '10 at 19:09

Cfreak

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wow tough crowd on this one. What exactly is wrong with this answer? – Cfreak Jul 09 '10 at 19:19
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It's thousands of times worse than using `tee` in my opinion. – Kristopher Ives Jul 09 '10 at 21:16
Requires I run process in background – Kristopher Ives Aug 19 '12 at 19:10
Never knew about this Awesome tip! – Adam Feb 13 '13 at 01:00
1

I don't think this is really the answer I was looking for when I came here, but the knowledge of that command and its uses is useful. +1 – vmrob Jun 26 '14 at 19:24

score 4 · Answer 4 · answered Aug 15 '12 at 19:31

another option is to use block based output capture from within the script (not sure if that is the correct technical term).

Example

#!/bin/bash 
{
  echo "I will be sent to screen and file"
  ls ~
} 2>&1 | tee -a /tmp/logfile.log

echo "I will be sent to just terminal"

I like to have more control and flexibility - so I prefer this way.

How to log output in bash and see it in the terminal at the same time?

4 Answers4