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.

Solution 1

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.

Solution 2

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)

Solution 3

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

Solution 4

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.

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