redirect COPY of stdout to log file from within ba

Question

I know how to redirect stdout to a file:

exec > foo.log
echo test

this will put the 'test' into the foo.log file.

Now I want to redirect the output into the log file AND keep it on stdout

i.e. it can be done trivially from outside the script:

script | tee foo.log

but I want to do declare it within the script itself

I tried

exec | tee foo.log

but it didn't work.

Answer 1

Easy way to make a bash script log to syslog. The script output is available both through /var/log/syslog and through stderr. syslog will add useful metadata, including timestamps.

Add this line at the top:

exec &> >(logger -t myscript -s)

Alternatively, send the log to a separate file:

exec &> >(ts |tee -a /tmp/myscript.output >&2 )

This requires moreutils (for the ts command, which adds timestamps).

Answer 2

Neither of these is a perfect solution, but here are a couple things you could try:

exec >foo.log
tail -f foo.log &
# rest of your script

or

PIPE=tmp.fifo
mkfifo $PIPE
exec >$PIPE
tee foo.log <$PIPE &
# rest of your script
rm $PIPE

The second one would leave a pipe file sitting around if something goes wrong with your script, which may or may not be a problem (i.e. maybe you could rm it in the parent shell afterwards).

Answer 3

The accepted answer does not preserve STDERR as a separate file descriptor. That means

./script.sh >/dev/null

will not output bar to the terminal, only to the logfile, and

./script.sh 2>/dev/null

will output both foo and bar to the terminal. Clearly that's not the behaviour a normal user is likely to expect. This can be fixed by using two separate tee processes both appending to the same log file:

#!/bin/bash

# See (and upvote) the comment by JamesThomasMoon1979 
# explaining the use of the -i option to tee.
exec >  >(tee -ia foo.log)
exec 2> >(tee -ia foo.log >&2)

echo "foo"
echo "bar" >&2

(Note that the above does not initially truncate the log file - if you want that behaviour you should add

>foo.log

to the top of the script.)

The POSIX.1-2008 specification of tee(1) requires that output is unbuffered, i.e. not even line-buffered, so in this case it is possible that STDOUT and STDERR could end up on the same line of foo.log; however that could also happen on the terminal, so the log file will be a faithful reflection of what could be seen on the terminal, if not an exact mirror of it. If you want the STDOUT lines cleanly separated from the STDERR lines, consider using two log files, possibly with date stamp prefixes on each line to allow chronological reassembly later on.