shell - redirect COPY of stdout to log file from within bash script itself

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

#!/usr/bin/env bash

# Redirect stdout ( > ) into a named pipe ( >() ) running "tee"
exec > >(tee -i logfile.txt)

# Without this, only stdout would be captured - i.e. your
# log file would not contain any error messages.
# SEE (and upvote) the answer by Adam Spiers, which keeps STDERR
# as a separate stream - I did not want to steal from him by simply
# adding his answer to mine.
exec 2>&1

echo "foo"
echo "bar" >&2

Note that this is bash, not sh. If you invoke the script with sh myscript.sh, you will get an error along the lines of syntax error near unexpected token '>'.

If you are working with signal traps, you might want to use the tee -i option to avoid disruption of the output if a signal occurs. (Thanks to JamesThomasMoon1979 for the comment.)

---

Tools that change their output depending on whether they write to a pipe or a terminal (ls using colors and columnized output, for example) will detect the above construct as meaning that they output to a pipe.

There are options to enforce the colorizing / columnizing (e.g. ls -C --color=always). Note that this will result in the color codes being written to the logfile as well, making it less readable.