Bash script global redirection with exec and write to screen

Question

I have a large bash script and I want to globally redirect all stdout/stderr to a file, except some progress indicators such specific echo messages. For example, consider the following script

#!/bin/bash

# all stdout and stderr go to out.log
exec &> out.log

special_echo "starting"

# many other commands go here
# which output both the stdout and stderr
# also some special_echo commands in here,
# for example
echo "msg1"
special_echo "middle"
echo "msg2" >&2

special_echo "finished"

When it is run the output should be

$ ./script
starting
middle
finished
$

But, out.log should contain

msg1
msg2

How to implement special_echo functionality? I've tried using a file descriptor and echoing to that but can't get it to display on the screen.

Is there a way to achieve this without appending redirection to every line or doing something like in this answer?

score 12 · Accepted Answer · answered Aug 07 '13 at 10:43

12

Yes, using another file descriptor is the way to go:

#!/bin/bash

exec 3>&1
special_echo () {
    echo "$@" >&3
}

exec &> out.log

special_echo "starting"
echo "msg1"
special_echo "middle"
echo "msg2" >&2
special_echo "finished"

answered Aug 07 '13 at 10:43

choroba

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Works perfectly. I had `exec &> out.log` first and then `exec 3>&1` which now I see was the wrong order. – user2660278 Aug 07 '13 at 11:02

score 1 · Answer 2 · answered Aug 07 '13 at 10:49

1

Redirect to /dev/tty, which is the controlling terminal. Works also for input.

answered Aug 07 '13 at 10:49

Tassos Bassoukos

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Is it POSIX to redirect to /dev/tty? – smarber Dec 11 '17 at 09:59

score 1 · Answer 3 · answered Dec 17 '18 at 00:44

At the start of your script save your stdout & stderr as custom descriptors.

# all stdout and stderr go to out.log
exec 3>&1 4>&2 &>out.log

^{you can use 3>/dev/stdout instead of 3>&1 et cetera, or split it into multiple exec redirect lines, if that's more readable to you.}

Then all you need to do is use them

echo "starting" >&3
echo "msg1"
echo "middle"   >&3
echo "msg2" >&2
echo "finished" >&3

^{you cant use >/dev/stdout because it actually got overridden with exec, custom descriptors are necessary.}

The best part about this is that you still have access to the original stderr, which you wont with /dev/tty

echo "special error" >&4

And you can completely undo your redirection for the rest of the script if that's something you need.

#restore original
exec >&3 2>&4

echo "this is special now"

No functions, no variables, just works.

Bash script global redirection with exec and write to screen

3 Answers3