Redirect output to less from inside script

Question

I can redirect the output of a script to a logfile from inside the script with exec:

#!/bin/bash
exec > stdout.log 2>&1
echo hello world

Is it possible to redirect the output to less instead of a file? I tried

#!/bin/bash
exec > >(less) 2>&1

# output some text
for (( i=1; i <= 500; i++ )); do echo "hello world $i"; done

but this fails in a strange way ... the less prompt is visible but you end back at the terminal.

I'd like to set this up a the start of the script (so it can be conditional, depending on arguments, tty, etc.).

Answer 1

You should have your script wait for the less child process, otherwise your script will terminate before it, and less will suddenly find itself outside the foreground process group, not able to read commands from the terminal or restore the terminal settings anymore.

Also, in order to prevent less for waiting forever for the end of its input, your script should close the pipe to it.

Putting all that together:

exec > >(less) 2>&1
trap 'exec >&- 2>&-; wait' EXIT
# >&- 2>&- => close stdout and stderr => cause EOF on less' stdin

seq 1 50000
# the rest of your script

But this is not very nice, not portable to most other shells, and relying on undocumented (and unreliable) behaviour of bash: the wait won't work fine if you have more than one exec > >(...) in your script, and it will also wait on other background processes started with &.

---

A better idea would be to have your script call itself, using an environment variable to avoid infinite recursion:

if [ ! "$CALLED_MYSELF" ]; then
    set -o pipefail # supported in bash, but not in all the shells  
    CALLED_MYSELF=1 "$0" "$@" 2>&1 | less
    exit
fi

seq 1 50000
# the rest of your script

Answer 2

Try just this (without the exec line):

for (( i=1; i <= 500; i++ )); do echo "hello world $i"; done | less

(update)

If you want to include the whole script, you can wrap the script with { ... }

#!/bin/bash
{
# output some text
for (( i=1; i <= 500; i++)) do echo "hello world $i"; done

# whatever output you want...

} | less