Bash : How to store STDOUT (&1) pointer in a variable

Question

Here is a simple (and maybe dummy) question in an as-simple-as-the-question scenario.

I want to store my output file in a variable :

#!/bin/bash
LOG=/var/log/myown/backup.log

So I can build my script this way:

#!/bin/bash
LOG=/var/log/myown/backup.log
echo "Backup créé le $(date)" >>$LOG

Also, I'd like to make this possible for the LOG variable to point to STDOUT.

Then I could simply change the value of LOG as in LOG=&1 so it does not writeout to a file but to STDOUT.

Answer 1

Is this what you need?

LOG=/dev/stdout

Or perhaps:

LOG=/var/log/myown/backup.log
exec 4> "$LOG"  ## exec >>4 "$LOG" to append
echo "Backup créé le $(date)" >&4

Or finally:

LOG=/var/log/myown/backup.log
exec 4> "$LOG"  ## exec >>4 "$LOG" to append
LOG_FD=4  ## Change to 1 to put back to stdout.
echo "Backup créé le $(date)" >&"$LOG_FD"

You can also use a function for convenience:

function log {
    echo "$1" >&"$LOG_FD"
}
log "Backup créé le $(date)"

Answer 2

In modern (4.1+) versions of bash, you can use expansions to refer to file descriptors:

log_fd=1                # hardcode FD 1 as output source
echo "Hello" >&$log_fd  # ...and use that value

This is extra handy with automatic file descriptor allocation, also added in 4.1:

exec {log_fd}>log.txt   # open log.txt on an automatically-assigned free FD
                        # ...and save that FD number in log_fd
echo "Hello" >&$log_fd  # write to log.txt on that FD

You can't put the & itself into the variable -- which is actually better design; otherwise, you couldn't redirect to a filename starting with a literal &, and any program wherein a user could specify a filename would allow the user to target stdout/stderr/etc -- but that's no loss of flexibility, since you can open any filename on any FD using exec in its redirection-enacting usage mode.