Home Write to terminal after redirecting stdout to a file without using stderr?
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Write to terminal after redirecting stdout to a file without using stderr?

Dagg Nabbit
1#
Dagg Nabbit Published in 2012-02-23 00:00:27Z

I have two shell scripts, one that serves as the main "program" and another that serves as a "library."

In several places in the "program," I'll do something like: log "$thing" >> "$logfile", where log is a function defined in the "library."

# program.sh

logfile="log.txt"
stuff="hahah heheh hoho"

. library.sh 

for thing in $stuff; do
  log "$thing" >> "$logfile"
done

My question: Is there a way to redirect some of the output from the function back to the terminal without using stderr?

# library.sh

log () {

  # This gets written to the log
  echo "`date --rfc-3339=seconds`: $1"

  # How to write this to the terminal *without* using stderr?
  echo "Info: Message written to log." >&2

}

I want to avoid the use of stderr because in my actual program, there's an option to redirect errors to a file, but the messages I want to send to the terminal are informational, not errors, and should always show up on the terminal.

Ignacio Vazquez-Abrams
2#
Ignacio Vazquez-Abrams Reply to 2012-02-23 00:05:42Z

Open /dev/tty on another FD.

exec 0< /dev/null
exec 1> /dev/null
exec 2> /dev/null
exec 3> /dev/tty
echo 'Hello, World!' >&3 
sarnold
3#
sarnold Reply to 2012-02-23 00:19:52Z

You can write directly to /dev/tty each time you want to write to the terminal:

echo "hello world" > /dev/tty

For a small example:

$ cat writer.sh 
#!/bin/sh

echo "standard output"
echo "standard error" >&2

echo "direct to terminal" > /dev/tty
$ ./writer.sh > /tmp/out 2> /tmp/err
direct to terminal
$ cat /tmp/out
standard output
$ cat /tmp/err
standard error
$ 
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