Shell – Is it possible to create sticky redirection from stdout/stderr

io-redirectionshell-scriptstderrstdout

Consider such code:

echo hello >> log.txt
echo world >> log.txt

I execute command with stdout redirection. Is it possible to create sticky redirection? Something like this:

redir stdout log.txt
echo hello
echo world
stop-redir

This would be equivalent to the first piece code.

What I really need — make such sticky redirection for stdout and stderr (at the same time), execute some commands, stop redirection, and read log file.

Best Answer

exec 3>&1 4>&2 >log.txt 2>&1
echo hello
echo world
exec 1>&3 2>&4
less log.txt

I.e.: make file descriptors 3 and 4 copies of the current 1 (stdout) and 2 (stderr), redirect 1 to log.txt and 2 to the same as 1; then perform commands; then restore stdout and stderr from the saved values in file descriptors 3 and 4.

Related Solutions

Bash – Use a Variable to Store stderr|stdout Redirection

Another solution could be the following:

#!/bin/bash

verbose=0

exec 3>&1
exec 4>&2

if ((verbose)); then
  echo "verbose=1"
else
  echo "verbose=0"
  exec 1>/dev/null
  exec 2>/dev/null
fi

echo "this should be seen if verbose"
echo "this should always be seen" 1>&3 2>&4

Then add 1>&3 2>&4 only to commands of which you want to see the output.

Bash – redirect and log script output

function startLogging {
    exec > >(gawk -v pid=$$ '{ print strftime("%F-%T"),pid,$0; fflush(); }' | tee -a $logfile)
    [ ! -z "$DEBUG" ] && exec 2>&1 || exec 2> >(gawk -v pid=$$ '{ print strftime("%F-%T"),pid,$0; fflush(); }' >>$logfile)
    echo "=== Log started for $$ at $(date +%F-%T) ==="
}

You need to have $logfile set to something

Best Answer

Related Solutions

Bash – Use a Variable to Store stderr|stdout Redirection

Bash – redirect and log script output

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