bash: how to redirect stdout/stderr then later revert fd's?

Question

I want a script to redirect stdout and stderr to a file, do a bunch of stuff, then undo those redirections and take action on the file contents. I'm trying:

function redirect(){
   exec 3>&1
   exec 4>&2
   exec 1>outfile 2>&1
}
function undirect(){
   exec 1>&3
   exec 2>&4
}
echo first
redirect
echo something
cat kjkk
undirect
if some_predicate outfile; then echo ERROR; fi

Which seems to do what I want, but it seems rather complex. Is there a cleaner/clearer way to do this?

Schlueter

Answer 1

If you really need to switch it back and forth, without knowing beforehand what will go where and when, that's pretty much the way to do it. Depending on your requirements though it might be neater to isolate the parts which need redirecting, and execute them as group, like so:

echo first
{
  echo something
  cat kjkk
} 1>outfile 2>&1
if some_predicate outfile; then echo ERROR; fi

the {} is called a group command, and the output from the entire group is redirected. If you prefer, you can do your execs in a subshell, as it only affects the subshell.

echo first
(
  exec 1>outfile 2>&1

  echo something
  cat kjkk
)
if some_predicate outfile; then echo ERROR; fi

Note that I'm using parentheses () here, rather than braces {} (which were used in the first example).

HTH

Leopardi

Or even in the current shell: { echo something; cat afile; } >outfile 2>&1 – Nearsighted

@glenn; right. the first one could be like that, probably better too.. I'll edit – Leopardi

Answer 2

It seems pretty clean to me. The only thing I'd to is to pass the "outfile" name as a parameter to the function, since you use the filename outside of the function

redirect() {
    exec 3>&1
    exec 4>&2
    exec 1>"$1" 2>&1
}
:
redirect outfile
:
if some_predicate outfile; ...

Nearsighted