Return a single value from a shell script function

Question

Example:

#!/bin/sh

a() {
R=f
ls -1 a*
[ "$?" == "1" ] && { R=t; }
echo $R
}

r=`a`
echo $r

$r contains t or f but also the output of the ls command.

I may write ls -1 a* >/dev/null 2>/dev/null, but if there is a more complex script that can be lead to errors.

Is there any way to return a single value from a() ?

Answer 1

A shell function can return a numeric value. Consider 0 and 1 rather than 'f' and 't'

#!/bin/sh

a() {
R=0
ls -1 a*
[ "$?" == "1" ] && { R=1; }
return $R
}

a
r=$?
echo $r

This will still write output from the ls -1 a* which you probably still want to dispose of, but the value of r will be either 0 or 1 and won't include the output.

The other examples of redirecting output either from a line or a whole block are good, and, as others suggested, you should get to know other ways to test for conditions (but I was assuming the ls was kind of an arbitrary example)

Answer 2

you don't have to use ls to check for files starting with a. Just use the shell

a() {
  shopt -s nullglob
  ret="f"
  for file in a*
  do   
    ret="t"
    break
  done
  echo "$ret"
}

Answer 3

You can put a redirection on a list of commands:

{
  command1
  command2
} >/dev/null

If at some point in the script you don't want any output from subsequent commands, you can redirect the shell's output with the exec builtin:

echo interesting
exec >/dev/null
echo boring

Note that this lasts until the end of the script, not just until the end of a function. This takes care of commands after the interesting one but not before.

There is a way to revert the effect of exec /dev/null, by using file descriptor manipulations. I don't necessarily recommend it though, because it can be tricky to work out in practice. The idea is to relocate whatever is connected to the standard output to a different descriptor, then redirect standard output to a different file, and finally relocate the original standard output back to standard output.

{
  exec 3>&1         # duplicate fd 3 to fd 1 (standard output)
  exec >/dev/null   # connect standard output to /dev/null
  echo boring
  exec 1>&3         # connect standard output back to what was saved in fd 3
  echo interesting
  exec >/dev/null   # connect standard output to /dev/null again
  echo more boring
} 3>/dev/null       # The braced list must have its fd 3 connected somewhere,
                    # even though nothing will actually be written to it.

Answer 4

a() { 
ls -1 a*  > /dev/null
[ "$?" == "0" ] && echo t  || echo f

} 

r=`a` 
echo $r 

Consider using [ -f filename ] and other file tests as well.