Returning value from called function in a shell script

A Bash function can't return a string directly like you want it to. You can do three things:

1. Echo a string
2. Return an exit status, which is a number, not a string
3. Share a variable

This is also true for some other shells.

Here's how to do each of those options:

1. Echo strings

lockdir="somedir"
testlock(){
    retval=""
    if mkdir "$lockdir"
    then # Directory did not exist, but it was created successfully
         echo >&2 "successfully acquired lock: $lockdir"
         retval="true"
    else
         echo >&2 "cannot acquire lock, giving up on $lockdir"
         retval="false"
    fi
    echo "$retval"
}

retval=$( testlock )
if [ "$retval" == "true" ]
then
     echo "directory not created"
else
     echo "directory already created"
fi

2. Return exit status

lockdir="somedir"
testlock(){
    if mkdir "$lockdir"
    then # Directory did not exist, but was created successfully
         echo >&2 "successfully acquired lock: $lockdir"
         retval=0
    else
         echo >&2 "cannot acquire lock, giving up on $lockdir"
         retval=1
    fi
    return "$retval"
}

testlock
retval=$?
if [ "$retval" == 0 ]
then
     echo "directory not created"
else
     echo "directory already created"
fi

3. Share variable

lockdir="somedir"
retval=-1
testlock(){
    if mkdir "$lockdir"
    then # Directory did not exist, but it was created successfully
         echo >&2 "successfully acquired lock: $lockdir"
         retval=0
    else
         echo >&2 "cannot acquire lock, giving up on $lockdir"
         retval=1
    fi
}

testlock
if [ "$retval" == 0 ]
then
     echo "directory not created"
else
     echo "directory already created"
fi

You are working way too hard. Your entire script should be:

if mkdir "$lockdir" 2> /dev/null; then 
  echo lock acquired
else
  echo could not acquire lock >&2
fi

but even that is probably too verbose. I would code it:

mkdir "$lockdir" || exit 1

but the resulting error message is a bit obscure.

If it's just a true/false test, have your function return 0 for success, and return 1 for failure. The test would then be:

if function_name; then
  do something
else
  error condition
fi

In case you have some parameters to pass to a function and want a value in return. Here I am passing "12345" as an argument to a function and after processing returning variable XYZ which will be assigned to VALUE

#!/bin/bash
getValue()
{
    ABC=$1
    XYZ="something"$ABC
    echo $XYZ
}


VALUE=$( getValue "12345" )
echo $VALUE

Output:

something12345

I think returning 0 for succ/1 for fail (glenn jackman) and olibre's clear and explanatory answer says it all; just to mention a kind of "combo" approach for cases where results are not binary and you'd prefer to set a variable rather than "echoing out" a result (for instance if your function is ALSO suppose to echo something, this approach will not work). What then? (below is Bourne Shell)

# Syntax _w (wrapReturn)
# arg1 : method to wrap
# arg2 : variable to set
_w(){
eval $1
read $2 <<EOF
$?
EOF
eval $2=\$$2
}

as in (yep, the example is somewhat silly, it's just an.. example)

getDay(){
  d=`date '+%d'`
  [ $d -gt 255 ] && echo "Oh no a return value is 0-255!" && BAIL=0 # this will of course never happen, it's just to clarify the nature of returns
  return $d
}

dayzToSalary(){
  daysLeft=0
  if [ $1 -lt 26 ]; then 
      daysLeft=`expr 25 - $1`
  else
     lastDayInMonth=`date -d "`date +%Y%m01` +1 month -1 day" +%d`
     rest=`expr $lastDayInMonth - 25`
     daysLeft=`expr 25 + $rest`
  fi
  echo "Mate, it's another $daysLeft days.."
}

# main
_w getDay DAY # call getDay, save the result in the DAY variable
dayzToSalary $DAY