BashFAQ/045/contrib

This example was contributed by Charles Duffy (who is now more than a little embarassed by same, and suggests using flock instead). It has been separated from the parent page because the code has several issues that make it dubious.

• Are we sure this code's correct? There seems to be a discrepancy between the names LOCK_DEFAULT_NAME and DEFAULT_NAME; and it checks for processes in what looks to be a race condition; and it uses the Linux-specific /proc file system and the GNU-specific egrep -o to do so.... I don't trust it. It looks overly complex and fragile. And quite non-portable. -- GreyCat

   LOCK_DEFAULT_NAME=$0
   LOCK_HOSTNAME="$(hostname -f)"
  
   ## function to take the lock if free; will fail otherwise
   function grab-lock {
     local PROGRAMNAME="${1:-$LOCK_DEFAULT_NAME}"
     local PID=${2:-$$}
     (
       umask 000;
       mkdir -p "/tmp/${PROGRAMNAME}-lock"
       mkdir "/tmp/${PROGRAMNAME}-lock/held" || return 1
       mkdir "/tmp/${PROGRAMNAME}-lock/held/${LOCK_HOSTNAME}--pid-${PID}" && return 0 || return 1
     ) 2>/dev/null
     return $?
   }
  
   ## function to nicely let go of the lock
   function release-lock {
     local PROGRAMNAME="${1:-$LOCK_DEFAULT_NAME}"
     local PID=${2:-$$}
     (
       rmdir "/tmp/${PROGRAMNAME}-lock/held/${LOCK_HOSTNAME}--pid-${PID}" || true
       rmdir "/tmp/${PROGRAMNAME}-lock/held" && return 0 || return 1
     ) 2>/dev/null
     return $?
   }
  
   ## function to force anyone else off of the lock
   function break-lock {
     local PROGRAMNAME="${1:-$LOCK_DEFAULT_NAME}"
     (
       [ -d "/tmp/${PROGRAMNAME}-lock/held" ] || return 0
       for DIR in "/tmp/${PROGRAMNAME}-lock/held/${LOCK_HOSTNAME}--pid-"* ; do
         OTHERPID="$(egrep -o '[0-9]+$' <<<"$DIR")"
         [ -d "/proc/${OTHERPID}" ] || rmdir "$DIR"
       done
       rmdir "/tmp/${PROGRAMNAME}-lock/held" && return 0 || return 1
     ) 2>/dev/null
     return $?
   }
  
   ## function to take the lock nicely, freeing it first if needed
   function get-lock {
     break-lock "$@" && grab-lock "$@"
   }