Lockable script

lockable_script_boilerplate.sh

#!/bin/bash
 
## Copyright (C) 2009  Przemyslaw Pawelczyk <[email protected]>
## License: GNU General Public License v2, v3
#
# Lockable script boilerplate
 
### HEADER ###
 
LOCKFILE="/var/lock/$(basename "$0")"

# PRIVATE
_lock()             {
  while flock -"$1" "$LOCKFD"; do
    INODE_FD="$(stat -Lc%i "/proc/$$/fd/$LOCKFD" 2>/dev/null)"
    INODE_FS="$(stat -c%i "$LOCKFILE" 2>/dev/null)"
    [ "$INODE_FD" = "$INODE_FS" ] && return 0 || exec {LOCKFD}>"$LOCKFILE"
  done
  return 1
}
_no_more_locking()  { _lock u; _lock xn && rm -f "$LOCKFILE"; }
_prepare_locking()  { exec {LOCKFD}>"$LOCKFILE"; trap _no_more_locking EXIT; }
 
# ON START
_prepare_locking
 
# PUBLIC
exlock_now()        { _lock xn; }  # obtain an exclusive lock immediately or fail
exlock()            { _lock x; }   # obtain an exclusive lock
shlock()            { _lock s; }   # obtain a shared lock
unlock()            { _lock u; }   # drop a lock
 
### BEGIN OF SCRIPT ###
 
# Simplest example is avoiding running multiple instances of script.
exlock_now || exit 1
 
# Remember! Lock file is removed when one of the scripts exits and it is
#           the only script holding the lock or lock is not acquired at all.