Linux flock, how to "just" lock a file?

Question

In Bash, I'm trying to make a function getLock to be used with different lock names.

function getLock
{
    getLock_FILE="${1}"
    getLock_OP="${2}"
    case "${getLock_OP}" in
        "LOCK_UN")
            flock -u "${getLock_FILE}"
            rm -fr "${getLock_FILE}"
            ;;
        "LOCK_EX")
            flock -x "${getLock_FILE}"
    esac
}

But flock says flock: bad number: myfilelock

How can I just lock a file, and release it when I want, without having to execute a command in the flock?

It is to be used like this:

getLock myfilelock LOCK_EX
somecommands
........
getLock myfilelock LOCK_UN

Answer 1

To lock the file:

exec 3>filename # open a file handle; this part will always succeed
flock -x 3      # lock the file handle; this part will block

To release the lock:

exec 3>&-       # close the file handle

You can also do it the way the flock man page describes:

{
  flock -x 3
  ...other stuff here...
} 3>filename

...in which case the file is automatically closed when the block exits. (A subshell can also be used here, via using ( ) rather than { }, but this should be a deliberate decision -- as subshells have a performance penalty, and scope variable modifications and other state changes to themselves).

---

If you're running a new enough version of bash, you don't need to manage file descriptor numbers by hand:

# this requires a very new bash -- 4.2 or so.
exec {lock_fd}>filename  # open filename, store FD number in lock_fd
flock -x "$lock_fd"      # pass that FD number to flock
exec $lock_fd>&-         # later: release the lock

---

...now, for your function, we're going to need associative arrays and automatic FD allocation (and, to allow the same file to be locked and unlocked from different paths, GNU readlink) -- so this won't work with older bash releases:

declare -A lock_fds=()                        # store FDs in an associative array
getLock() {
  local file=$(readlink -f "$1")              # declare locals; canonicalize name
  local op=$2
  case $op in
    LOCK_UN)
      [[ ${lock_fds[$file]} ]] || return      # if not locked, do nothing
      exec ${lock_fds[$file]}>&-              # close the FD, releasing the lock
      unset lock_fds[$file]                   # ...and clear the map entry.
      ;;
    LOCK_EX)
      [[ ${lock_fds[$file]} ]] && return      # if already locked, do nothing
      local new_lock_fd                       # don't leak this variable
      exec {new_lock_fd}>"$file"              # open the file...
      flock -x "$new_lock_fd"                 # ...lock the fd...
      lock_fds[$file]=$new_lock_fd            # ...and store the locked FD.
      ;;
  esac
}

If you're on a platform where GNU readlink is unavailable, I'd suggest replacing the readlink -f call with realpath from sh-realpath by Michael Kropat (relying only on widely-available readlink functionality, not GNU extensions).