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).