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
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
flock -x "$lock_fd"
exec $lock_fd>&-
...now, for your function, we're going to need associative arrays and automatic FD allocation (and, to allow the same file to be locked and unlocked from different paths, GNU readlink) -- so this won't work with older bash releases:
declare -A lock_fds=() # store FDs in an associative array
getLock() {
local file=$(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).