# Lock your script (against parallel execution)
## Why lock?
Sometimes there's a need to ensure only one copy of a script runs, i.e
prevent two or more copies running simultaneously. Imagine an important
cronjob doing something very important, which will fail or corrupt data
if two copies of the called program were to run at the same time. To
prevent this, a form of `MUTEX` (**mutual exclusion**) lock is needed.
The basic procedure is simple: The script checks if a specific condition
(locking) is present at startup, if yes, it's locked - the scipt
doesn't start.
This article describes locking with common UNIX(r) tools. There are
other special locking tools available, But they\'re not standardized, or
worse yet, you can't be sure they\'re present when you want to run your
scripts. **A tool designed for specifically for this purpose does the
job much better than general purpose code.**
### Other, special locking tools
As told above, a special tool for locking is the preferred solution.
Race conditions are avoided, as is the need to work around specific
limits.
- `flock`:
- `solo`:
## Choose the locking method
The best way to set a global lock condition is the UNIX(r) filesystem.
Variables aren't enough, as each process has its own private variable
space, but the filesystem is global to all processes (yes, I know about
chroots, namespaces, \... special case). You can \"set\" several things
in the filesystem that can be used as locking indicator:
- create files
- update file timestamps
- create directories
To create a file or set a file timestamp, usually the command touch is
used. The following problem is implied: A locking mechanism checks for
the existance of the lockfile, if no lockfile exists, it creates one and
continues. Those are **two separate steps**! That means it's **not an
atomic operation**. There's a small amount of time between checking and
creating, where another instance of the same script could perform
locking (because when it checked, the lockfile wasn't there)! In that
case you would have 2 instances of the script running, both thinking
they are succesfully locked, and can operate without colliding. Setting
the timestamp is similar: One step to check the timespamp, a second step
to set the timestamp.
[**Conclusion:**]{.underline} We need an
operation that does the check and the locking in one step.
A simple way to get that is to create a **lock directory** - with the
mkdir command. It will:
* create a given directory only if it does not exist, and set a successful exit code
* it will set an unsuccesful exit code if an error occours - for example, if the directory specified already exists
With mkdir it seems, we have our two steps in one simple operation. A
(very!) simple locking code might look like this:
``` bash
if mkdir /var/lock/mylock; then
echo "Locking succeeded" >&2
else
echo "Lock failed - exit" >&2
exit 1
fi
```
In case `mkdir` reports an error, the script will exit at this point -
**the MUTEX did its job!**
*If the directory is removed after setting a successful lock, while the
script is still running, the lock is lost. Doing chmod -w for the parent
directory containing the lock directory can be done, but it is not
atomic. Maybe a while loop checking continously for the existence of the
lock in the background and sending a signal such as USR1, if the
directory is not found, can be done. The signal would need to be
trapped. I am sure there there is a better solution than this
suggestion* --- *[sn18](sunny_delhi18@yahoo.com) 2009/12/19 08:24*
**Note:** While perusing the Internet, I found some people asking if the
`mkdir` method works \"on all filesystems\". Well, let's say it should.
The syscall under `mkdir` is guarenteed to work atomicly in all cases,
at least on Unices. Two examples of problems are NFS filesystems and
filesystems on cluster servers. With those two scenarios, dependencies
exist related to the mount options and implementation. However, I
successfully use this simple method on an Oracle OCFS2 filesystem in a
4-node cluster environment. So let's just say \"it should work under
normal conditions\".
Another atomic method is setting the `noclobber` shell option
(`set -C`). That will cause redirection to fail, if the file the
redirection points to already exists (using diverse `open()` methods).
Need to write a code example here.
``` bash
if ( set -o noclobber; echo "locked" > "$lockfile") 2> /dev/null; then
trap 'rm -f "$lockfile"; exit $?' INT TERM EXIT
echo "Locking succeeded" >&2
rm -f "$lockfile"
else
echo "Lock failed - exit" >&2
exit 1
fi
```
Another explanation of this basic pattern using `set -C` can be found
[here](http://pubs.opengroup.org/onlinepubs/9699919799/xrat/V4_xcu_chap02.html#tag_23_02_07).
## An example
This code was taken from a production grade script that controls PISG to
create statistical pages from my IRC logfiles. There are some
differences compared to the very simple example above:
- the locking stores the process ID of the locked instance
- if a lock fails, the script tries to find out if the locked instance
still is active (unreliable!)
- traps are created to automatically remove the lock when the script
terminates, or is killed
Details on how the script is killed aren't given, only code relevant to
the locking process is shown:
``` bash
#!/bin/bash
# lock dirs/files
LOCKDIR="/tmp/statsgen-lock"
PIDFILE="${LOCKDIR}/PID"
# exit codes and text
ENO_SUCCESS=0; ETXT[0]="ENO_SUCCESS"
ENO_GENERAL=1; ETXT[1]="ENO_GENERAL"
ENO_LOCKFAIL=2; ETXT[2]="ENO_LOCKFAIL"
ENO_RECVSIG=3; ETXT[3]="ENO_RECVSIG"
###
### start locking attempt
###
trap 'ECODE=$?; echo "[statsgen] Exit: ${ETXT[ECODE]}($ECODE)" >&2' 0
echo -n "[statsgen] Locking: " >&2
if mkdir "${LOCKDIR}" &>/dev/null; then
# lock succeeded, install signal handlers before storing the PID just in case
# storing the PID fails
trap 'ECODE=$?;
echo "[statsgen] Removing lock. Exit: ${ETXT[ECODE]}($ECODE)" >&2
rm -rf "${LOCKDIR}"' 0
echo "$$" >"${PIDFILE}"
# the following handler will exit the script upon receiving these signals
# the trap on "0" (EXIT) from above will be triggered by this trap's "exit" command!
trap 'echo "[statsgen] Killed by a signal." >&2
exit ${ENO_RECVSIG}' 1 2 3 15
echo "success, installed signal handlers"
else
# lock failed, check if the other PID is alive
OTHERPID="$(cat "${PIDFILE}")"
# if cat isn't able to read the file, another instance is probably
# about to remove the lock -- exit, we're *still* locked
# Thanks to Grzegorz Wierzowiecki for pointing out this race condition on
# http://wiki.grzegorz.wierzowiecki.pl/code:mutex-in-bash
if [ $? != 0 ]; then
echo "lock failed, PID ${OTHERPID} is active" >&2
exit ${ENO_LOCKFAIL}
fi
if ! kill -0 $OTHERPID &>/dev/null; then
# lock is stale, remove it and restart
echo "removing stale lock of nonexistant PID ${OTHERPID}" >&2
rm -rf "${LOCKDIR}"
echo "[statsgen] restarting myself" >&2
exec "$0" "$@"
else
# lock is valid and OTHERPID is active - exit, we're locked!
echo "lock failed, PID ${OTHERPID} is active" >&2
exit ${ENO_LOCKFAIL}
fi
fi
```
## Related links
- [BashFAQ/045](http://mywiki.wooledge.org/BashFAQ/045)
- [Implementation of a shell locking
utility](http://wiki.grzegorz.wierzowiecki.pl/code:mutex-in-bash)
- [Wikipedia article on File
Locking](http://en.wikipedia.org/wiki/File_locking), including a
discussion of potential
[problems](http://en.wikipedia.org/wiki/File_locking#Problems) with
flock and certain versions of NFS.