Category: Script

Using lock files in a bash shell script

This post is old (2011), there are many better ways to do this.
See https://www.unix.com/man-page/linux/1/flock/ for one example.
Also pgrep and lsof examples here:
https://www.baeldung.com/linux/bash-ensure-instance-running

—

Wrote this script recently – I had written a simple shell script that updated an HTML page with its output, then realised it would be all too easy for simultaneous writes to clobber the file.

This kind of concurrency can & should really be solved properly by using a database obviously, but it got me thinking and playing around and I ended up with the below – it’s clearly very “happy path” with loads of room for improvements – please feel free to suggest some or add to it 🙂


#!/bin/bash

#
# Example script that uses lock files to avoid concurrent writes
# TODO: loads more validation and error handling!
#
# www.DonaldSimpson.co.uk
# 25th May 2011

setup_lockfile(){
# set name of this program’s lockfile:
MY_NAME=`basename $0`
LOCKFILE=/tmp/lock.${MY_NAME}.$$
# MAX_AGE is how long to wait until we assume a lock file is defunct
# scary stuff, with loads of scope for improvement…
# could use fuser and see if there is a process attached/not?
# maybe check with lsof? or just bail out?
MAX_AGE=5
echo “My lockfile name is ${LOCKFILE}”
sleep 1
}

check_lock(){
# Check for an existing lock file
while [ -f /tmp/lock.${MY_NAME}* ]
do
# A lock file is present
if [[ `find /tmp/lock.* -mmin +${MAX_AGE}` > “0” ]]; then
echo “WARNING: found and removing old lock file…`ls /tmp/lock.${MY_NAME}*`”
rm -f /tmp/lock.${MY_NAME}*
else
echo “A recent lock file already exists : `ls /tmp/lock.${MY_NAME}* | awk -F. {‘print $2″.”$3″, with PID: ” $4’}`”
echo “Will wait until the lock file is over ${MAX_AGE} minutes old then remove it…”
fi
sleep 5
done
}

create_lock(){
# ok to carry on… create a lock file – quickly 😉
touch ${LOCKFILE}
# check we managed to make it ok…
if [ ! -f ${LOCKFILE} ]; then
echo “Unable to create lockfile ${LOCKFILE}!”
exit 1
fi
echo “Created lockfile ${LOCKFILE}”
}

cleanup_lock(){
echo “Cleaning up… ”
rm -f ${LOCKFILE}
if [ -f ${LOCKFILE} ]; then
echo “Unable to delete lockfile ${LOCKFILE}!”
exit 1
fi
echo “Ok, lock file ${LOCKFILE} removed.”
}

setup_lockfile
check_lock
create_lock

# Any calls to exit() from here on should first call cleaup_lock
# Do main processing tasks here…
sleep 20


# All Done.
cleanup_lock

Serving WordPress as the default page

Here’s a note of what I needed to do in order to get WordPress serving as the default site on my domain – it was originally at www.donaldsimpson.co.uk/wordpress/ and I wanted it to just be www.donaldsimpson.co.uk

A bit of a Google shows there are many ways to do this, but here’s how I did it:

vi /opt/bitnami/apache2/conf/httpd.conf

then comment the current entry and add a new one pointing to the htdocs dir for WordPress:

#DocumentRoot “/opt/bitnami/apache2/htdocs”
DocumentRoot “/var/www/html/donwp.freemyip.com”

Then restart Apache (/opt/bitnami/apache2/bin/apachectl restart or similar) after which you just need to go to the WordPress Admin General Settings page and change these values to point to the root of your site/domain:

WodPress address (URL) www.donaldsimpson.co.uk

Site address (URL) www.donaldsimpson.co.uk

And that should be that – you can now delete that backup you made at the start…

 

Update:

It’s may be a good idea to define your WP_HOME and WP_SITEURL in your wp-config.php file too, like so:

define(‘WP_HOME’, ‘https://www.donaldsimpson.co.uk’);
define(‘WP_SITEURL’, ‘https://www.donaldsimpson.co.uk’);

This avoids a database lookup to get these details, which should speed things up fractionally too 🙂

 

 

Quick directory listing for large file systems

 

Useful bit of Perl code – folk at work found this useful approach on the web somewhere – it’s much quicker than doing a recursive find apparently:

 

my @dirlist = ();

sub process_files

{

my $path = shift;

opendir (DIR, $path) or die “Unable to open $path: $!”;

my @files =

map { $path . ‘\’ . $_ }

grep { !/^.{1,2}$/ }

readdir (DIR);

closedir (DIR);

for (@files)

{

if (-d $_))

{

print $_.”n”;

push @dirlist, $_;

push @files, process_files ($_);

}

}

}

process_files(“.”);


Pardon the indentation/formatting 😉

Cheers,

 

Don