If you haven’t read my article on Automated Installer for Nginx, you should (not to toot my own horn). It is a script containing around 200 lines of code, including the templates. I wrote it in about 2 hours, mostly, because I had a great environment for testing. I’m going to share my story and my clean up script, so that hopefully, it helps you while you write your scripts.
Test Often
I find BASH’s syntax to be aggravating, to say the least. However, there are a few ways to make life a bit easier. The most obvious is to use an editor that supports BASH syntax, so it can highlight as you write your code. I prefer nano. It helps point out things like unterminated quotation marks.
Once you a few lines written, you should test for syntatical errors. It is very simple! Just type:
bash -n scriptname.sh
Wasn’t that easy? I suggest doing it for every 10 lines or so. I’ve been writing BASH scripts for a couple years and still find myself making minor errors here and there. It is much easier to catch them as you write, rather than at the end. I’ve certainly had days where I spent over an hour hunting a minor syntactical error.
Exit on Error
If you have a long script, where each part relies on the previous part working, there is no reason not to add “set -e” at the top. This will exit if any statement returns anything other than 0 (BASH for completed successfully or true). Basically, if a single command has an error, it will quit the whole script, rather than leaving you with a half working system. This is especially important for automation scripts, where a broken script can break the whole system. Another (possibly better) option is to leave off “set -e”, and do sanity checking after each procedure. For example, if you create a configuration file for a daemon, then restart the daemon, you could kill the daemon. In this instance, it is probably better to revert to the previous configuration file, and re-restart the daemon, so it is still running. I’d almost require this for a production script. An example of “set -e” would be:
#!/bin/bash set -e ....
Check All Variables Are Set
Much like “set -e”, “set -u”, or “no unset” is a great thing to prevent your scripts from exiting for silly reasons. This simply checks to make sure all variables are set before they are used, and exits if they aren’t. It sometimes is quite hard to debug a problem where one variable just shows up as a 0 length string. This works great for production scripts, and I’d say it is almost required. Here’s how you do it!
#!/bin/bash set -eu ....
Know Which Commands Are Being Executed
A great way to flip on debugging and see exactly which command is throwing that annoying error. If you add “set -v”, it will print out the command it is executing right before it executes. This is handy for shorter scripts and longer scripts alike, however, longer scripts can get a bit tiresome. Obviously, don’t leave this on for production! Let’s see all three together:
#!/bin/bash set -euv ....
Cleanup Scripts Save Time and Headaches
If your script is the kind that builds a lot of configuration files or does stuff with databases, or anything like that, a clean up script will prevent you from boring yourself to death every time you script does something other than expected. Basically, you want to undo what you just did. Delete new files, reload daemons again, drop created database users, etc. Here’s an example I used for the nginx script:
#!/bin/bash # We won't use set -eu, because this is likely to error our if the error in the script happened early. # Remove created files rm -rf /ebs/www/$1/ rm /etc/php5/fpm/pools/$2 rm /etc/nginx/sites-available/$2 rm /etc/nginx/sites-enabled/$2 # Reload daemons so deletions take effect /etc/init.d/nginx reload /etc/init.d/php5-fpm reload # Remove created user deluser $1 # Remove created MySQL database and user echo "drop database $1; drop user $1@localhost;" > clear.sql mysql -u root -pPASSWORD < clear.sql rm clear.sql
Just like the script, this takes a username and a domain name for its arguments. Similar scripts will make your life much, much easier.
Have any good BASH script tutorials? Post them in the comments!
About Josh Gachnang
