Config files for your script

For this task, you don't have to write fat parser routines (unless you want it 100% secure or you want a special file syntax) - you can use Bash's source command. It doesn't execute a script (as in execute a command), but it includes the file contents of the sourced file into the current shell, just as if they were present directly:

#!/bin/bash
echo "Reading config...." >&2
source /etc/cool.cfg
echo "Config for the username: $cool_username" >&2
echo "Config for the target host: $cool_host" >&2

So, where do these variables come from? If everything works fine, they are defined in /etc/cool.cfg which is a file that's sourced into the current script or shell. Note that this is not the same as executing this file as a script! The sourced file most likely contains something like:

cool_username="guest"
cool_host="foo.example.com"

These are normal statements understood by Bash, nothing special to do. Of course (and that is a big disadvantage under normal circumstances) the sourced file can contain everything that Bash understands, including evil code!

The source command also is available under the name . (dot). The usage of the dot is identical:

#!/bin/bash
echo "Reading config...." >&2
. /etc/cool.cfg
echo "Config for the username: $cool_username" >&2
echo "Config for the target host: $cool_host" >&2

There's also a way to provide a system-wide config file in /etc and a custom one in ~/ (user's home) to override some system-wide defaults. The user-specific config will only be used when present, in the following example:

#!/bin/bash
echo "Reading system-wide config...." >&2
. /etc/cool.cfg
if [ -r ~/.coolrc ]; then
  echo "Reading user config...." >&2
  . ~/.coolrc
fi

As mentioned earlier, the sourced file can contain everything, it's basically an included Bash script. That raises security issues, like the configuring person is able to “execute” arbitrary code when your script is sourcing its config file. You might want to only allow constructs in the form NAME=VALUE in that file (variable assignment syntax) and maybe comments (though, comments are technically unimportant, of course). Imagine the following “config file”, containing some “evil” code:

# cool config file for my even cooler script (eh?)
username=god_only_knows
hostname=www.example.com
# hey look, weird code follows...
echo "I am the skull virus..."
echo rm -fr ~/*
mailto=netadmin@example.com

I guess you don't want these echo-commands (which could be any other commands!) to be executed. One way to be a bit safer is to filter only the constructs you want, write the filtered results to a new file and source the new file. Egrep (grep -E) will help us here, it filters by description of how a line should look:

#!/bin/bash
configfile='/etc/cool.cfg'
configfile_secured='/tmp/cool.cfg'

# check if the file contains something we don't want
if egrep -q -v '^#|^[^ ]*=' "$configfile"; then
  echo "Config file is unclean, cleaning it..." >&2
  # filter the original to a new file
  egrep '^#|^[^ ]*='  "$configfile" > "$configfile_secured"
  configfile="$configfile_secured"
fi

# now source it, either the original or the filtered variant
source "$configfile"

To make clear what it does: It checks if the file contains something we don't want, if yes, it filters it and writes the filtered contents to a new file. If done, it changes the name stored in the variable configfile from the original name to the name of the secured file. Then, as usual, it sources the file named by that variable, if it is the original one or the secured one.

This filter only allows NAME=VALUE and comments in the file, though it doesn't prevent all methods of executing code. I will address that later.