Bash Shell Scripting/Intro Exercise

We will look at a few example to get a taste as to what Bash scripting can be used. Do not fret if you do not understand these just yet.

Hello World

Let's start with a simple "hello world" program:

echo 'Hello, world!'

We can either type this directly at the Bash prompt, or else save this as a file (say, hello_world.sh) and run it by typing bash hello_world.sh or ./hello_world.sh at the Bash prompt. (Later we will see some more sophisticated ways to create and run a Bash script file.) In either case, it will print Hello, world!:

$ echo 'Hello, world!'
Hello, world!

Here we have used the $ symbol to indicate the Bash prompt: after $, the rest of the line shows the command that we typed, and the following line shows the output of the command.

Check File

Here is a slightly more complex script:

if [[ -e readme.txt ]] ; then
  echo 'The file "readme.txt" exists.'
else
  echo 'The file "readme.txt" does not exist.'
fi

This script tests whether there exists a file named readme.txt in the current directory, and uses an if statement to control, based on that test, which commands are run. It, too, could be typed directly at the prompt — any script can — but in this case that is not likely to be useful.

Both of the above are entirely "within Bash", in that they do not require Bash to run any external programs. (The commands echo, if … then … else … fi, and [[ -e … ]] are all built-in commands, implemented by Bash itself.) But, being a shell-scripting language, a large part of Bash's purpose is to run external programs. The following script demonstrates this ability:

if [[ -e config.txt ]] ; then
  echo 'The file "config.txt" already exists. Comparing with default . . .'
  diff -u config-default.txt config.txt > config-diff.txt
  echo 'A diff has been written to "config-diff.txt".'
else
  echo 'The file "config.txt" does not exist. Copying default . . .'
  cp config-default.txt config.txt
  echo '. . . done.'
fi

Here diff and cp are two common utility programs that, while not part of Bash, are found on most systems that have Bash. The above script assumes the presence of a default configuration file named config-default.txt, and checks for the presence of a configuration file named config.txt. If config.txt exists, then the script uses the external program diff to produce a "diff" (a report of the differences between, in this case, two files), so that the user can see what non-default configurations are in place. If config.txt does not exist, then the script uses the external program cp ("copy") to copy the default configuration file to config.txt.

As you can see, the external programs are run using the same sort of syntax as the built-in commands; they're both just "commands".

The above version of this script is very "verbose", in that it generates a great deal of output. A more typical script would likely not include the echo commands, since users are unlikely to need this level of information. In that case, we might use the # notation to include comments that are completely ignored by Bash, and do not appear to the user. Such comments are simply informative notes for someone reading the script itself:

if [[ -e config.txt ]] ; then
  # if config.txt exists:
  diff -u config-default.txt config.txt > config-diff.txt # see what's changed
else
  # if config.txt does not exist:
  cp config-default.txt config.txt # take the default
fi

But the above is simply for demonstration's sake. In reality, a script this simple does not require any comments at all.