The-Unix-Shell-Teaching-Notes

6.3 Variables in Shell Scripts

In the molecules directory, imagine you have a shell script called script.sh containing the following commands:

head -n $2 $1
tail -n $3 $1

While you are in the molecules directory, you type the following command:

$ bash script.sh '*.pdb' 1 1

Which of the following outputs would you expect to see?

  1. All of the lines between the first and the last lines of each file ending in .pdb in the molecules directory
  2. The first and the last line of each file ending in .pdb in the molecules directory
  3. The first and the last line of each file in the molecules directory
  4. An error because of the quotes around *.pdb
Solution The correct answer is 2. The special variables $1, $2 and $3 represent the command line arguments given to the script, such that the commands run are: 
$ head -n 1 cubane.pdb ethane.pdb octane.pdb pentane.pdb propane.pdb
$ tail -n 1 cubane.pdb ethane.pdb octane.pdb pentane.pdb propane.pdb
The shell does not expand '*.pdb' because it is enclosed by quote marks. As such, the first argument to the script is '*.pdb' which gets expanded within the script by head and tail.

Episode 6 Exercise 4