What if we want to select lines from an arbitrary file?
We could edit middle.sh each time to change the filename,
but that would probably take longer than typing the command out again
in the shell and executing it with a new file name.
Instead, let’s edit middle.sh and make it more versatile:
$ nano middle.sh
Now, within “nano”, replace the text octane.pdb with the special variable called $1:
head -n 15 "$1" | tail -n 5
Inside a shell script,
$1 means ’the first filename (or other argument) on the command line’.
We can now run our script like this:
$ bash middle.sh octane.pdb
ATOM 9 H 1 -4.502 0.681 0.785 1.00 0.00
ATOM 10 H 1 -5.254 -0.243 -0.537 1.00 0.00
ATOM 11 H 1 -4.357 1.252 -0.895 1.00 0.00
ATOM 12 H 1 -3.009 -0.741 -1.467 1.00 0.00
ATOM 13 H 1 -3.172 -1.337 0.206 1.00 0.00
or on a different file like this:
$ bash middle.sh pentane.pdb
ATOM 9 H 1 1.324 0.350 -1.332 1.00 0.00
ATOM 10 H 1 1.271 1.378 0.122 1.00 0.00
ATOM 11 H 1 -0.074 -0.384 1.288 1.00 0.00
ATOM 12 H 1 -0.048 -1.362 -0.205 1.00 0.00
ATOM 13 H 1 -1.183 0.500 -1.412 1.00 0.00
For the same reason that we put the loop variable inside double-quotes,
in case the filename happens to contain any spaces,
we surround $1 with double-quotes.
Currently, we need to edit middle.sh each time we want to adjust the range of
lines that is returned.
Let’s fix that by configuring our script to instead use three command-line arguments.
After the first command-line argument ($1), each additional argument that we
provide will be accessible via the special variables $1, $2, $3,
which refer to the first, second, third command-line arguments, respectively.
Knowing this, we can use additional arguments to define the range of lines to
be passed to head and tail respectively:
$ nano middle.sh
head -n "$2" "$1" | tail -n "$3"