Now that we know a few basic commands,
we can finally look at the shell’s most powerful feature:
the ease with which it lets us combine existing programs in new ways.
We’ll start with the directory called shell-lesson-data/molecules
that contains six files describing some simple organic molecules.
The .pdb extension indicates that these files are in Protein Data Bank format,
a simple text format that specifies the type and position of each atom in the molecule.
$ cd ~/shell-lesson-data
$ ls molecules
cubane.pdb ethane.pdb methane.pdb
octane.pdb pentane.pdb propane.pdb
Let’s go into that directory with cd and run an example command wc -l instead of just wc,
the output shows only the number of lines per file:
$ cd molecules
$ wc -l *.pdb
20 cubane.pdb
12 ethane.pdb
9 methane.pdb
30 octane.pdb
21 pentane.pdb
15 propane.pdb
107 total
The -m and -w options can also be used with the wc command, to show
only the number of characters or the number of words in the files.
What happens if a command is supposed to process a file, but we don’t give it a filename? For example, what if we type:
$ wc -l
but don’t type *.pdb (or anything else) after the command?
Since it doesn’t have any filenames, wc assumes it is supposed to
process input given at the command prompt, so it just sits there and waits for us to give
it some data interactively. From the outside, though, all we see is it
sitting there: the command doesn’t appear to do anything.
Hint: Press Ctrl+C to cancel the executable when the code hangs… If you make this kind of mistake, you can escape out of this state by holding down the control key (Ctrl) and typing the letter C once and letting go of the Ctrl key. Ctrl+C