The shell prompt changes from $ to > and back again as we were
typing in our loop. The second prompt, >, is different to remind
us that we haven’t finished typing a complete command yet. A semicolon, ;,
can be used to separate two commands written on a single line.
When the shell sees the keyword for,
it knows to repeat a command (or group of commands) once for each item in a list.
Each time the loop runs (called an iteration), an item in the list is assigned in sequence to
the variable, and the commands inside the loop are executed, before moving on to
the next item in the list. Inside the loop, we call for the variable’s value by putting $
in front of it. The $ tells the shell interpreter to treat the variable as a variable
name and substitute its value in its place, rather than treat it as text or an external command.
In this example, the list is three filenames: basilisk.dat, minotaur.dat, and unicorn.dat.
Each time the loop iterates, it will assign a file name to the variable filename
and run the head command. The first time through the loop, $filename is basilisk.dat.
The interpreter runs the command head on basilisk.dat and pipes the first two lines to
the tail command, which then prints the second line of basilisk.dat. For the second iteration, $filename becomes minotaur.dat. This time, the shell runs head on minotaur.dat
and pipes the first two lines to the tail command, which then prints the second line of minotaur.dat.
For the third iteration, $filename becomes unicorn.dat, so the shell runs the head command
on that file, and tail on the output of that. Since the list was only three items, the shell exits the for loop.
Here we see > being used as a shell prompt, whereas > is also
used to redirect output.
Similarly, $ is used as a shell prompt, but, as we saw earlier,
it is also used to ask the shell to get the value of a variable.
If the shell prints > or $ then it expects you to type something,
and the symbol is a prompt.
If you type > or $ yourself, it is an instruction from you that
the shell should redirect output or get the value of a variable.
When using variables it is also possible to put the names into curly braces to clearly
delimit the variable name: $filename is equivalent to ${filename}, but is different from
${file}name. You may find this notation in other people’s programs.
We have called the variable in this loop filename in order to make its purpose clearer
to human readers. The shell itself doesn’t care what the variable is called;
if we wrote this loop as:
$ for x in basilisk.dat minotaur.dat unicorn.dat
> do
> head -n 2 $x | tail -n 1
> done
or:
$ for temperature in basilisk.dat minotaur.dat unicorn.dat
> do
> head -n 2 $temperature | tail -n 1
> done
it would work exactly the same way.
Don’t do this!
Programs are only useful if people can understand them,
so meaningless names (like x) or misleading names (like temperature)
increase the odds that the program won’t do what its readers think it does.