On a cross-country plane trip, you sometimes reach an altitude from which you can see where you’ve been. This month’s column on the topic of loops is a high-altitude point in our exploration of the bash shell. Much of what has been explained so far is a prerequisite for understanding loops. Computers are powerful because they can quickly and accurately perform repetitive operations, and the loop is the shell construct that puts this power right into your hands.
On a cross-country plane trip, you sometimes reach an altitude from which you can see where you’ve been. This month’s column on the topic of loops is a high-altitude point in our exploration of the bash shell. Much of what has been explained so far is a prerequisite for understanding loops. Computers are powerful because they can quickly and accurately perform repetitive operations, and the loop is the shell construct that puts this power right into your hands.
The while Command
The shell provides three different loop commands:
while
until
for
The while command, the simplest of the three, has this form:
while test do body done
Notice that the command includes three keywords (while, do, and done) and two user-supplied components (test and body). The command is often written as shown, on four lines. However, the command can also be written on a single line by inserting semi-colons that enable the shell to parse it:
while test; do body; done
Figure One: The syntactic structure of the while command.
Computer scientists sometimes refer to keywords and punctuation as syntactic sugar. In this respect, the while command is indeed sugar-laden. Figure One presents the command’s syntax visually, using a simple railroad track…
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