Anyone Can Program the Shell (Unix Power Tools, 3rd Edition)

1.8. Anyone Can Program the Shell

One of the really wonderful things about the shell is that it doesn't just read and execute the commands you type at a prompt. The shell is a complete programming language.

The ease of shell programming is one of the real highlights of Unix for novices. A shell program need be no more than a single complex command line saved in a file -- or a series of commands.

For example, let's say that you occasionally need to convert a Macintosh Microsoft Word file for use on your Unix system. Word lets you save the file in ASCII format. But there's a catch: the Mac uses a carriage return ASCII character 015 to mark the end of each line, while Unix uses a linefeed (ASCII 012). As a result, with Unix, the file looks like one long paragraph, with no end in sight.

That's easy to fix: the Unix tr (Section 21.11) command can convert every occurrence of one character in a file to another:

bash-2.04$ tr '\015' '\012' < file.mac > file.unix

But you're a novice, and you don't want to remember this particular piece of magic. Fine. Save the first part of this command line in a file called mac2unix in your personal bin directory (Section 7.4):

tr '\015' '\012'

Make the file executable with chmod (Section 50.5):

bash-2.04$ chmod +x mac2unix

Now you can say:

bash-2.04$ mac2unix < file.mac > file.unix

But why settle for that? What if you want to convert a bunch of files at once? Easy. The shell includes a general way of referring to arguments passed to a script and a number of looping constructs. The script:

for Section 35.21, $x Section 35.9

for x
do
    echo "Converting $x"
    tr '\015' '\012' < "$x" > "tmp.$x"
    mv "tmp.$x" "$x"
done

will convert any number of files with one command, replacing each original with the converted version:

bash-2.04$ mac2unix file1 file2 file3 ...

As you become more familiar with Unix, it quickly becomes apparent that doing just a little homework can save hours of tedium. This script incorporates only two simple programming constructs: the for loop and variable substitution (Section 35.9, Section 35.3).[1] As a new user with no programming experience, I learned these two constructs by example: I saved a skeleton for loop in a file and simply filled in the blanks with whatever commands I wanted to repeat. Section 35.2 has more about shell programming.

[1][Tim is keeping this article simple, as an illustration of how easy writing a shell program can be. If you're writing this little script for general use, you can make it work like a filter (Section 1.5) by adding four or five more lines of code: a case (Section 35.10) or if (Section 35.13) statement that tests the number of command-line arguments. With no filename arguments, the script would simply run tr '\015' '\012'. -- JP]

In short, Unix is sometimes difficult because it is so rich and complex. The user who doesn't want to learn the complexity doesn't have to -- the basic housekeeping commands are simple and straightforward. But the user who wants to take the time to investigate the possibilities can uncover a wealth of useful tools.

-- TOR