Unix Power Tools, 3rd Edition

Chapter 36. Shell Programming for the Initiated

Beyond the Basics

This chapter has a bunch of tricks and techniques for programming with the Bourne shell. Some of them are documented but hard to find; others aren’t documented at all. Here is a summary of this chapter’s articles:

• The first group of articles is about making a file directly executable with #! on the first line. On many versions of Unix, an executable file can start with a first line like this:

  #!/path/to/interpreter

  The kernel will start the program named in that line and give it the file to read. Chris Torek’s Usenet classic, Section 36.2, explains how #! started. Section 36.3 explains that your “shell scripts” may not need a shell at all.

• The next bunch of articles are about processes and commands. The exec command, Section 36.5, replaces the shell with another process; it can also be used to change input/output redirection (see below). The : (colon) operator evaluates its arguments and returns a zero status — Section 36.6 explains why you should care.

• Next are techniques for handling variables and parameters. Parameter substitution, explained in Section 36.7, is a compact way to test, set, and give default values for variables. You can use the $0 parameter and Unix links to give the same script multiple names and make it do multiple things; see Section 36.8. Section 36.9 shows the easy way to get the last command-line argument. Section 36.10 has an easy way to remove all the command-line arguments.

• Four articles cover ...