Classic Shell Scripting

Summary

The read command reads lines and splits the data into fields, for assigning to named shell variables. The -r option provides some control over how data is read.

I/O redirection allows you to change the source or destination of one program, or multiple programs running together in a subshell or code block. Besides redirecting to or from files, pipelines let you hook multiple programs together. Here documents provide inline input.

File descriptor manipulation, particularly of file descriptors 1 and 2, is a fundamental operation, used repeatedly in everyday scripting.

printf is a flexible, albeit somewhat complicated, command for generating output. Most of the time, it can be used in a simple manner, but its power is occasionally needed and valuable.

The shell performs a number of expansions (or substitutions) on the text of each command line: tilde expansion (if supported) and wildcards; variable expansion; arithmetic expansion; and command substitution. Wildcarding now includes POSIX character classes for locale-dependent matching of characters in filenames. By convention, "dot files" are not included in wildcard expansions. Variable and arithmetic expansion were described in Chapter 6. Command substitution has two forms: `...` is the original form, and $(...) is the newer, easier-to-write form.

Quoting protects different source-code elements from special treatment by the shell. Individual characters may be quoted by preceding them with a backslash. Single quotes protect all enclosed ...