Classic Shell Scripting

Summary

Variables are necessary for any serious programming. Shell variables hold string values, and a large array of operators for use in ${ var...} lets you control the results of variable substitution.

The shell provides a number of special variables (those with nonalphanumeric names, such as $? and $!), that give you access to special information, such as command exit status. The shell also has a number of special variables with predefined meanings, such as PS1, the primary prompt string. The positional parameters and special variables $* and $@ give you access to the arguments used when a script (or function) was invoked. env, export, and readonly give you control over the environment.

Arithmetic expansion with $((...)) provides full arithmetic capabilities, using the same operators and precedence as in C.

A program's exit status is a small integer number that is made available to the invoker when the program is done. Shell scripts use the exit command for this, and shell functions use the return command. A shell script can get the exit status of the last command executed in the special variable $?.

The exit status is used for control-flow with the if, while, and until statements, and the !, && and || operators.

The test command, and its alias [...], test file attributes and string and numeric values, and are useful in if, while, and until statements.

The for loop provides a mechanism for looping over a supplied set of values, be they strings, filenames, or whatever else. while