7.9. An Introduction to Functions

Although the Bourne shell does not have an alias mechanism for abbreviating commands, it does support functions (introduced to the shell in SVR2). Functions are used to execute a group of commands with a name . They are like scripts, only more efficient. Once defined, they become part of the shell's memory so that when the function is called, the shell does not have to read it in from the disk as it does with a file. Often functions are used to improve the modularity of a script (discussed in the programming section of this chapter). Once defined, they can be used again and again. Functions are often defined in the user 's initialization file, .profile . They must be defined before they are invoked, and cannot be exported.

7.9.1 Defining Functions

The function name is followed by a set of empty parentheses. The definition of the function consists of a set of commands separated by semicolons and enclosed in curly braces. The last command is terminated with a semicolon. Spaces around the curly braces are required.

FORMAT

 function_name () { commands ; commands; } 

Example 7.35.

 1   $  greet () { echo "Hello $LOGNAME, today is `date`; }  2   $  greet   Hello ellie, today is Thu Oct 21 19:56:31 PDT 2004  

EXPLANATION

1. The function is called greet .

2. When the greet function is invoked, the command(s) enclosed within the curly braces are executed.

Example 7.36.

 1   $  fun () { pwd; ls; date; }  2   $  fun   /home/jody/ellie/prac   abc      abc123   file1.bak   none       nothing   tmp   abc1     abc2     file2       nonsense   nowhere   touch   abc122   file1    file2.bak   noone      one   Sat Feb 21 11:15:48 PST 2004  3   $  welcome () { echo "Hi  and "; }  4   $  welcome tom joe   Hi tom and joe  5   $  set jane nina lizzy  6   $  echo $*   jane nina lizzy  7   $  welcome tom joe   hi tom and joe  8   $  echo   jane nina  

EXPLANATION

1. The function fun is named and defined. The name is followed by a list of commands enclosed in curly braces. Each command is separated by a semicolon. A space is required after the first curly brace or you will get a syntax error. A function must be defined before it can be used.

2. The function behaves just like a script when invoked. Each of the commands in the function definition are executed in turn .

3. There are two positional parameters used in the function welcome . When arguments are given to the function, the positional parameters are assigned those values.

4. The arguments to the function, tom and joe , are assigned to $1 and $2 , respectively. The positional parameters in a function are private to the function and will not interfere with any used outside the function.

5. The positional parameters are set at the command line. These variables have nothing to do with the ones set in the function.

6. $* displays the values of the currently set positional parameters.

7. The function welcome is called. The values assigned to the positional parameters are tom and joe .

8. The positional variables assigned at the command line are unaffected by those set in the function.

7.9.2 Listing and Unsetting Functions

To list functions and their definitions, use the set command. The function and its definition will appear in the output, along with the exported and local variables. Functions and their definitions are unset with the unset command.