UNIX Shells by Example (4th Edition)

Shell scripts can take command-line arguments. Arguments are used to modify the behavior of the program in some way. The C shell assigns command-line arguments to positional parameters and enforces no specific limit on the number of arguments that can be assigned (the Bourne shell sets a limit of nine positional parameters). Positional parameters are number variables . The script name is assigned to $0 , and any words following the scriptname are assigned to $1 , $2 , $3 . . . ${10} , ${11} , and so on. $1 is the first command-line argument. In addition to using positional parameters, the C shell provides the argv built-in array.

10.5.1 Positional Parameters and argv

If using the argv array notation, a valid subscript must be provided to correspond to the argument being passed in from the command line or the error message Subscript out of range is sent by the C shell. The argv array does not include the script name. The first argument is $argv[1] , and the number of arguments is represented by $#argv . (There is no other way to represent the number of arguments.) See Table 10.4.

Example 10.10.

(The Script) #!/bin/csh f # The greetings script # This script greets a user whose name is typed in at the command line. 1 echo

(The Script) #!/bin/csh “f # The greetings script # This script greets a user whose name is typed in at the command line. 1 echo $0 to you $1 $2 $3 2 echo Welcome to this day `date awk '{print $1, $2, $3}'` 3 echo Hope you have a nice day, $argv[1] \! 4 echo Good “bye $argv[1] $argv[2] $argv[3] (The Command Line) % chmod +x greetings % greetings Guy Quigley 1 greetings to you Guy Quigley 2 Welcome to this day Fri Aug 28 3 Hope you have a nice day, Guy! 4 Subscript out of range to you 2 echo Welcome to this day `date awk '{print , , }'` 3 echo Hope you have a nice day, $argv[1] \! 4 echo Goodbye $argv[1] $argv[2] $argv[3] (The Command Line) % chmod +x greetings % greetings Guy Quigley 1 greetings to you Guy Quigley 2 Welcome to this day Fri Aug 28 3 Hope you have a nice day, Guy! 4 Subscript out of range

Table 10.4. Command-Line Arguments

EXPLANATION

1. The name of the script and the first three positional parameters are to be displayed. Because there are only two positional parameters coming in from the command line, Guy and Quigley , $1 becomes Guy , $2 becomes Quigley , and $3 is not defined.

2. The awk command is quoted with single quotes so that the shell does not confuse awk 's field numbers $1 , $2 , and $3 with positional parameters. (Do not confuse awk 's field designators $1 , $2 , and $3 with the shell's positional parameters.)

3. The argv array is assigned values coming in from the command line. Guy is assigned to argv[1] and its value is displayed. You can use the argv array to represent the command-line arguments within your script, or you can use positional parameters. The difference is that positional parameters do not produce an error if you reference one that has no value, whereas an unassigned argv value causes the script to exit with the Subscript out of range error message.

4. The shell prints the error Subscript out of range because there is no value for argv[3] .