The semicolon, back apostrophe, dollar, double dollar, and question mark are special characters that are used primarily in scripts to add comments, execute commands from within strings, introduce environment variables, reference the current Shell number, and accept input redirection.
Semicolons add comments to command lines. All characters to the right of a semicolon are ignored, allowing you to place descriptive comments on the same line with AmigaDOS commands. For example:
ASSIGN T: RAM: t ;set up T: directory for
;scripts
Comments can continue onto additional lines if they are too long to fit on one line. New lines must begin with a semicolon and should be indented to the same level as the previous comment for clarity.
Back apostrophes are used to execute commands from within a string. If a string containing a command enclosed in back apostrophe is printed, the enclosed command is executed. For example:
1> ECHO "The date and time are: `date`"
prints "The date and time are: " followed by the output of the DATE command. When a command such as DIR that produces multiple lines of output is embedded in an ECHO statement, the output is not properly formatted; all of it appears on one line.
The dollar sign is used in two ways: as an operator that introduces an environment variable (which also works outside of a script) and in a bracketed statement to separate a variable value from a default value.
For example, with an environment variable:
1> ECHO "Current Kickstart version: $Kickstart"
Current Kickstart version: 40.70
As a default separator in a script:
COPY foo.library TO <LIBS:$userlibdir>
You can change the character that provides this function from the dollar sign to something else with the .DOLLAR scripting keyword.
A bracketed double dollar sign (<$$>) substitutes the current process number. You can reference the current Shell number by the double dollar sign character string <$$> including brackets since it always returns the current process number as a string. When you create temporary files in a multitasking environment, it is essential for these files to have unique names so that processes do not interfere with each other. Adding the <$$> string to file names creates unique names for temporary files, logical assignments, and PIPEs. A .KEY statement is required in any script that uses <$$>. .KEY is described on page 5-7. To avoid conflict with the redirection arguments, <$$>'s angle brackets can be redefined using the .BRA and .KET commands. .BRA and .KET are described on page 5-8.
The question mark, when used as a separate argument in a command, instructs the command to accept input redirection.
