System Debug Reference Manual > Chapter 2 User Interface

Command Line Substitutions

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Command line scanning proceeds from left to right and is done in two phases. The first preprocessing phase scans a command line for the vertical bar character (|), which introduces the following syntax:


   |expression[:fmtspec][~]

When the command preprocessor recognizes the above syntax, it removes all the characters associated with it from the command line and replaces them with text representing the value of the expression. The expression part of the substitution syntax may be any valid expression as previously described in this manual. In particular, there are no special restrictions placed on command line substitution expressions.

The optional :fmtspec represents special formatting directives that may be used to control the formatting of the value of the expression when it is converted to characters and inserted back into the command line. Fmtspec is always specified as a string literal and is fully defined by the W (WRITE) command in chapter 4.

The optional closing tilde (~) character is used to terminate the command line substitution string when it appears adjacent to text that is not to participate in the substitution. The tilde is always removed as part of the substitution.

During the preprocessing phase, a command line is scanned repeatedly until no command line substitutions are performed. Note that, after an individual substitution is performed, scanning continues after the point of substitution. If the substituted text causes another substitution (by containing a new vertical bar character), it is processed during the next scan of the command line.

The special meanings of both the vertical bar and the tilde are cancelled when they are immediately preceded by the backslash (\) escape character. After the preprocessing phase of command line scanning is finished, the escape characters are removed, leaving the following vertical bar or tilde by itself. The practice of using the escape character to remove the special meaning of some other character is known as escaping, and is often used in string literals, particularly in regular expressions. Refer to appendix A for a discussion of how patterns and regular expression can be constructed for use in pattern matching.

Command line substitutions are performed on every command line, including those which define macros. If a macro definition is to contain a command line substitution to be performed when the macro is executed, it should be escaped to prevent it from being performed when the macro is defined.

Command line substitution is subject to the current state of the CMDLINESUBS environment variable. If set to FALSE, command line substitutions are not performed. Examples of command line substitutions are listed below:

Assuming the following declarations have been entered,


   var grp   = 'PUB'
   var acnt  = 'SYS'
   var cmd   = 'SYMOPEN'
   var const = $20
   var n     = $1

the following examples demonstrate command line substitutions:


  symopen myfile.|grp~.|acnt

becomes


  symopen myfile.PUB.SYS

while


  while n < |const:"#" do {cmd1;cmd2;cmd3}

becomes


  while n < #32 do {cmd1;cmd2;cmd3}

which saves many searches for the constant. And


  while |n < |const do {cmd1;cmd2;cmd3}

becomes


  while $1 < $20 do {cmd1;cmd2;cmd3}

which will loop infinitely. Next consider the following:


   $nmdebug > var n "mom"
   $nmdebug > wl "|n"
   mom
   $nmdebug > wl "\|n"
   |n

Note how the presence of the backslash cancels the command line substitution.

Operand Lookup Precedence

Aliases

Command Line Substitutions

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