Parsing

The command line parsing is done by pBat in a precise order. Knowing the order of different parsing step is really useful to a batch programmer.

Parsing steps

During parsing, pBat execute steps in the following order:

• Line reading: During this step pBat reads text lines, either from commands typed in the prompt or from a file. The line reading can occur in two different ways, depending on the CMDLYCORRECT setting of SETLOCAL command:

  • If CMDLYCORRECT is disabled, the file is read entirely at start-up, pre-parsing lines. and labels. This behaviour is specific to pBat, this enables saving up execution time compared to the historical reading method of batch files. Before every new line execution, pBat checks that the source file has not been modified, it is, it loads it again and continues execution at same line number.

  • If CMDLYCORRECT is enabled, pBat uses the same mechanics as cmd.exe. That is, before every new line reading, the script file is opened, the line is read, and then the script is closed. This method is inherited from older versions of ms-dos which had very few memory. Even though this strategy saves up memory, it causes a high penalty on execution speed. Thus this method is not the default any more for pBat

  A change of CMDLYCORRECT setting during script execution leads to a restart of the script from the beginning. Thus, any CMDLYCORRECT change is discouraged during a script execution.

• Conventional expansion: When lines are read from a file or the prompt, pBat perform the very first step of variable expansion, following rules described in variables expansion.

• Command line and conditional operators parsing: Once variable are expanded, pBat does two tasks simultaneously. Conditional operators are parsed while blocks are matched to check there is the same number of block opening and closing.

• Redirections parsing: pBat tries to detect active redirections of the standard streams on the command-line and applies it.

• Command detection and parsing: After redirections parsing, pBat searches command boundaries, and tries to extract command arguments according to argument parsing policies.

• Special variables expansion: In each of the detected argument, pBat then performs special variable expansion.

• Delayed expansion: Once special variable expanded, the delayed expansion takes finally places. This is the final step before execution.

• Execution: After all the preceding step, the command is finally executed by the appropriate command.

See also

Command line, Conditional operators