Ian & Stuart's Australian Mac 1

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Text File | 1994-09-20 | 12KB | 279 lines

8 Batches and Programs MacDOS recognises four types of "executable" files: • Applications: Files of type 'APPL'. • Compiled AppleScripts: Files of type 'osas'. • Text AppleScripts, of type 'TEXT' and creator 'ToyS'. • Batch programs, of type 'TEXT' containing MacDOS commands to be interpreted and executed one line at a time. You have two ways of telling MacDOS what program to execute: • by writing its file specification at the beginning of a command line; • by executing the command call fileSpec . The two strategies are only different if you are already executing in batch: in the first case, control is simply transferred to the new program; with CALL, execution of the current batch resumes after the application has been launched or the new batch program has terminated. Instead of having to type full file specifications, you can take advantage of the system variable PATH and just type the name of the application or batch program. If you keep all applications and batch programs (or their aliases) in a small number of folders, you will never have to type a full file specification at all. Note that MacDOS, unlike DOS, does not automatically add the extension .EXE when looking for executable files. Neverthess, similarly to DOS, MacDOS automatically adds the extension .BAT when looking for batch programs. Introduction to Batch Programs After opening a batch file for reading, MacDOS reads and attempts to execute one line of text at a time. In its simplest form, a batch program is therefore just a way of storing a series of MacDOS commands so that you do not need to type them over and over again. Nevertheless, there are a series of features which make batch programs very useful: • Several MacDOS commands are only available within batch programs. • You can control the execution of batch programs through launch parameters. • You can setup a batch program in such a way that it behaves like a Macintosh application. • You can document your algorithms and the reasoning behind them by adding lines of comments. To add lines of comments, you can use the command REM (for REMark). REM tells MacDOS to ignore the rest of the line and continue with the next one. You can also use an exclamation mark ('!') in the same way. If you redirect the output of a batch program, MacDOS automatically applies the redirection to each individual command in the batch file, including nested batches. Note that the output of batch programs cannot be piped. Jumps and Labels Normally, MacDOS executes the lines of a batch program in sequence, as they appear in the file. You can direct MacDOS to interrupt the sequence and resume execution somewhere else in the file by using the command GOTO label. Almost any word or number can be used as a label but, to be recognised and accepted as such, it must satisfy the following conditions: • It cannot contain spaces or tabs. • It cannot begin with a percent sign. • It must be the first word in a line which begins with a colon. • It must be unique within the file. Before executing a batch program, MacDOS scans the whole file looking for lines which begin with a colon. For each one of such lines, MacDOS builds a "label entry" containing the first word after the colon and the file offset of the next line. Note that MacDOS ignores whatever follows the label, so that you can use the rest of the line for commenting. Also note that the colon identifies the label but is not part of it. When MacDOS encounters a GOTO during batch execution, it scans the label entries looking for the requested label. If it finds an entry with the matching label, it then resumes execution of the batch program from the line with the file offset found in the entry. Replaceable Parameters Whether you just type a batch filename or use the command CALL, you can pass to the batch program a series of parameters. The parameter %0 stores the name of the batch file itself, while the parameters %1 to %9 are available for you to define as you need. Each one of these replaceable parameters consists of a string and is accessible from within the batch program through a number preceded by a percent sign. For example, if you type: myBatch name "a full address" the program myBatch will be able to access the string name as %1 and the string a full address as %2 . The string myBatch will be available as %0 . Despite the limitation in the number of parameters, MacDOS lets you pass to a batch program up to 24 strings. You can access the additional 15 strings by using the command SHIFT. SHIFT copies %1 to %0, %2 to %1, ..., and %9 to %8; it then stores the 10th string into %9. By "pushing in" one new string at a time in this way, you can acess all 15 additional strings. Conditional Statements MacDOS lets you implement conditional statements with the command IF. The general format of IF is if condition do aCommand , where aCommand is executed only if condition is true. Valid conditions are: string1 == string2 exist fileSpec existdir folderSpec not existdir folderSpec As you can see from the last example, you can also invert a condition by inserting a 'not' between the IF and the condition. Loops Very often, a particular sequence of commands needs to be repeated for each file belonging to a particular set or until certain special conditions are verified. To serve this purpose, MacDOS provides the command: FOR. FOR lets you define a set of files and then operate on each one of them. In its simplest form, FOR executes a single command included in the same line: for %name in (set) do aCommand where set is a comma-separated list of file specifications, and aCommand can refer to the current file through the special variable %name. To execute several commands for each file, you can use an extended format of FOR: for %name in (set) do begin ... several commands which can refer to the current file through the global variable %name% ... next name The most general way of implementing loops is with a combination of an IF and a GOTO. For example, if you need to repeat certain operations until a certain condition is verified, you can do as follows (equivalent a WHILE or a FOR in C or a WHILE-DO in Pascal): set k = 1 :LOOP_LBL if k == 5 goto BREAK_LBL ... here you do what you need to do ... incr k goto LOOP_LBL :BREAK_LBL Alternatively, you can place the IF at the end of the loop (equivalent to a DO in C or Fortran or a REPEAT-UNTIL in Pascal): set control=initial :LOOP_LBL ... here you do what you need to do ... ... and also set the control variable in order to exit the loop ... if control==done goto BREAK_LBL goto LOOP_LBL :BREAK_LBL Error Handling Beside reporting error messages in clear, MacDOS also stores error codes into the global variable doserr. To convert the error codes to their corresponding messages, you can use the command SHOW. The command ONERROR lets you trap all the errors which occur while executing a batch program. This is particularly useful when you execute a batch program with ECHO OFF, because in that case MacDOS suppresses error and warning messages. ONERROR lets you specify the label of an error handling procedure. For example, onerror label ... first command for which you want to trap errors ... ... second command for which you want to trap errors ... ... onerror is functionally equivalent to the following less readable construct: set doserr=0 ... first command for which you want to trap errors ... if not %doserr% == 0 goto label ... second command for which you want to trap errors ... if not %doserr% == 0 goto label ... set doserr=0 User Interface While executing in batch, MacDOS normally displays all commands as they are executed. With the command ECHO OFF, you can direct MacDOS to suppress both the echoing of the commands and their output, including warning and error messages. You can then re-enable the display with ECHO ON. If you only want to suppress the output of a single command, you can do so by attaching an '@' to the front of the command name. By typing as first line of a batch program the command @echo off , you make the program completely silent. In order to display variable values and other messages while echoing is disabled, you can use ECHO as illustrated in the following examples: echo Now processing file No %fileNum% echo File %1 found: it is in folder %foldSpec% After ECHOing an intermediate result, you might need to suspend execution of a batch program and wait for confirmation before continuing. For this purpose you can use the command PAUSE. When MacDOS encounters PAUSE, it displays the message: press any key to continue ... and waits for your reply. If you type cmd-dot or cntl-C, MacDOS then asks Terminate batch job (Y/N) ? You can then terminate the job by replying 'Y' or 'y'. Batches that behave like Applications Any file of type 'TEXT' is accepted by MacDOS as a valid batch program. Nevertheless, if you want to automatically launch MacDOS by double clicking on the icon of a batch file, you need to set its creator to 'mDOS'. By using the command EXIT as last line, you can also automatically terminate MacDOS and return to the Finder at the end of the batch program. Here is a simple program that is actually used every day to back up this User's Guide while it is being written: confirm off copy "\MacDOS folder\product documents\manual" 2: /u/v exit It copies to a floppy all files which have been updated since the last backup. You can preset to 'mDOS' the creator of text files by setting the system variable CREATOR: set creator=mDOS Alternatively, you can change to 'mDOS' the creator of existing files: ren/c!mDOS fileSpec Note that MacDOS executes the batch program autoexec.bat (see below) before executing the batch program that you have double clicked. If you keep MacDOS running all the times, instead of aliasing the MacDOS application into the folder "Startup Items", you might like to replace all the items in that folder with a small batch program which CALLs them all. They would be launched in the order in which you call them (MacDOS will always be the first one, though).