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BAT-HINT # 10 ************************************************************************** from the BATHINTS library... part of the BATPOWER CONFERENCE from: THE PAINFRAME OPUS/FIDO 261/1004 Baltimore, Maryland 1-301-488-7461 ************************************************************************** The IF subcommand The IF subcommand is not unique to batch file processing in DOS... the command may be issued from either a batch file or from the command line, though it finds its greatest use from within the batch file. Through the use of IF a user may check for several things: 1. whether a file exists 2. whether a specific errorlevel has been returned from the keyboard or from a program 3. whether a variable exists 4. the value of a variable or string The general syntax of the IF command is: if [not] {true statement} {perform command} where [not] is optional, {true statement} is a legal statement to test with the IF command and {perform command} is any executable command. Errorlevel Testing ------------------ Many of you have no doubt already used the IF command to test for errorlevels, so lets start with a simple example of an errorlevel test: sample1.bat ----------- echo off cls :start echo Press a key... (you must press the right one!) key if errorlevel 65 goto correct echo Sorry, that was the wrong key... goto start :correct echo You pressed uppercase A echo That was the letter I was waiting for! This example uses the KEY.COM utility available from BATPOWER.ARC, a utility that pauses the execution of a batch file and waits for a single key to be pressed at the keyboard... and then returns an errorlevel code equal to the scan code of the key pressed (all keys and key combinations have an associated scan code used by DOS; for details on the specific scan codes see BATHINT ? ?... bh_?.doc; for details on the use of KEY.COM see the doc file KEY.DOC packaged with the program in BATPOWER.ARC). In this example, the IF command is waiting for the uppercase (capital) A to be pressed... its scan code is 65. Lets examine the command in more detail: if errorlevel 65 goto correct We can divide the command statement into three parts according to the syntax already given. First, IF, well thats what we are talking about. Second, errorlevel 65, that is the {true statement} that we are testing... is the errorlevel 65, or more precisely, is the errorlevel equal to or greater than 65? Thats the way testing for errorlevels works. If a statement is made like: if errorlevel 65 {perform command} then the IF command tests whether the errorlevel returned is equal to or greater than the specified number. In sample1.bat we could narrow down the specificity of the test with a statement like: if errorlevel 65 if not errorlevel 66 {perform command} AhAh! A nested IF command. Examine closely what this statement says: If the errorlevel is 65 or greater AND if the errorlevel is NOT 66 or greater then {perform command}... so the errorlevel must be equal to 65 (or more) but not equal to 66 (or more)... thus the errorlevel MUST be 65 for the statement to be true. If the statement is true (if an A was pressed) then {perform command}, which in sample1.bat is GOTO CORRECT, is executed. If it is false (if the errorlevel is less than 65) then {perform command} is not executed and the batch file ignores {perform command} and jumps to the next line ( the line below). So with "if errorlevel XX if not errorlevel XX+1" we may single out a specific errorlevel for testing. Examining the third part of the statement "goto correct", we see the command to be executed IF THE STATEMENT IS TRUE. GOTO is batch file specific command in DOS which instructs DOS to jump to a specified label. In our example, the label is "correct" (though it could be anything, so long as that label exists). Notice that further down in the batch file is the following line: :correct and notice also that when specifying a label with the GOTO command it is not necessary to include the colon (:) before the label (but the label must have a colon where it actually appears as a directive in the batch file). So if everything is true (you actually did press the letter A), sample1.bat performs the "GOTO CORRECT" command and searches the batch file for the label :correct then continues executing each line, one at a time, from the label :correct. In our example, two echo commands are executed indicating that that was the right key to press... and the batch file ends. If the statement was FALSE (you did not press A; the errorlevel was less than and not equal to 65) then the batch file ignores the "goto correct" command and skips to the next line, which echos that you pressed the wrong key and then performs the "goto start" command, which jumps back to the :start label and asks you to press a key again (the letter A!). So great eh? What can I do with it? Well there are many applications, only one of which is a small menu of possible selections. As a last example, consider the following scenerio: You have three programs from which to choose and you have written a batch file which starts each application by its name. The applications are: Yorgi (a fanatstic game of fun and skill?!), RipWrite (a great word processor) and Snarf (a utility program that you cannot live without). Lets assume that each batch file (yorgi.bat, ripwrite.bat and snarf.bat) are available through the normal DOS command line by virtue of the fact that they are in the c:\bat subdirectory that is named in your path command... so that they be found when called upon). Rather than type the name of the batch file which starts these great programs, you have created a simple menu from which with a single keystroke you may select any one of them. That batch file may be written like this: menu.bat -------- echo off cls echo Main Menu: echo 1. Yorgi echo 2. RipWrite echo 3. Snarf echo Press a number according to your selection... key if errorlevel 51 if not errorlevel 52 goto select3 if errorlevel 50 if not errorlevel 51 goto select2 if errorlevel 49 if not errorlevel 50 goto select1 goto sorry :select1 yorgi :select2 ripwrite :select3 snarf :sorry echo You must press a number between 1 and 3 pause goto start cls Notice that the IF ERRORLEVEL statements are numbered backwards from the scan code for number 3 (51) to the scan code for number 1 (49). Following the logic given for sample1.bat, you should be able to see that you must start from the highest number down (if you have questions just ask!!!). Testing for the Presence of a File ---------------------------------- IF can test for the existance of a file. Examine the following batch file: sample2.bat ----------- echo off cls if exist myword.doc echo myword.doc exists! cls Simple enough... sample2.bat tests for the existance of a file called "myword.doc" and if the statement is true (that is, if myword.doc does exist), it executes the command "echo myword.doc exists!". If the file is not found the remainder of the command line is ignored and the batch file continues processing at the next line, which in this example clears the screen. Note that the "if exist" command only recognizes files in the current directory. To test for a file located elsewhere you should log onto the drive and into the subdirectory where the file is presumed to exist. For example, if the file was located on drive B in a subirectory called "document", you would need to change sample2.bat to read: echo off cls b: cd\document if exist b:\document\myword.doc echo myword.doc exists! Occasionally, the test for a specific file (as in sample2.bat) may be useful, but more commonly the filename is substituted for a REPLACEABLE PARAMETER that the user can specify when the batch file is initially called into action. Our example could be made more general by making the changes shown below: sample3.bat ----------- echo off cls if exist %1 echo %1 exists! cls In this example, the %1 parameter would be entered at the command line when the batch file is started. For example, to test for the existance of a file called "another.doc" the user would start the batch file with the following command: sample3 another.doc The %0 parameter is always the name of the batch file (sample3 in this case) and "another.doc" is the %1 parameter. When the "if exist" line is executed, the term "another.doc" will be substituted for %1. More often than not, it is more useful to utilize a "goto" statement after the IF command. In sample2.bat and sample3.bat the "echo filename.ext!" message would only be flashed on the screen momentarily if the statement was true since the next and last line in these batch files issues the CLS (clear screen) command. A more refined way to write these batch files is shown below: sample4.bat ----------- echo off cls if exist %1 goto foundit echo Sorry, %1 does not exist. pause goto fini :foundit echo %1 exists! pause :fini cls In this example, if %1 exists batch file processing jumps to the "foundit" label and echos that fact... then pauses for a keystroke before clearing the screen and terminating. If %1 does not exist it echos "Sorry...", waits for a keystroke and jumps to the "fini" label then clears the screen and terminates. At first glance, the usefullness of checking for the existance of a file doesn't seem to have much use... but consider this example: You normally store some of your programs as compressed archives to save on disk space. One such program, a game called "WASTER" is archived into a single file called "waster.arc" on drive C in the subdirectory GAMES. You have a batch file that starts the program by unarchiving the program onto a large RAM disk (say drive H), then runs the game (waster.exe) from the RAM drive. Here is a batch file that might do such processing... and also check to see if waster.arc is already unarchived on drive H... and if so, does not try to unarc it again, but rather justs runs the program from the RAM drive. This example uses the program PKXARC.EXE from PKWARE to do the unarchiving: waster.bat ---------- echo off cls if not exist h:\waster.exe pkxarc c:\games\waster.arc h:\ h: waster Equivalence Testing ------------------- The correct syntax for testing equivalence is: if stringA==stringB {perform command} where stringA and stringB are ASCII characters and may be substituted for replaceable parameters and/or environmental variables. Very often one may wish to include a parameter after the name of a batch file so as to direct some special function. For example, password.bat tests for the presence of a password given as the %1 replaceable parameter. In this example, the password is "eyesonly": password.bat ------------ echo off cls if %1==eyesonly goto correct echo Your password was incorrect. goto end :correct {perform some commands here} :end In order that the command that you would place on line 7 to be invoked, you would have to enter the following statement at the command line: password eyesonly Nothing else would work. If the incorrect parameter is given, or if no parameter was given, the IF statement would not be true and the file would display "Your password was incorrect". Of course, such a batch file would not provide any substantial level of security, since a user would only need to view the batch file with an editor or with the type command to see what the correct password should be. Note that the password must match exactly. If the command was given as: password EYESONLY or password eysOnly or any other variation of upper and lower case letters, the IF statement would be false. Often it is of benefit to test for the absence of a parameter, especially when you may have forgotten the correct syntax for a complicated batch file written some time ago. Note the following statement: if "%1"=="" {perform command} This statement will be true when the %1 parameter is a NULL string... the %1 parameter was not given on the command line which started the batch file. Note that the NULL string is a real string... it is just a string with a size of zero. If the quotation marks were left out: if %1=="" {perform command} a syntax error message would be generated. The quotation marks can just as easily be replaced with dollar signs, exclamation marks... almost anything. As an example several statements are shown below. If no parameter is given they are all true, if a parameter was given, they are all false, and one works just as well as any of the others: if "%1"=="" {perform command} if $%1$==$$ {perform command} if !%1!==!! {perform command} if x%1==x {perform command} if %1x==x {perform command} if .%1==. {perform command} Use whichever style you prefer. Tests for equivalence may also employ environmental variables (see BATHINT? for a discussion of environmental variables). As always, the environmental variable must be enclosed within quotation marks for its value to be correctly substituted. For example, if the following command had been issued: set name=Bimbo so that the variable "name" contained the value "Bimbo", the value of the variable could be tested with the following statements: if %name%==Bimbo goto correct echo The name was not Bimbo. goto end :correct echo The name was Bimbo. :end *********************************************************** David Creasey