Conditional branching with "If" statements
If something is true then do an action (otherwise do a different action)The second part of the statement (in parentheses) is optional. Otherwise, the system just goes to the next line in the batch file if the first condition isn't met. The actual syntax is
If (condition) (command1) Else (command2)The "Else" part is optional. The form "If not" can also be used to test if a condition is false. Note that "If" tests for true or false in the Boolean sense.
"If exist" statement
There is a special "If exist" statement that can be used to test for the existence of a file, followed by a command. An example would be:
If exist somefile.ext del somefile.ext You can also use a negative existence test:
if not exist somefile.ext echo no file
"If defined" statement
Another special case is "if defined ", which is used to test for the existence of a variable. For example:
if defined somevariable somecommand This can also be used in the negative form, "if not defined".
"If errorlevel" statement
Yet another special case is "if errorlevel", which is used to test the exit codes of the last command that was run. Various commands issue integer exit codes to denote the status of the command. Generally, commands pass 0 if the command was completed successfully and 1 if the command failed. Some commands can pass additional code values. For example, there are five exit code values for
Xcopy. These exit code values are stored in the special variable errorlevel. An example command would be:
if errorlevel n somecommand where "n" is one of the integer exit codes. Note that the comparison is done by checking if errorlevel is greater than or equal to n. If used with "not" the comparison checks
if errorlevel is less than n.
|NEQ||not equal to|
|LEQ||less than or equal to|
|GEQ||greater than or equal to|
In some cases the condition to be met is obtained by comparing strings. For example
if string1 == string2Note that the "equals" sign is written twice. This condition is met if the two strings are exactly identical, including case. To render comparisons insensitive to case, use the switch "/i". For more general comparisons, use the operators in Table I. (The operators are given in
upper case in the table but they are not case-dependent.) Numerical comparisons only work with all-digit strings. Otherwise, the comparison is done alphabetically. For example "a" is less than "b". For case independence, use the switch "/i". An example command might read:
if /i string1 gtr string2 somecommand
When comparing variables that are strings, it may be best to enclose the variable name in quotes. For example, use:
if "%1" == somestring somecommand
The "goto" command
Generally, the execution of a batch file proceeds line-by-line with the command(s) on each line being run in turn. However, it is often desirable to execute a particular section of a batch file while skipping over other parts. The capability to hop to a particular section is provided by the appropriately named "goto" command (written as one word). The target section is labeled with a line at
the beginning that has a name with a leading colon. Thus the script looks like
... Execution will skip over "some
commands" and start with "some other commands". The label can be a line anywhere in the script, including before the "goto" command.
...some other commands
"Goto" commands often occur in "if" statements. For example you might have a command of the type:
if (condition) goto :label
The "End of File" (:eof) label for exiting a script
Sometimes it is desirable to terminate a script if a certain condition is met (or not met). One way to exit is to use the special label :eof in a goto command. The label is not actually placed in the batch file. Windows XP and later recognize :eof without any label explicitly placed at the end of the batch file. Thus if you need to test for a
particular condition that makes script termination desirable, you can write:
if (condition) goto :eofNote that this terminates the script but does not necessarily close the command shell.
An ancient method for doing repetitive tasks uses a counter,"if" statements, and the "goto" command. The counter determines how many times the task is repeated, the "if" statement determines when the desired number of repetitions has been reached, and the "goto" command allows for an appropriate action to be executed, either the repetitive task or exiting. Generally, the more elegant methods provided by the powerful "for...in...do" command are preferable and they are discussed on the next page. However, for completeness and to illustrate some of what we have discussed, I will give an example that uses the clumsier method.
The simple script below creates the numbers 1 to 99 and sends them to a file. It uses the "set" command to create a variable that is also the counter for how many times we have iterated.
@echo off(Best programming practice would dictate that the variable %counter% be localized or destroyed at the
end but for simplicity I have omitted the several extra lines needed to do that. As written, this environment variable would persist until the command shell itself, not just the script, was closed.)
set /a counter=0
set /a counter=%counter%+1
if %counter% ==100 (goto :eof) else (echo %counter% >> E:\count.txt)
In anticipation, I can note that the same result as the script above can be achieved with a two-line script using the "for" statement discussed on the next page:
for /l %%X in (1,1,99) do (echo %%X >> E:\count.txt)
|Back to top||Next: Iteration methods|