Batch File String Processing

As you gain experience with batch files, you're likely to find that you 
need to manipulate text strings.  You may need to prompt a user for a name 
or password, process a list of files, or find a name in a phone list.  All 
of these are examples of string processing  the manipulation of lines of 
readable text. 

CMD.EXE include several features that make string processing easier.  For 
example, you can use the INKEY and INPUT commands for user input; the 
ECHO, SCREEN, SCRPUT, and VSCRPUT commands for output; and the FOR command 
or the @FILEREAD function to scan through the lines of a file.  In 
addition, variable functions offer a wide range of string handling 
capabilities. 

For example, suppose you need a batch file that will prompt a user for a 
name, break the name into a first name and a last name, and then run a 
hypothetical LOGIN program.  LOGIN expects the syntax /F:first /L:last 
with both the first and last names in upper case and neither name longer 
than 8 characters.  Here is one way to write such a program: 

        @echo off
        setlocal
        unalias *
        input Enter your name (no initials):  %%name

        set first=%@word[0,%name]
        set flen=%@len[%first]
        set last=%@word[1,%name]
        set llen=%@len[%last]

        iff %flen gt 8 .or. %llen gt 8 then
           echo First or last name too long
           quit
        endiff

        login /F:%@upper[%first] /L:%@upper[%last]
        endlocal

The SETLOCAL command at the beginning of this batch file saves the 
environment and aliases.  Then the UNALIAS * command removes any existing 
aliases so they won't interfere with the behavior of the commands in the 
remainder of the batch file.  The first block of lines ends with an INPUT 
command which asks the user to enter a name.  The user's input is stored 
in the environment variable NAME. 

The second block of lines extracts the user's first and last names from 
the NAME variable and calculates the length of each.  It stores the first 
and last name, along with the length of each, in additional environment 
variables.  Note that the @WORD function numbers the first word as 0, not 
as 1. 

The IFF command in the third block of lines tests the length of both the 
first and last names.  If either is longer than 8 characters, the batch 
file displays an error message and ends.  Finally, in the last block, the 
batch file executes the LOGIN program with the appropriate parameters, 
then uses the ENDLOCAL command to restore the original environment and 
alias list.  At the same time, ENDLOCAL discards the temporary variables 
that the batch file used (NAME, FIRST, FLEN, etc.). 

When you're processing strings, you also need to avoid some common traps. 
 The biggest one is handling special characters. 

Suppose you have a batch file with these two commands, which simply accept 
a string and display it: 

        input Enter a string:  %%str
        echo %str

Those lines look safe, but what happens if the user enters the string "some 
> none" (without the quotes).  After the string is placed in the variable 
STR, the second line becomes: 

        echo some > none

The ">" is a redirection symbol, so the line echoes the string "some" and 
redirects it to a file called NONE - probably not what you expected.  You 
could try using quotation marks to avoid this kind of problem (see 
Argument Quoting), but that won't quite work.  If you use back-quotes 
(ECHO `%STR`), the command will echo the four-character string %STR. 
 Environment variable names are not expanded when they are inside 
back-quotes. 

If you use double quotes (ECHO "%STR"), the string entered by the user 
will be displayed properly, and so will the quotation marks.  With double 
quotes, the output would look like this: 

        "some > none"

As you can imagine, this kind of problem becomes much more difficult if 
you try to process text from a file.  Special characters in the text can 
cause all kinds of confusion in your batch files.  Text containing 
back-quotes, double quotes, or redirection symbols can be virtually 
impossible to handle correctly. 

One way to overcome these potential problems is to use the SETDOS /X 
command to temporarily disable redirection symbols and other special 
characters.  The two-line batch file above would be a lot more likely to 
produce the expected results if it were rewritten this way: 

        setdos /x-15678
        input Enter a string:  %%str
        echo %str
        setdos /x0

The first line turns off alias processing and disables several special 
symbols, including the command separator (see Multiple Commands) and all 
redirection symbols.  Once the string has been processed, the last line 
re-enables the features that were turned off in the first line. 

If you need advanced string processing capabilities beyond those provided 
by CMD.EXE you may want to consider using the REXX language.  Our products 
support external REXX programs for this purpose.