Quite a number of programmers learned scripting on a PC running DOS. Even the crippled DOS batch file language allowed writing some fairly powerful scripts and applications, though they often required extensive kludges and workarounds. Occasionally, the need still arises to convert an old DOS batch file to a UNIX shell script. This is generally not difficult, as DOS batch file operators are only a limited subset of the equivalent shell scripting ones.
Table M-1. Batch file keywords / variables / operators, and their shell equivalents
|Batch File Operator||Shell Script Equivalent||Meaning|
|%||$||command-line parameter prefix|
|/||-||command option flag|
|\||/||directory path separator|
|==||=||(equal-to) string comparison test|
|!==!||!=||(not equal-to) string comparison test|
|@||set +v||do not echo current command|
|*||*||filename "wild card"|
|>||>||file redirection (overwrite)|
|>>||>>||file redirection (append)|
|NOT||!||negate following test|
|NUL||/dev/null||"black hole" for burying command output|
|ECHO||echo||echo (many more option in Bash)|
|ECHO.||echo||echo blank line|
|ECHO OFF||set +v||do not echo command(s) following|
|FOR %%VAR IN (LIST) DO||for var in [list]; do||"for" loop|
|GOTO||none (use a function)||jump to another location in the script|
|PAUSE||sleep||pause or wait an interval|
|CHOICE||case or select||menu choice|
|IF EXIST FILENAME||if [ -e filename ]||test if file exists|
|IF !%N==!||if [ -z "$N" ]||if replaceable parameter "N" not present|
|CALL||source or . (dot operator)||"include" another script|
|COMMAND /C||source or . (dot operator)||"include" another script (same as CALL)|
|SET||export||set an environmental variable|
|SHIFT||shift||left shift command-line argument list|
|SGN||-lt or -gt||sign (of integer)|
|PRN||/dev/lp0||(generic) printer device|
|LPT1||/dev/lp0||first printer device|
|COM1||/dev/ttyS0||first serial port|
Batch files usually contain DOS commands. These must be translated into their UNIX equivalents in order to convert a batch file into a shell script.
Table M-2. DOS commands and their UNIX equivalents
|DOS Command||UNIX Equivalent||Effect|
|ASSIGN||ln||link file or directory|
|ATTRIB||chmod||change file permissions|
|COMP||diff, comm, cmp||file compare|
|DELTREE||rm -rf||delete directory recursively|
|DIR||ls -l||directory listing|
|EXIT||exit||exit current process|
|FC||comm, cmp||file compare|
|FIND||grep||find strings in files|
|MORE||more||text file paging filter|
|PATH||$PATH||path to executables|
|TIME||date||display system time|
|TYPE||cat||output file to stdout|
|XCOPY||cp||(extended) file copy|
Virtually all UNIX and shell operators and commands have many more options and enhancements than their DOS and batch file counterparts. Many DOS batch files rely on auxiliary utilities, such as ask.com, a crippled counterpart to read.
DOS supports only a very limited and incompatible subset of filename wild-card expansion, recognizing just the * and ? characters.
Converting a DOS batch file into a shell script is generally straightforward, and the result ofttimes reads better than the original.
Example M-1. VIEWDATA.BAT: DOS Batch File
REM VIEWDATA REM INSPIRED BY AN EXAMPLE IN "DOS POWERTOOLS" REM BY PAUL SOMERSON @ECHO OFF IF !%1==! GOTO VIEWDATA REM IF NO COMMAND-LINE ARG... FIND "%1" C:\BOZO\BOOKLIST.TXT GOTO EXIT0 REM PRINT LINE WITH STRING MATCH, THEN EXIT. :VIEWDATA TYPE C:\BOZO\BOOKLIST.TXT | MORE REM SHOW ENTIRE FILE, 1 PAGE AT A TIME. :EXIT0
The script conversion is somewhat of an improvement. 
Example M-2. viewdata.sh: Shell Script Conversion of VIEWDATA.BAT
#!/bin/bash # viewdata.sh # Conversion of VIEWDATA.BAT to shell script. DATAFILE=/home/bozo/datafiles/book-collection.data ARGNO=1 # @ECHO OFF Command unnecessary here. if [ $# -lt "$ARGNO" ] # IF !%1==! GOTO VIEWDATA then less $DATAFILE # TYPE C:\MYDIR\BOOKLIST.TXT | MORE else grep "$1" $DATAFILE # FIND "%1" C:\MYDIR\BOOKLIST.TXT fi exit 0 # :EXIT0 # GOTOs, labels, smoke-and-mirrors, and flimflam unnecessary. # The converted script is short, sweet, and clean, #+ which is more than can be said for the original.
Ted Davis' Shell Scripts on the PC site has a set of comprehensive tutorials on the old-fashioned art of batch file programming. Certain of his ingenious techniques could conceivably have relevance for shell scripts.
Various readers have suggested modifications of the above batch file to prettify it and make it more compact and efficient. In the opinion of the ABS Guide author, this is wasted effort. A Bash script can access a DOS filesystem, or even an NTFS partition (with the help of ntfs-3g) to do batch or scripted operations.
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