CMD Shell Tips and Pitfalls

External References

• Excellent site on http://www.ss64.com/nt/
  • Also check pages on syntax http://www.ss64.com/nt/syntax.html
• On redirection http://www.robvanderwoude.com/index.html

Frequent Mistakes

• WRONG - Use = for string comparison.
• CORRECT - Use == for string comparison.
• BETTER - EQU is less confusing and less exposed to mistake.

if "0" = "0" echo Hello    # WRONG
if "0" == "0" echo Hello   # CORRECT
if "0" EQU "0" echo Hello  # BETTER

• WRONG - Closing bracket will close the IF statement!
• CORRECT - Use [] instead
• BETTER - Escape parenthesis with ^ instead

if "0" EQU "0" (
  rem do something
  echo I am doing something (and something)...     # WRONG
)
if "0" EQU "0" (
  rem do something
  echo I am doing something [and something]...     # CORRECT
)
if "0" EQU "0" (
  rem do something
  echo I am doing something ^(and something^)...   # BETTER
)

• WRONG - Closing bracket will close the IF statement even in variable expansion!
• CORRECT - Always quote variables with "..."
• CORRECT - Quotes " not necessary with %% variable

set MYVAR=Closing)
if "0" EQU "0" (
  rem do something
  echo My beautiful var %MYVAR%...        # WRONG
)

set MYVAR=Closing)
if "0" EQU "0" (
  rem do something
  echo My beautiful var "%MYVAR%"...      # CORRECT
)

for /F %%i in ("bracket)") do (
  echo My Closing bracket: %%i            # CORRECT
)

• Some Variable Expansion:
  • !!! Use %% for FOR statement in batch file only
  • !!! Use single % for command line parameters only

set MYVAR=My Var
echo %MYVAR       # expands to MYVAR
echo %%MYVAR      # expands to %MyVar
echo %MYVAR%      # expands to My Var
echo "%MYVAR"     # expands to "MYVAR"
echo "%%MYVAR"    # expands to "%MYVAR"
echo "%MYVAR%"    # expands to "My Var"

• WRONG - %VAR% are expanded when read not when executed (early expansion)!
• CORRECT - Use a subroutine !
• BETTER - Use Variable Delayed Expansion and syntax !VAR!(enabled with cmd.exe /V or setlocal enabledelayedexpansion)!

setlocal ENABLEDELAYEDEXPANSION
set MYVAR=
for /F %%i in ("TEST") do (
  echo TEST: %%i           # expands to TEST: TEST
  set MYVAR=%%i
  echo MYVAR: "%MYVAR%"    # WRONG   - expands to TEST: ""
  call :myset %%i          # CORRECT - expands to TEST: "TEST" - call subroutine
  echo MYVAR: "!MYVAR!"    # BETTER  - expands to TEST: "TEST" - delayed expansion
)
goto :EOF

:myset
set MYVAR=%1
echo MYVAR: "%MYVAR%"      # CORRECT - within sub, expands to TEST: "TEST"
goto :EOF

• WRONG - Possible syntax error if MYVAR is empty in IF ... EQU expression
• CORRECT - Always quote variables (we'll never repeat this enough) !

set MYVAR=
if MYVAR EQU YES echo failed       # WRONG
if "MYVAR" EQU "YES" echo failed   # CORRECT

• WRONG - SET command includes trailing blanks
• CORRECT - Remove all trailing blanks in SET command
• BETTER - Configure your favorite editor to always remove trailing blanks on every line on save

# WRONG - set with trailing blanks
set MYDIR=C:\TEMP\  
# CORRECT - set without trailing blanks
set MYDIR=C:\TEMP\

• WRONG - trailing blanks inside path makes %~n1 to fail (!!! very frequent when using an env_var with trailing blanks !!!)
• CORRECT - remove trailing blanks when using %~n1

EDIT: This bug seems fixed in Windows XP now.

call :strip "C:\TEMP  \MYFILE.ext"    # WRONG... will output 'FILE'
rem

call :strip "C:\TEMP\MYFILE.ext"      # CORRECT... will output 'MYFILE'

goto :EOF
:strip
echo %~n1
goto :EOF

• WRONG - %* not modified by SHIFT
• CORRECT - Use a loop instead with %~1

echo %*
SHIFT
echo %*                               # WRONG... will output the same string

:getFileList
if "%~1" EQU "" goto :noMoreParam
echo "%~1"                            # CORRECT... Loop with %~1
SHIFT
goto :getFileList
:noMoreParam

• WRONG - Never use "%1" or %1
• CORRECT - Always use "%~1" (always correctly quoted)

type "%1"             # WRONG
type "%~1"            # CORRECT

• WRONG - in W2K, FOR /R skips SYSTEM files/directories (in 4NT, SYSTEM & HIDDEN files are skipped).
• CORRECT - Use DIR instead (here we list all files that is not a directory...)
• CORRECT - Use DIR instead (here we list all directories)

for /r "." %%i in (*.*) do @echo %%i                        # WRONG - Skips SYSTEM files/directories
for /F "usebackq" %%i in (`dir /a:-d /s /b`) do @echo %%i   # CORRECT (all files that is not a directory)
for /F "usebackq" %%i in (`dir /a:d /s /b`) do @echo %%i    # CORRECT (all directories)

• WRONG - cmd.exe remove surrounding quotes, except in some very specific cases (see CMD /?).
• CORRECT - Always surround with quotes and always use flag cmd.exe /S to force quote removal.

rem
set MYBAT="C:\Program Files\MyBat\MyBat.bat"
set MYPARAM="parameter 1" "parameter 2"
cmd /V:F /C %MYBAT% %MYPARAM%                # WRONG - quotes will be removed from parameters

set MYBAT="C:\Program Files\MyBat\MyBat.bat"
set MYPARAM="parameter 1" "parameter 2"
cmd /V:F /S /C "%MYBAT% %MYPARAM%"           # CORRECT - First quote in %MYBAT% and last quote in %MYPARAM% will be removed.

• WRONG - FOR /F with other delims NOK if usebackq used (it seems there is a buffer overflow in cmd.exe)
• CORRECT - FOR /F with other delims is ok if usebackq not used

EDIT: This bug seems fixed in Windows XP now.

set STRING=MaskOSLib Maker~1:dir:1
for /F "usebackq tokens=1-4 delims=:~" %%i in ('%STRING%') do echo %%i-%%j-%%k-%%l    # WRONG
goto :EOF
set STRING="MaskOSLib Maker~1:dir:1"
for /F "tokens=1-4 delims=:~" %%i in (%STRING%) do echo %%i-%%j-%%k-%%l               # CORRECT
goto :EOF

• WRONG - Use FOR /F to enumerate items in a list
• CORRECT - Use FOR to enumerate items in a list (space, comma and semi-colon are used as separators)

set STRING=value1 value2 value3
for /F %%i in ("%STRING%") do echo %%i   # WRONG
for %%i in (%STRING%) do echo %%i        # CORRECT
goto :EOF

• WRONG - returning an ERRORLEVEL to VBS with an ending goto :EOF (or whatever)
• CORRECT - Set ERRORLEVEL as the very last operation executed

:: following set ERRORLEVEL to 1, and then exit
VERIFY OTHER 2>/NUL
goto :EOF                               # WRONG - Calling script will see an ERRORLEVEL 0

::-----------------------------

setlocal
:: ...
if ... goto :FINISHFAIL
:: Do some stuffs and finish...
endlocal
goto :EOF

:FINISHFAIL
:: close locals and set ERRORLEVEL
endlocal
VERIFY OTHER 2>/NUL                     # CORRECT - Calling script will see an ERRORLEVEL > 0

Hints and Tips

Miscellaneous

• To bypass all pause commands, or to answer next set /P command

echo Y| batchpgm.bat

• Enumerate elements in a list

set SOURCEFILEEXT=.c .h
set TEXTFILEEXT=%SOURCEFILEEXT% .txt .ddf
for %i in (%TEXTFILEEXT%) do echo %i

• Alternate rem

:: This is also a remark

• Inlined rem

echo Hello world      &rem ampersand must be used for inlined rem (all space on the left are part of echo command!)

• Make errorlevel persistent - env. var override ERRORLEVEL if defined

if ERRORLEVEL 1 set ERRORLEVEL=%ERRORLEVEL%
echo %ERRORLEVEL%

• Change dir and drive

cd /D "%TEMP%"

• Parsing command result

for /F "usebackq tokens=*" %%i in (`isText /sv %1`) do echo ... invalid ASCII: "%%i"

• Echo an empty line

echo.

• Scan directory and subdir - %%~pnxi is used to remove trailing dot

for /R "%~1\" %%i in (.) do echo  "%%~pnxi"

• Ask user input

set /P CONFIRM=... Empty files found [might crash GNU/Indent]... Delete [Y/N/Abort]?
if /I "%CONFIRM:~0,1%" EQU "Y" (
  echo user says YES
) else (
  echo user says NO
)

• Set errorlevel in subroutine

Call :MySub "1"
goto :Continue

:MySub
if "%1" NEQ "1" exit /b 1
goto :EOF

:Continue

• Remove surrounding quotes in argument

echo with or without quote: %1
echo always without quote: %~1
echo always with quote: "%~1"

• Use `echo %ENV_VAR%` instead of '%ENV_VAR%' in FOR loops

rem   Assume ENV_VAR is set to a very long list of items delimited by ;
rem   The following fails when ENV_VAR is too long
for /F "usebackq delims=;" %%i in ('%ENV_VAR%') do echo %%i
rem   The following works when ENV_VAR is too long
for /F "usebackq delims=;" %%i in (`echo %ENV_VAR%`) do echo %%i

• Enumerate items in a list separated by a blank - !!! The list may not contain joker like * or & !!!

set TEXTFILEEXT=.txt .ddf .c .h
for %%i in (%TEXTFILEEXT%) do echo *%%i

• Enumerate items in a list separated by an another token (list can be quite long)

set ENV_VAR="item1";"item2";"item3"
for /F "usebackq delims=;" %%i in (`echo %ENV_VAR%`) do call :enumerate %%i
rem ... For loop is only to translate ; into blanks
goto :EOF
:enumerate
if "%~1" equ "" goto :EOF
echo "%~1"
SHIFT
goto :enumerate

• Yet another solution To enumerate items in a list separated by an another token (list can be quite long)

set ENV_VAR="item1";"item2";"item3"; my item
call :enumerate %ENV_VAR:;= %
goto :EOF
:enumerate
if "%~1" equ "" goto :EOF
echo "%~1"
SHIFT
goto :enumerate

• Enable/disable Command Processor Extension or Delayed Extension

SETLOCAL ENABLEEXTENSIONS
SETLOCAL DISABLEEXTENSIONS
SETLOCAL ENABLEDELAYEDEXPANSION
SETLOCAL DISABLEDELAYEDEXPANSION
rem ... setlocal set errorlevel to 0 if successful - can be used to detect if extensions are supported

• Know whether a name refers to an existing file or to an existing dir

if exist "%~1" goto :isFile
pushd "%~1"
if errorlevel 1 goto :doesntexist
popd
:isDir
rem %1 is a directory
:isFile
rem %1 is a file
:doesntexist
rem %1 does not exist

• An optimized sub-routine that returns the length of a string (requires Delayed Expansion to be enabled)

::------ GetLength ----------------------------------------------------------------------------------------------------------------
:: %1     = string whose length must be returned
:: RESULT = length
:GetLength
set _STR=_%~1
set RESULT=0
set _STEP=128
:nextchar
if "%_STEP%" EQU "0" goto :EOF
set /A _I=RESULT + _STEP
if "!_STR:~%_I%!" EQU ""  (
  set /A "_STEP>>=1"
) ELSE (
  set /A RESULT=RESULT + _STEP
  if "%_STEP%" NEQ "128" set /A "_STEP>>=1"
)
goto :nextchar

• Escaping characters - Use ^) to escape parenthesis in a command block

for %%i in (*.*) do (
        echo first one: %%i (hello^)
        echo second one: %%i (hello again^)
)

Redirections

Info from http://www.robvanderwoude.com/index.html

command > file          Write standard output of command to file
command 1> file         Write standard output of command to file (same as previous)
command 2> file         Write standard error of command to file (OS/2 and NT)
command > file 2>&1     Write both standard output and standard error of command to file (OS/2 and NT)
command >> file         Append standard output of command to file
command 1>> file        Append standard output of command to file (same as previous)
command 2>> file        Append standard error of command to file (OS/2 and NT)
command >> file 2>&1    Append both standard output and standard error of command to file (OS/2 and NT)
commandA ¦ commandB     Redirect standard output of commandA to standard input of commandB
command < file          Command gets standard input from file
command 2>&1            Command's standard error is redirected to standard output (OS/2 and NT)
command 1>&2            Command's standard output is redirected to standard error (OS/2 and NT)

Location of 2>&1 is critical. It must be placed at the end of the line, or right before the next pipe.

To redirect both standard output and standard error to a file, use 2>&1 at the end:

echo Everything from stdout and stderr to a single file > file 2>&1

Redirection can be placed at the beginning of the line to increase readability, but beware of side effects:

ECHO Directory of all files on C: >> LOG1.LOG
DIR C:\ /S >> LOG1.LOG
rem is the same as the more readable version:
>> LOG1.LOG   ECHO Directory of all files on C:
>> LOG1.LOG   DIR C:\ /S
rem But the following
VER ¦ TIME > LOG1.LOG
rem is not the same as the line below (where it is VER that is redirected to LOG1.LOG !!!)
> LOG1.LOG VER ¦ TIME

Each redirection device exists in every directory on every drive so redirection to a device like NUL, AUX, LPTn, COMn, PRN COST 1 file handle per device per directory where the redirection is done !!! To avoid this, avoid redirect to NUL, but redirect to \NUL or better %TEMP%\NUL. Also use PRINT instead of redirecting to LPTn.

echo A way to trash output>NUL
echo This is a better way >\NUL

Using SED in a CMD batch file

Main problem of using SED is escaping the quote " in sed command.

• In general, use \" to escape "

sed "s!\"Micro!\"Macro!g;" "temp.txt"               ::Change '"Micro' into '"Macro'

• ... but it doesn't work anymore if query contains \"[^\"]*

sed "s!\"[^\"]*soft!hard!g;" "temp.txt"             ::DOESN'T WORK

• Solution: Don't quote the sed command (Ok if no space in it)!

sed s!\"[^\"]*soft!hard!g; "temp.txt"               ::WORKS !

• Solution (cont'd): if there is space, escape them using \x20 or \t.

• Note that [^\"]*\" works even if quoted:

sed "s![^\"]*\"Micro!Macro!g;" "temp.txt"           ::WORKS !

• Escaping " and redirecting output
  • \" is not recognised by cmd.exe, but considered as a backslash followed by an opening/closing quote
  • → so redirection will work only if not enclosed in quotes according to cmd.exe:

sed "s![^\"]*\"Micro!Macro!g;" "temp.txt" > "result.txt"        ::WORKS !           (even number of \")
sed "s![^\"]*\"Micro!\"Macro!g;" "temp.txt" > "result.txt"      ::DOESN'T WORK !    (odd number of \")

• Solution: Use a sub-routine:

call :DOIT > "result.txt"
rem ...

:DOIT
sed "s![^\"]*\"Micro!\"Macro!g;" "temp.txt" > "result.txt"