Commands usually set error level at the end of their execution. In Windows NT and later, it is a 32-bit signed integer; in MS DOS, it used to be an integer from 0 to 255. Keywords: return code, exit code, exit status.

The conventional meaning of the error level:

• 0 - success
• not 0 - failure
• The error levels being set are usually positive.
• If the command does not distinguish various kinds of failure, the error level on failure is usually 1.

Uses of the error level:

• It can be tested using && and ||; see also syntax.
• It can be tested using IF.
• The value can be accessed from ERRORLEVEL variable.

Examples:

• dir >NUL && echo Success
  • The part after && is executed only if the error level is zero.
• color 00 || echo Failure
  • The part after || is executed only if the error level is non-zero, whether positive or negative.
• color 00 || ( echo Failure)
  • Multi-line bracketing works as well.
• echo %ERRORLEVEL%
  • Displays the error level without changing it.
• if %errorlevel% equ 0 echo The error level is zero, meaning success.
• if %errorlevel% neq 0 echo The error level is non-zero, meaning failure.
• if errorlevel 1 echo The error level is >= 1, meaning failure via positive error level.
  • Does not cover failure via negative error level. Note the >= part: this is not the same as if %errorlevel% equ 1.
• exit /b 1
  • Returns a batch file, setting the error level to 1.
• cmd /c "exit /b 10"
  • In the middle of a batch file or on the command line, sets the error level to 10.
• (cmd /c "exit /b 0" && Echo Success) & (cmd /c "exit /b -1" || Echo Failure)
  • As above, showing the error level is indeed affected.
• (cmd /c "exit /b 0" & cmd /c "exit /b 1") || Echo Failure
  • The error level of a chain created by & is the error level of the last command of the chain.
• cmd /c "exit /b -1" & if not errorlevel 1 echo Would-be success
  • The if not errorlevel 1 test, which might appear to test for success, passes on negative numbers: it tests on "not error level >= 1", which is "error level <= 0".
• set myerrorlevel=%errorlevel%
  • Remembers the error level for later.
• set errorlevel=0
  • To be avoided: overshadows the built-in errorlevel variable. Ensures that subsequent accesses via %ERRORLEVEL% return 0 rather than the actual error level.
• cmd /c "exit /b 0"if 1 equ 1 ( cmd /c "exit /b 1" & echo %errorlevel% )
  • Displays 0, since %errorlevel% gets expanded before cmd /c "exit /b 1" gets executed.

Links:

• Error level at ss64

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