Errorlevels

The correct name for errorlevels would be return codes.
But since the DOS command to determine the return code is IF ERRORLEVEL, most people use the name errorlevel.

Errorlevels are not a standard feature of every command.
A certain errorlevel may mean anything the programmer wanted it to.
Most programmers agree that an errorlevel 0 means the command executed successfully, and an errorlevel 1 or higher usually spells trouble.
But there are many exceptions to this general rule.

IF ERRORLEVEL construction has one strange feature, that can be used to our advantage: it returns TRUE if the return code was equal to or higher than the specified errorlevel.

This means most of the time we only need to check IF ERRORLEVEL 1 ... and this will return TRUE for every positive, non-zero return code.
Likewise, IF NOT ERRORLEVEL 0 ... will return TRUE for every negative, non-zero return code.

In COMMAND.COM (MS-DOS, DOS-box, Windows 9*/ME), to determine the exact return code the previous command returned, we could use a construction like this:

@ECHO OFF
IF ERRORLEVEL 1 SET ERRORLEV=1
IF ERRORLEVEL 2 SET ERRORLEV=2
IF ERRORLEVEL 3 SET ERRORLEV=3
IF ERRORLEVEL 4 SET ERRORLEV=4
 •
 •
 •
IF ERRORLEVEL 254 SET ERRORLEV=254
IF ERRORLEVEL 255 SET ERRORLEV=255
ECHO ERRORLEVEL = %ERRORLEV%

This is perfectly OK if we only have to check, say, 15 consecutive errorlevels.
If we need to check every errorlevel, though, there are better alternatives.

In DOS (COMMAND.COM), we can use FOR loops to determine the errorlevel:

@ECHO OFF
REM Reset variables
FOR %%A IN (1 10 100) DO SET ERR%%A=
REM Check error level hundredfolds
FOR %%A IN (0 1 2) DO IF ERRORLEVEL %%A00 SET ERR100=%%A
IF %ERR100%==2 GOTO 200
IF %ERR100%==0 IF NOT "%1"=="/0" SET ERR100=
REM Check error level tenfolds
FOR %%A IN (0 1 2 3 4 5 6 7 8 9) DO IF ERRORLEVEL %ERR100%%%A0 SET ERR10=%%A
IF "%ERR100%"=="" IF %ERR10%==0 SET ERR10=
:1
REM Check error level units
FOR %%A IN (0 1 2 3 4 5) DO IF ERRORLEVEL %ERR100%%ERR10%%%A SET ERR1=%%A
REM Modification necessary for errorlevels 250+
IF NOT ERRORLEVEL 250 FOR %%A IN (6 7 8 9) DO IF ERRORLEVEL %ERR100%%ERR10%%%A SET ERR1=%%A
GOTO End
:200
REM In case of error levels over 200 both
REM tenfolds and units are limited to 5
REM since the highest DOS error level is 255
FOR %%A IN (0 1 2 3 4 5) DO IF ERRORLEVEL 2%%A0 SET ERR10=%%A
IF ERR10==5 FOR %%A IN (0 1 2 3 4 5) DO IF ERRORLEVEL 25%%A SET ERR1=%%A
IF NOT ERR10==5 GOTO 1
:End
REM Clean up the mess and show results
SET ERRORLEV=%ERR100%%ERR10%%ERR1%
FOR %%A IN (1 10 100) DO SET ERR%%A=
ECHO ERRORLEVEL %ERRORLEV%

This example still handles only 255 error levels (that's all there is in DOS), but it can be easily adjusted once you understand the basic principles.

To check errorlevels during batch file development, use either COMMAND /Z yourbatch.bat to display the errorlevel of every command executed in MS-DOS 7.* (Windows 95/98), or PROMPT Errorlevel$Q$R$_$P$G in OS/2 Warp (DOS) sessions.

Setting errorlevels

MS-DOS & Windows 9x:

Use ERRORLVL.EXE from OzWoz Software, or SETERLEV.COM 1.0 from Jim Elliott to test batch files that (are supposed to) check on errorlevels.
The syntax couldn't be simpler:

ERRORLVL number

or

SETERLEV number

where number can be any number from 0 to 255.

A small Kix "one liner" can be used too:

EXIT $ErrLev

If called by a batch like this:

KIX32 ERRORLEVEL.KIX $ErrLev=23

it will return an errorlevel 23 (ERRORLEVEL.KIX would be the name of the kix script mentioned above).

Or use CHOICE.COM, available in all DOS 6.* and up versions, to set an errorlevel:

ECHO 5 | CHOICE /C:1234567890 /N

and

ECHO E | CHOICE /C:ABCDEFGHIJ /N

will both result in errorlevel 5 since both 5 and E are the fifth choice (/C:...) in the corresponding CHOICE commands.

Windows NT 4 and later:

In NT 4 use either

COLOR 00

or

VERIFY OTHER 2> NUL

to set an errorlevel 1.

Windows 2000 and later:

In Windows 2000 and later the EXIT command accepts an optional exit code, a.k.a. return code or errorlevel, e.g. EXIT /B 1 for errorlevel 1:

EXIT

Quits the CMD.EXE program (command interpreter) or the current batch script.

EXIT [ /B ] [ exitCode ]

[ Brought to my attention by Maor Conforti. Thanks ]

If you want to set an errorlevel inside a batch file, for example to test an external command used by that batch file, you can use CMD.EXE /D /K EXIT 6 to set errorlevel 6 and continue.
CMD's /D switch disables any command processor AutoRun commands you may have set on your computer (you can check this by looking for HKEY_LOCAL_MACHINE\Software\Microsoft\Command Processor\AutoRun in the registry).
Do NOT use SET ErrorLevel=6 as this will render the Errorlevel variable static (SET ErrorLevel= will restore it to a dynamic variable again).

[ Addition of /D to disable AutoRun by Isidro. Thanks ]