The -b switch tells sendmail in what mode to operate. For example, sendmail can "become" a daemon listening for incoming SMTP connections, or it can run in a mode that tells it to simply print the contents of the queue and exit. The form of the -b switch is:

-bmode

If mode is more than a single letter, all but the first letter is silently ignored. If the mode is missing or not one of those allowed, sendmail prints the following error message and exits:

Invalid operation mode bad letter here

If the -b command-line switch is omitted altogether, the default mode becomes -bm (deliver mail and exit).

Beginning with V8.7 sendmail, the letter that selected the operating mode is assigned at startup to the ${opMode} sendmail macro (${opMode}).