The ! character can be used to create a pipeline between an MDB command and the user's shell. If the $SHELL environment variable is set, MDB will fork and exec this program for shell escapes; otherwise /bin/sh is used. The shell is invoked with the -c option followed by a string formed by concatenating the words after the ! character.
The ! character takes precedence over all other metacharacters, except semicolon (;) and NEWLINE. After a shell escape is detected, the remaining characters up to the next semicolon or NEWLINE are passed “as is” to the shell. The output of shell commands cannot be piped to MDB dcmds. Commands executed by a shell escape have their output sent directly to the terminal, not to MDB.