sendmail, 4th Edition

Delivery Via Programs

When any of the addresses to the right of a colon in the alias list begin with a | character, delivery is made by piping the mail message through a program. This is automatic with modern configuration files.^[200] Beginning with V8.7 sendmail, any delivery agent for which the F=| flag (F=| (vertical bar) on page 765) is set can also pipe messages through programs. To disable this ability, simply remove the F=| flag from all delivery agent declarations in your configuration file.

The forms that a program address can legally take in the aliases(5) file (or ~/.forward file; see Piping Through Programs on page 504) are as follows:

|prg
"|prg args"
|"prg args"

Here, prg is the full path of the program to be run (the environment variable PATH is not available). If command-line arguments are needed for the program, they must follow prg, and the entire expression must be quoted. The leading full quotation mark can either precede or follow the |. If the address is quoted with full quotation marks, the leading quotation mark is ignored in determining the leading | character.

To execute the program, sendmail executes the command in the P= equate of the prog delivery agent. That command is usually one of the following:

/bin/sh -c
/usr/bin/smrsh -c

These tell sendmail to run /bin/sh (the Bourne shell) or /usr/bin/smrsh (the sendmail restricted shell) to execute the program specified by prg. The -c tells that shell to take any arguments that follow and execute them as though they ...