sendmail, 4th Edition

Piping Through Programs

The ~/.forward file can contain the names of programs to run. A program name is indicated by a leading pipe (|) character, which might or might not be quoted (Delivery Via Programs on page 468). For example, a user might be away on a trip and want mail to be handled by the vacation(1) program:

\user, "|/usr/ucb/vacation user"

Recall that prefixing a local address with a backslash tells sendmail to skip additional alias transformations. For \user, this causes sendmail to deliver the message (via the local delivery agent) directly to the user’s spool mailbox.

The quotes around the vacation program are necessary to prevent the program and its single argument (user) from being viewed as two separate addresses. The vacation program is run with the command-line argument user, and the mail message is given to it via its standard input.

Beginning with V8 sendmail, a user must have a valid shell to run programs from the ~/.forward file and to write files via the ~/.forward file. See The /etc/shells File on page 180 for a description of this process and for methods to circumvent it at the system level.

Because sendmail sorts all addresses and deletes duplicates before delivering to any of them, it is important that programs in ~/.forward files be unique. Consider a program that doesn’t take an argument and suppose that two users both specified that program in their ~/.forward files:

user 1 →  \user1, "|/bin/notify"
user 2 →  \user2, "|/bin/notify"

Prior to V8 sendmail, when ...