Re: where does command output go
On Sun, Nov 09, 2025 at 15:36:12 +0100, Nicolas George wrote:
> fxkl47BF@protonmail.com (HE12025-11-09):
> > i execute
> >
> > some_long_running_command | mail billy_bob@localhost
> >
> > where is the output of the command stored while it is running before it's mailed
>
> Under the responsibility of the mail command. Probably in memory.
This command is called a "pipeline", and it has two smaller commands
inside it. Both of the smaller commands run simultaneously, and the
output of the first command is connected to the input of the second
command via a pipe(2). For those who don't have the developer man
pages installed:
DESCRIPTION
pipe() creates a pipe, a unidirectional data channel that can be used
for interprocess communication. [...]
Data written to the write end of the pipe is buffered by the ker‐
nel until it is read from the read end of the pipe. For further de‐
tails, see pipe(7).
The first command is writing to the "write end of the pipe", and any
output it produces will be buffered in memory until it's read by
the second command, mail.
Depending on how quickly the mail command reads input, the amount of
data buffered in memory may be small, or large, but it will never
exceed the kernel's maximum allocated pipe buffer size. pipe(7) goes
into some detail about the "Pipe capacity" on various kernel versions.
If the amount of data buffered in the pipe reaches the kernel's limit,
the writing process will "block" (be put on pause, essentially) until
the reading process has consumed some data to make room for more.
In practice, the mail command probably reads its input very quickly,
so the amount of data held in the pipe will probably never reach the
limit. With other readers, this is not always going to be true.
There is no temporary file involved in the pipeline itself. The data
written by the first command is read in pieces as it's being written.
What the reader (mail) does with the data it reads will depend on how
it was implemented. The mail command may decide to hold all the input
in memory, or it may write it to a file. Different implementations of
mail(1) may make different decisions about this.
Once mail has received all of the input data, it will typically open
its own pipe(2) to /usr/sbin/sendmail, to which it will write a full
email message, including headers generated by the mail program, a
blank line, and finally the input that it received from the first
program.
>From that point, treatment of the message will depend on your local MTA
(whatever implements /usr/sbin/sendmail on your system). It's very
likely that your MTA will write the message to a file in what it calls a
"queue", which is full of messages pending delivery. When the message is
ultimately delivered by the local delivery agent, it will typically end up
in the recipient's "inbox", which could be a single file or a directory
full of files, depending on how you've configured your mail system.
Or, the recipient may have configured it so that the incoming message
is piped through a program, or forwarded to an outside email address,
or simply discarded. There are lots of possibilities.
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