Ygtjki

What happens in the following two examples? Why is only the second command in each pipeline executed? How does the pipe work here?

example 1

$ cat /var/log/messages | date
Mon Apr 8 17:10:02 IST 2019

example 2

$ cat /var/log/messages | w
17:10:34 up 8 days, 1:24, 6 users, load average: 0.63, 0.49, 0.61

Both commands in both examples are executed. Only the output of the command after the pipe symbol is actually shown.

In your pipeline, the contents of the /var/log/messages file is piped into the input of the date command by running cat on the left hand side of the pipeline.

The date command does not care about its standard input stream and will ignore any data arriving there. It will, however, produce its own output, which it does on its standard output stream.

The net visible result is that the output of the cat is ignored and discarded, while the output of date is show in the terminal (or wherever the output of the pipeline goes).

What actually happens with the output of the cat command is that, since date won't read it, the cat command is temporarily blocked, waiting for its output to be read (after successfully outputting however much the pipe's buffer can hold; it does not know date will never read it). The date command does its thing and outputs its string, after which it terminates.

At that point, when date terminates and the standard input stream of date is closed, the cat command receives a PIPE signal by the shell. This signal tells cat that whatever data it's trying to write to its standard output is never going to be read, and it too terminates (terminating on PIPE is the default action for this signal). The rest of the file that cat was reading from is never read, and the data in the pipe's buffer is discarded when the shell releases the memory associated with it.

The exact same thing happens with date replaced by w, or any other command that does not read its standard input stream.

You can compare this with a using a command that actually does read its standard input stream:

cat /var/log/messages | tr '[:lower:]' '[:upper:]'

or, without the pipe (as the cat is actually not needed in any of these examples),

tr '[:lower:]' '[:upper:]' </var/log/messages

That's how pipes work: The left hand side command's output is send as input into the right hand side command.

If you want to run two commands one after another, use a semicolon:

cat /var/log/messages ; date

If you want to run the second command only if the first one is successful, use &&:

cat /var/log/messages && date

If you want to run the second command only if the first one fails, use ||:

cat /var/log/messages || date

If you want to run the first command in the background and run the second one at the same time, use &

cat /var/log/messages & date