Follow a pipe using less?

Question

Can less follow (by pressing F) a piped input (similarly to a file)? For a file that is being written to, the command

less <file>

will follow the file when pressing F.

But if I have a command that pipes output directly into less, like this

command | less

pressing F will do nothing.

So it looks like pipes cannot be followed like files can? Or maybe it has to do with command also writing to STDERR? The effect I'm trying to achieve is always see the latest output of the command: just like keeping PageDown pressed!

A related remark holds for G (go to end): when piping directly to less, it won't work.

Answer 1

Pressing F or G makes less try to reach input EOF. If the input is a pipe, less hangs until the pipe is closed on the other side (and not "does nothing").

This can be worked around by saving the command output to a temporary file in the background, and then by using it as input for less:

command > /tmp/x &
less +F /tmp/x; kill %; rm /tmp/x

There is no option to do this in less only; however, I admit it would be useful.

Answer 2

Can less follow (by pressing F) a piped input (similarly to a file)?

Yes, starting with version 474.

Related: Is there any way to exit “less” follow mode without stopping other processes in pipe?

For reference, the issue with F not working with pipes has reference number 300 on the list of known bugs and is titled The F command does not work on piped input.

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A related remark holds for G (go to end): when piping directly to less, it won't work.

It works starting from version 466. Citing from release notes for this version:

New command ESC-G goes to end of currently buffered data in a pipe

Answer 3

From the less man page

[Keyboard] COMMANDS [...]

   F      Scroll  forward, and keep trying to read when the end of file is reached.  Normally this command would be used when already
          at the end of the file.  It is a way to monitor the tail of a file which is growing while it is being viewed.  (The  behav‐
          ior is similar to the "tail -f" command.)

so this should work, and it actually works for me.

Answer 4

For older versions of less (many distros still do not have v581 or later as default) you can use the F-key ("Follow") on a pipe in less as if you were performing less on a file directly by using this one-liner (no tmp files):

The problem is that pressing ctrl+c sends a SIGINT to all commands in the pipeline, aborting not only your less-follow, but your generating command as well. To work around this I usually put the generating command in a subshell and trap SIGINT there. Like this:

(trap '' SIGINT; command) | less

The parenthesis give you a subshell and trap with empty command '' is a special invocation that throws away the signal entirely within that subshell, without propagating it to the currently executing command. SIGINT is the ctrl+c signal. Now you can hit ctrl+c and only less will receive it, letting you use less on a pipe just like you would on a log-file or other growing file. Once you quit less (by pressing q) the pipeline is broken and the generating command gets sent a SIGPIPE, which almost always has the same effect as being sent a SIGINT, but the trap isn't set up to catch it.

Newer versions of less from v581 and onwards allow you to use ctrl+x in place of ctrl+c to abort the follow command (F-key).