32

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.

1

4 Answers 4

27

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.

9
  • 3
    After pressing F or G on pipe input less not only does a blocking read, but does it in a loop waiting for EOF. And an EOF on a pipe happens only if its other side is closed.
    – mik
    Feb 12, 2015 at 22:29
  • 3
    If less would update screen in that loop, there would not be a problem. Blocking read has little to do with this issue.
    – mik
    Feb 12, 2015 at 22:46
  • 1
    @Flow this is not a hang the question was about, but merely a wait which is expected in this situation - a wait for more data in the file when EOF is reached (which will not happen for a closed pipe, by the way), or for an interrupt to exit the follow mode
    – mik
    Jun 27, 2016 at 9:21
  • 1
    @PiotrDobrogost with blocking read less wouldn't be able to update screen, if there is no data; when some data appear, blocking read will return it, and less would be able to update screen without a separate thread
    – mik
    Jan 19, 2017 at 13:11
  • 1
    @PiotrDobrogost yes, it would block waiting for new data or EOF, but my point is that the screen does not need to be updated during this waiting (just because there is no new data)
    – mik
    Jan 21, 2017 at 11:44
10

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.


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

11
  • The ESC-G command was introduced in version 466, released on 23 Aug 2014.
    – mik
    Feb 12, 2015 at 23:02
  • @mik Looks like an error in release notes for version 471 then. Thanks, fixed. Feb 12, 2015 at 23:11
  • Not an error, they just list the changes incrementally since a stable release, version 458 in this case. However, there is no stable release with the ESC-G command.
    – mik
    Feb 12, 2015 at 23:56
  • The ESC-G command is now in a stable release (481): "16 Oct 2015 less-481 has been released for general use".
    – mik
    Feb 18, 2016 at 16:41
  • Update: Regarding the F command on pipes, this is also fixed in less-474. Instead of seeking to EOF, the F command seeks to the end of buffered input and starts reading there. However it's not really usable because when you hit ctrl-C to stop the F command, it kills the process producing output. I'm not sure how to fix that. – Mark Nudelman, maintainer of less May 8, 2017 at 13:57
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.

1
  • 1
    This command behaves differently when used with pipe as described by @mik and is clearly not what OP is looking for. Jan 29, 2015 at 15:41
2

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).

2
  • 1
    Just to clarify: since version 581 (released on 18 Apr 2021 for general use) ctrl+X can be used to exit F mode.
    – mik
    Feb 4 at 22:08
  • Thanks. Yes, newer versions of less have ctrl-X, though many machines are still running older versions of less. I will edit my answer to mention this. Mar 19 at 15:15

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.