SSH output isn't line buffered?

Question

I'm running a script on a remote machine like this:

ssh $host "pip install -r /path/to/requirements.txt"

But the output isn't line buffered; instead of seeing one line returned at a time, all the lines (~10) are all printed at once as the connection terminates.

What's up with this? Is there any way to force them to be line buffered?

(also, to state the obvious: when I ssh into $host and run the command “manually”, the output is line buffered, as expected)

score 54 · Accepted Answer · answered Oct 03 '11 at 02:55

54

Use ssh -t ... to force a pseudo-tty allocation (which is what you get when you log in normally via ssh.)

answered Oct 03 '11 at 02:55

Ryan Fox

656

didn't help, unfortunately. This also might be problem with MTU, but neither of it helped – Nick Roz Sep 09 '16 at 23:48
See Magnus' comment below ref using -tt - that worked for me when -t didn't. – Tom Dalton Mar 26 '18 at 13:35

score 25 · Answer 2 · edited Oct 03 '11 at 10:59

25

To expand a little bit on Ryan Fox's answer: Many programs (most? - it's the default for any C program) line-buffer stdout when they're talking to a terminal, but fully buffer it otherwise. (The C standard specifies that stdout is initially fully buffered when it "can be determined not to refer to an interactive device".)

So what you're seeing is that the output of the program you're running remotely (as given to stdout) isn't line-buffered; ssh is just passing through what it gets when it gets it. (I think ssh actually does no buffering at all on its output - that would be the least magical way to make sure that the user sees what the remote program intended.)

edited Oct 03 '11 at 10:59

David Wolever

4,854

answered Oct 03 '11 at 03:25

Dave Vandervies

251

An anonymous user suggests that stdout is fully buffered, not line buffered – Michael Mrozek Oct 03 '11 at 03:53
Dave Vandervies has told me out-of-band that he was the one who made the edit, and that it is correct (ie, stdout is initially fully buffered, not line buffered). – David Wolever Oct 03 '11 at 06:05

score 9 · Answer 3 · edited Feb 23 '19 at 04:30

9

To expand even more on Ryan Fox's answer, ssh -t didn't work for me either, but ssh -tt did. See the ssh man page about -t:

Multiple -t options force tty allocation, even if ssh has no local tty

edited Feb 23 '19 at 04:30

Prvt_Yadav

5,882

answered Nov 23 '17 at 08:06

Magnus Berg

91

SSH output isn't line buffered?

3 Answers3

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