'read' not timing out when reading from pipe in bash

Question

I create a pipe using

mkfifo /tmp/foo.pipe

Now, I want to try reading from the pipe for a maximum of 2 seconds, so I execute

read -t 2 line < /tmp/foo.pipe

The timeout does not occur. Read just sits there waiting for input from the pipe.

The manuals say that 'read' is supposed to work with named pipes. Does anyone have an idea why this is happening?

ls -al /tmp/foo.pipe
prw-r----- 1 foo bar 0 Jun 22 19:06 /tmp/foo.pipe

Welcome to Stack Overflow! If you accepted answers to your questions by clicking the green check mark next to the best answer (if there is one), people would be more likely to give answers to your questions. — Adam Rosenfield, Jun 23 '11 at 02:25

score 18 · Accepted Answer · edited May 23 '17 at 12:10

18

Your shell is blocking on the open() call before invoking the read builtin.

On Linux, you can open the FIFO for both read and write at the same time to prevent blocking on open; this is non-portable, but may do what you want.

read -t 2 <>/tmp/foo.pipe

Adapted from: Bash script with non-blocking read

edited May 23 '17 at 12:10

Community

1
1

answered Jun 23 '11 at 02:35

MZS

573
4
12

score 1 · Answer 2 · answered Nov 08 '12 at 19:35

1

If you just want to flush (and discard) the data from the FIFO:

dd iflag=nonblock if=/tmp/foo.pipe of=/dev/null &> /dev/null

answered Nov 08 '12 at 19:35

Ken A

371
2
4

score 0 · Answer 3 · answered Jun 23 '11 at 02:26

0

Your shell is the one that is holding it up, it is attempting to read from the pipe to feed the data into the read command, and since it is not getting anything it just sits there waiting.

answered Jun 23 '11 at 02:26

X-Istence

16,324
6
57
74

score -1 · Answer 4 · edited Jul 31 '12 at 01:04

-1

TMOUT=2
read line < /tmp/foo.pipe

edited Jul 31 '12 at 01:04

DavidO

13,812
3
38
66

answered Oct 20 '11 at 00:07

Marti

1

That just sets the default timeout, it doesn't change the way `read` actually implements the timeout. – Barmar Nov 09 '12 at 02:07

'read' not timing out when reading from pipe in bash

4 Answers4