Bash howto write/read to/from a named pipe without aborting after first sending

Question

GIVEN:

Bash command line (Terminal 1):

 > mkfifo pipo
 > cat pipo

Bash command line (Terminal 2):

 > echo -e "Hello World\nHi" > pipo

RESULT:

The bash in (Terminal 1) prints:

 Hello World
 Hi

and aborts.

QUESTION:

How can I achieve that it does not abort, but allows to send another echo through pipo?

pynexj · Accepted Answer · 2023-06-29T15:25:30.610

6

It's because echo ... > fifo opens and then closes the fifo. As a workaround you can do like this:

# open for writing
exec 20> fifo
echo foo >&20
echo bar >&20
...
# to close it
exec 20>&-

A bit explanation:

exec 20> fifo opens fifo for writing with FD (file descriptor) 20.
command >&20 redirects the output to FD 20.
exec 20>&- closes FD 20.

The following are excerpts from man bash:

exec [-cl] [-a name] [command [arguments]]

[...] If command is not specified, any redirections take effect in the current shell, and the return status is 0. If there is a redirection error, the return status is 1.

[n]>word

Redirection of output causes the file whose name results from the expansion of word to be opened for writing on file descriptor n, or the standard output (file descriptor 1) if n is not specified. If the file does not exist it is created; if it does exist it is truncated to zero size. [...]

[n]>&word

[...] If word evaluates to -, file descriptor n is closed. [...]

edited Jun 29 '23 at 15:25

answered Feb 18 '21 at 13:43

pynexj

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Would you, please, elaborate on the solution? What does ``exec 20> fifo`` do, for example? – Frank-Rene Schäfer Feb 18 '21 at 20:22
updated the answer. there's quite a lot about this. `man bash` for [detailed doc](https://man7.org/linux/man-pages/man1/bash.1.html). – pynexj Feb 19 '21 at 01:37
Does file descriptor 20 have a special meaning? – Frank-Rene Schäfer Feb 19 '21 at 09:22
20 is not special. usually the shell only uses 0 (stdin), 1 (stdout) and 2 (stderr). – pynexj Feb 19 '21 at 10:28

score 1 · Answer 2 · answered Feb 18 '21 at 18:54

1

In Terminal 1 do

tail -f pipo

instead of cat pipo.

answered Feb 18 '21 at 18:54

pjh

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Bash howto write/read to/from a named pipe without aborting after first sending

2 Answers2