47
votes
Accepted
Linux named pipes: not as FIFO as thought
This has nothing to do with FIFO semantics of pipes, and doesn’t prove anything about them either way. It has to do with the fact that FIFOs block on opening until they are opened for both writing and ...
- 423k
33
votes
Linux named pipes: not as FIFO as thought
Pipes are first-in first-out. Your problem is that you misunderstand when the “in” happens. The “in” event is writing, not opening.
Removing useless punctuation, your code is:
echo a > fifo & ...
32
votes
Accepted
Are the named pipe created by `mknod` and the FIFO created by `mkfifo` equivalent?
Yes, it's equivalent, but obviously only if you tell mknod to actually create a FIFO, and not a block or character device (rarely done these days as devtmpfs/udev does it for you).
mkfifo foobar
# ...
- 48.3k
25
votes
Accepted
Prevent automatic EOFs to a named pipe, and send an EOF when I want it
As others have indicated, the reader of a pipe receives EOF once there are no writers left. So the solution is to make sure there is always one writer holding it open. That writer doesn't have to send ...
- 71.3k
25
votes
Accepted
How does a FIFO (named pipe) differs from a regular pipe (unnamed pipe)?
"Named pipe" is actually a very accurate name for what it is — it is just like a regular pipe, except that it has a name (on a filesystem).
A pipe — the regular, un-named ("anonymous") one used in ...
- 109k
17
votes
A virtual file containing the concatenation of other files
wrote a fuse driver today, if someone is interested in the fuse solution
(the device mapper as well as the nbd-solution above will create block devices not regular files - which will break, if you ...
- 181
16
votes
Accepted
“Leaky” pipes in linux
Easiest way would be to pipe through some program which sets nonblocking output.
Here is simple perl oneliner (which you can save as leakybuffer) which does so:
so your a | b becomes:
a | perl -...
- 3,071
16
votes
Accepted
How to get an average pipe flow speed
With pv 1.2.0 (December 2010) and above, it's with the -a option:
Here with both current and average, line-based:
$ find / 2> /dev/null | pv -ral > /dev/null
[6.28k/s] [70.1k/s]
With 1.3.8 (...
- 534k
16
votes
Accepted
Change buffer size of named pipe
A fifo file is just a type of file which when opened for both reading and writing instantiates a pipe like a pipe() system call would.
On Linux at least, the data that transits though that pipe is ...
- 534k
14
votes
Accepted
Named pipes, file descriptors and EOF
It has to do with the closing of the file descriptor.
In your first example, echo writes to its standard output stream which the shell opens to connect it with f, and when it terminates, its ...
- 327k
13
votes
Accepted
Under what conditions exactly does SIGPIPE happen?
Your example is using a fifo not a pipe, so is subject to fifo(7). pipe(7) also tells:
A FIFO (short for First In First Out) has a name within the
filesystem (created using mkfifo(3)), and ...
- 33.5k
12
votes
Accepted
Reading a named pipe: tail or cat?
When you do:
cat fifo
Assuming no other process has opened the fifo for writing yet, cat will block on the open() system call. When another process opens the file for writing, a pipe will be ...
- 534k
12
votes
What is the purpose of using a FIFO vs a temporary file or a pipe?
APUE says “FIFOs can be used to duplicate an output stream”, it doesn’t say that FIFOs actually duplicate the output stream. As you point out, the duplication is done by tee in the example.
mkfifo ...
- 423k
11
votes
Accepted
Program output redirection
This supposed program will write to file descriptor number you specified.
consider the following hello world program:
#include <stdio.h>
main()
{
ssize_t i = 0 ;
printf ("hello world\n") ...
- 31.4k
11
votes
Accepted
How to implement "generators" like $RANDOM?
ksh93 has disciplines which are typically used for this kind of thing. With zsh, you could hijack the dynamic named directory feature:
Define for instance:
zsh_directory_name() {
case $1 in
(n)...
- 534k
11
votes
Accepted
Write to FIFO only if it exists
According to the BASH manual:
-p file
True if file exists and is a named pipe (FIFO).
So:
if [[ -p /tmp/my_fifo ]]; then
# commands to execute
fi
The question has the tag, bash. In context, ...
- 15.7k
11
votes
Accepted
How to save an output of airodump-ng to a file?
Check
man airodump-ng.
You want the -w option.
airodump-ng -w myOutput --output-format csv mon0
Generates a .csv file of the screendump with the output from airodump-ng one line per station.
- 4,718
11
votes
Accepted
How to cat named pipe without waiting
To prevent cat from hanging in the absence of any writer (in which case it's the opening of the fifo, not reading from it, that hangs), you can do:
cat 0<> "$my_named_pipe" <"$my_named_pipe"
...
- 534k
10
votes
Accepted
Why doesn't file redirection to and from a named pipe work, but piping to cat does?
This is because the netcat command has not even started yet! The shell when trying to open the fifo for input will block. Try
strace cat >fifo <fifo
and you will see nothing. Instead use, ...
- 50.5k
10
votes
Program output redirection
The numbers represent file descriptors (handles to files that have been opened).
The shell usually has 3 set automatically,
0 - stdin
1 - stdout
2 - stderr
But further files can be opened, and the ...
- 10.4k
9
votes
Accepted
How to read a webcam that is already used by a background capture?
Do this:
sudo modprobe v4l2loopback devices=1
If you get similar error like modprobe: FATAL: Module v4l2loopback not found in directory /lib/modules/4.6.0-kali1-amd64, just install v4l2loopback-dkms ...
- 5,016
9
votes
Can I increase the system pipe buffer max?
Your command changes the maximum buffer size, not the default one.
From the pipe(7) manpage:
/proc/sys/fs/pipe-max-size (since Linux 2.6.35)
The maximum size (in bytes) of individual pipes ...
- 17.6k
8
votes
Program output redirection
Those numbers are file descriptors. As you noted, there are several that are created automatically. As other files or file-like things are opened, they will get other numbers.
The numbers that are ...
- 18.3k
8
votes
Read named pipe once every time input is written
In your case, you can do simply:
tail -f $pipe | sh &
no need for loops.
- 975
8
votes
Named pipes, file descriptors and EOF
There's not much to it: when there are no writers to the pipe, it looks closed to readers, i.e. returns EOF when read and blocks when opened.
From the Linux man page (pipe(7), but see also fifo(7)):
...
- 135k
8
votes
Accepted
Split output and rejoin again with named pipes on linux
mkfifo thepipe
cmd3 <( cmd1 | tee thepipe ) <( cmd2 thepipe )
This uses a named pipe, thepipe, to transfer data between tee and cmd2.
Using your diagram:
cmd1 ---(tee)---(thepipe)--- cmd2 ---&...
- 327k
Only top scored, non community-wiki answers of a minimum length are eligible