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Questions tagged [file-descriptors]

The tag has no usage guidance.

1
vote
2answers
36 views

Flush the pipe/printf buffers externally for already running process with known PID

I am writing a data logging app, all programs are started like: ./program > out.bin The data collector periodically pools the stdout output files and reads the data. The issue is that the IO ...
1
vote
1answer
32 views

How to source inline text

When you are creating tutorials online for people to use in a single terminal sometimes it's nice to source inline code...however none of these worked as I would expect. I would expect foo to be "bar" ...
0
votes
1answer
13 views

Process substitution to script in order to monitor iotop

I am actively trying to get the output of the iotop for monitoring. As you know, the iotop is interactive, meaning that once started, it'll provide live updates on disk usage on the system. I'm ...
-1
votes
1answer
28 views

hive + what is the meaning of nofile and nproc

regarding to hive in hadoop cluster what is the meaning of the following hive - nofile 30000 hive - nproc 18000 the file: more /etc/security/limits.d/hive.conf # Licensed to the Apache ...
-3
votes
0answers
36 views

how to share stdout between two C- and bash- processes

i need to create a pipe run the child process(exec to bash) and write some data into this pipe(from bash-script) and read data from another end of pipe in parenting C-process later i can't use pipe ...
0
votes
1answer
49 views

Difference between file descriptor and soft/hard link

If soft link and hard link are pointer to a file. And file descriptor is a file handle pointing to an open file. Then what exact difference is between the two if both of them are a kind of pointer ...
0
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0answers
12 views

Locale does not work with “file” command output

I have some JPEG files with Description metadata I added in MAC OS. exiftool (in MAC) displays them in correct locale. I copied those files to a Raspbian OS. From command line or bash script ...
0
votes
2answers
113 views

What happens when redirecting with 3>&1 1>/dev/null?

I'm playing around with file descriptors to better grok them and I'm failing to understand the following. $ grep "..." 3>&1 1>/dev/null 1 12 13 123 321 3 The above does not show in the ...
-1
votes
0answers
23 views

Read and write in tun/tap interface

I am using a tap interface. I will give a brief of what I am trying to do. I created a tap interface. Running a thread to listen for the data sent by the applications. When any applications send ...
2
votes
1answer
72 views

Understanding pipes and redirections in dash

Someone asked how to pass the output of two commands as files to another command and they got the answer below. ( cmd1 | ( cmd2 | ( main_command /dev/fd/3 /dev/fd/4 ) 4<&0 ) 3<&0 ) I ...
1
vote
1answer
87 views

Understanding File Streams and File Descriptors

For extra context, I asked this question earlier, and thought I understood things, but not anymore. I know that echo ignores stdin. I know that stdin, stderr and stdout exist and, in this context, I ...
1
vote
2answers
297 views

exec command in bash loop for webscrapping

Here is a simple script which is curling https://unix.stackexchange.com/ and storing the result into an array, which is working fine. #!/usr/local/bin/bash [ -f pgtoscrap ] && { rm pgtoscrap; ...
1
vote
1answer
69 views

File Descriptors and Redirection

Several resources state the file descriptor 0 is connected to the keyboard. I have learned that echo does not receive input from stdin (or 0). This feels contradictory to me. When I run echo foo I'm ...
1
vote
1answer
34 views

Redirection to named file descriptor buggy inside functions

Bash has a feature where it will pick a file descriptor number for you and assign it to a variable. $ exec {fd}>foo.txt $ echo "$fd" 11 $ echo "bar" >&$fd $ cat foo.txt bar This feature ...
1
vote
1answer
66 views

Why do I need 'exec' if 'while read' can open a 'File Descriptor'

I want to read a file line by line using bash, tail and using a diferent file descriptor. all the guides use this method: method 1: echo 1111111 > z.txt exec {newFD}< <(tail -f -c +0 "z.txt"...
0
votes
0answers
20 views

Monitoring data flow to /dev/null for a given process

I have a process which reads out data from a custom PCI-e device using a custom driver. The program takes a file descriptor as an argument and writes the raw data to it. I would like to test the data ...
0
votes
1answer
72 views

Maximum value for /proc/sys/fs/file-max

Today my server crashed with Too many open files in system: but I already have high values, what is a maximum value for this? I feel like there aren't good results on google for this, explaining ...
2
votes
1answer
75 views

concurrent processing in bash using process expansion, and redirection

Apart from possible races that have to be controlled by using proper synchronisation, it's possible in bash to feed a data source concurrently into multiple pipelines and collect all their outputs ...
3
votes
3answers
125 views

tee into different variables

From the bash code command1 | tee >(command2) | command3 I want to capture the output of command2 in var2 and the output of command3 in var3. command1 is I/O-bound and the other commands are ...
0
votes
0answers
31 views

Is a mmaped file considered an open file in linux/unix?

Hi I feel confused about open file and mmapped file. I know that if a file is accessed using system call such as read/write will return a file descriptor, and be considered as an open file? How about ...
0
votes
1answer
57 views

Why is my files “last modified” timestamp only showing the year with the 'ls' command? [duplicate]

I'm building a java service that modifies the "Last Modified" field of some of the files it works with. The service keeps copies of files in a folder, it periodically checks a database for changes, ...
2
votes
1answer
148 views

lsof versus /proc/$PID/fd versus ulimit -n

I am trying to find the reason why my long-running app sometimes busts the maximum open file descriptor limit (ulimit -n). I would like to periodically log how many file descriptors the app has open ...
1
vote
0answers
146 views

Listen in to data read from file descriptor to ttyS

I have a serial device under /dev/ttyS0 and an application that is reading from the serial device (but not writing to it). I wanted to listen in to the data that is coming in on the serial device but ...
4
votes
2answers
123 views

How to periodically check if STDIN is open?

Some programs will exit when their STDIN is closed. Those that do this work nicely with Erlang/Elixir supervision via "ports". For those that don't, the Elixir docs suggest this wrapper script: #!/...
5
votes
2answers
600 views

Safest way to force close a file descriptor

Sometimes you need to unmount a filesystem or detach a loop device but it is busy because of open file descriptors, perhaps because of a smb server process. To force the unmount, you can kill the ...
0
votes
1answer
91 views

Multiple fifos over SSH

Shell script allow multiple file descriptors (up to 9 for POSIX compliance, but even more for bash): $ install -m 755 /dev/stdin /tmp/test <<\EOF #!/bin/sh echo hi1 >&1 echo hi2 >&...
2
votes
1answer
31 views

Why the output of ls is different for the same directory /dev/fd/ based on the method of referencing this directory?

cd /dev/fd/; ls Gives me output that I expect: 0 1 2 255 But: ls /dev/fd/ Gives different output that I don't expect: 0 1 2 3 Why? What happens here? What happened to file descriptor 255? ...
0
votes
0answers
232 views

Cannot pass argument through /proc/<pid>/fd/0

I have written a basic C++ program as such: #include <sstream> #include <unistd.h> #include <cstdio> #include <iostream> int main(void) { int foo; std::cin >> foo; ...
0
votes
2answers
77 views

STDOUT + STDERR output … is there any difference between considering the output to be an empty string vs NULL

I'm writing some application code that is used to execute Linux shell commands, and it then logs the command details into an SQL database. This includes the output of STDOUT + STDERR (separately). ...
4
votes
1answer
280 views

Testing if a file descriptor is valid (for input)

In the question "Testing if a file descriptor is valid", a test is sought for testing whether a file descriptor is opened or not. The answers all focus on testing whether the file descriptor is ...
1
vote
1answer
38 views

A command wants file paths. How can I give it stdin for the “infile” and stdout for the “outfile”?

Consider a command like foo -in /path/to/infile -out /path/to/outfile, that you'd like to pass strings to rather than using temp files. It may be called a lot causing much disk access, or is related ...
10
votes
4answers
2k views

Temporary folder that automatically destroyed after process exit

Can we use temporary folders like temporary files TMP=$(mktemp ... ) exec 3<>$TMP rm $TMP cat <&3 which will be destroyed automatically after this shell exit?
5
votes
2answers
241 views

Why doesn't sed -i work with /proc/PID/fd/FD

I have this file and fd: exec 88<>abc Why $ sed -i "s/cd/II/g" /proc/$$/fd/88 sed: couldn't open temporary file /proc/26194/fd/sedS1D1FT: No such file or directory but this work: $ cat /...
4
votes
2answers
999 views

Difference between ulimit, lsof, cat /proc/sys/fs/file-max

I've been getting java.io.IOException: Too many open files while running a Kafka instance and using one topic with 1000 partitions so I started investigating the file descriptors limits in my ec2 vm. ...
0
votes
1answer
72 views

Number of file descriptors - pidstat

As mentioned here: On RHEL 7.x, below command: pidstat -C syslog-ng -v does not recognise -v option Alternative: There are two process 417880 & 417881 for syslog-ng: # ls /proc/`pidof syslog-...
3
votes
1answer
719 views

How copytruncate actually works? [closed]

we would like to understand copytruncate before rotating the file using logrotate with below configuration: /app/syslog-ng/custom/output/all_devices.log { size 200M copytruncate dateext ...
6
votes
2answers
1k views

In Bash, what is file descriptor 255 for, can I use it?

I understand file descriptor (or file handler) is an file IO technique in Linux systems. I also know that each process has 3 standard streams (namely stdin, stdout and stderr) that are represented ...
0
votes
1answer
97 views

How does the act of closing shared file descriptors with a write-blocked process cause it to unblock?

Given multiple processes, forked from a parent process with shared file descriptors (representing STDOUT/STDERR), if one of the processes writes to STDOUT and exceeds the ~64K buffer it will block (as ...
0
votes
1answer
475 views

display the file descriptors of a process using python [closed]

How can I display the open file descriptors of a process in linux using python script? I tried using readlink /proc/PID/fd/* in python script but I am getting error. /bin/sh: 2: /fd/*: not found ...
2
votes
0answers
38 views

Get the path of song playing in VLC using python script

I am trying to get the path of song playing in VLC using python script. I have already find out the Process ID of vlc using pgrep -x vlc And I can also display the all file descriptors of vlc ...
1
vote
1answer
51 views

Why I can't see port opened by a process in file descriptors?

I have a rabbitmq management process running, which opens a port 15672, tcp 0 0 0.0.0.0:15672 0.0.0.0:* LISTEN 5431/beam As you can see the process has a PID ...
0
votes
0answers
225 views

ss command: file descriptor information when using the -p option

In ss command, when using the -p option, this presumably provides info about the process (owner, pid an fd) root@Cassiopeia:~# ss -tap | tail -1 ESTAB 0 0 [2a0i:527:9981:8511:f64e:...
0
votes
2answers
539 views

pass stdout fd to 'read' system call, but it still works fine [duplicate]

I pass 1(stdout)/2(stderr) to read system call but it still works fine. Then I pass 0(stdin) to write system call and find out it works too! int main(int argc, char** argv){ char buf[1024] = "...
-1
votes
2answers
49 views

Shell script to match row number from one file and put it as comments in another file when match

I have two files. The content of both files has dynamic and generated by the system when required. The first file contains the meaning for specific row number as below : head simdb.txt MSISDN ...
0
votes
0answers
25 views

send stdin to stderr for a time, then stop after timeout and discard: gnu textutils?

We have a very chatty application which says useful things during startup but then refuses to be quiet (useful logs are sent elsewhere). This interacts problematically with log rotation, snapshotting, ...
0
votes
2answers
265 views

Is a file descriptor ever NOT open?

Of course a file can be open, or not open. However, a file descriptor by definition refers to an open file (right?). (Well, except when it refers to something besides a file, like a pipe or what ...
0
votes
2answers
250 views

How to I tail the STDOUT of a cat command [duplicate]

Let's say that I start a cat process to wait for some input. e.g., $ cat > out.log In another terminal, I can identify its PID and feed data into its File Descriptor 0 (zero = STDIN), like this: ...
10
votes
3answers
2k views

Named pipes, file descriptors and EOF

Two windows, same user, with bash prompts. In window-1 type: $ mkfifo f; exec <f So bash is now attempting to read from file descriptor 0, which is mapped to named pipe f. In window-2 type: $ ...
0
votes
1answer
229 views

using fifo for daemons

While studying from Richard Stevens' book Unix network programming, I came across the following lines that speak about using FIFO between a client and a server. Client processes are started and ...
2
votes
1answer
396 views

Trap and collect script output, “input file is output file” error?

I need to upload the output of the current script, so I added a trap and set -ex, e.g #!/bin/bash exec &> /tmp/error.log trap 'cat /tmp/error.log; curl http://127.0.0.1/error.php?hostname=$(...