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1
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1answer
21 views

Propose additional file descriptor “stdmeta” [on hold]

I know that one can define additional file descriptors for ad-hoc use. However, I see real use for a "stdmeta" file descriptor that would be supported by common CLI tools. This file descriptor would ...
1
vote
2answers
34 views

why nginx holds file descriptions of logs?

on Red Hat Enterprise Linux Server release 6.6 (Santiago) nginx version: nginx/1.0.15 I use common nginx logrotate config, logrotate works fine and nginx creates new log file like access.log or ...
2
votes
4answers
64 views

What is an open file description?

When you fork a process, the child inherits its parent's file descriptors. I understand that when this happens, the child receives a copy of the parent's file descriptor table with the pointers in ...
-1
votes
1answer
49 views

Accessing array elements within process substitution bash

I've got a text file which looks like this: b4238ca2-cb8d-11e4-8731-1681e6b88ec1,https,username,password,ipaddress b4238f0e-cb8d-11e4-8731-1681e6b88ec1,https,username,password,ipaddress ...
1
vote
3answers
82 views

Store stdout and stderr in file, then recreate the output later

I've redirected output to a file like so: $ ./test.bash 2> >(sed 's/^/stderr: /' >> output) > >(sed 's/^/stdout: /' >> output) $ cat output stdout: Hello World! stderr: ...
10
votes
1answer
138 views

find out which file descriptors share the same “open file description”

If I do (in a Bourne-like shell): exec 3> file 4>&3 5> file 6>> file File descriptors 3 and 4, since 4 was dup()ed from 3, share the same open file description (same properties, ...
4
votes
1answer
277 views

Why did sshd delete my /dev/zero?

When I do lsof +c 0 | grep /dev/zero, I see the following two lines: sshd 19064 root DEL REG 0,4 497862 /dev/zero sshd 19064 root DEL REG 0,4 498725 /dev/zero ...
3
votes
5answers
280 views

Reading from file descriptor fails

This question is about reading and writing on a file descriptor. See the following example: #!/bin/sh file='somefile' # open fd 3 rw exec 3<> "$file" # write something to fd 3 printf ...
3
votes
1answer
25 views

termios settings after closing file

Are the settings altered by functions from termios.h kept after file is closed and open again? I want to use serial communication in a higher level language (Racket) which exposes only reading and ...
2
votes
1answer
88 views

Behaviour of 1>&0 in bash

So I am trying to learn more about file descriptors and their redirections. At the moment I am exploring an edge case of doing 1>&0 and I cannot quite understand what is happening there. $ ...
3
votes
2answers
372 views

How does Bash get the output of an executed program?

When I execute a program in Bash, for example, [ls][2], it sends its output to standard output (fd &1). And the ouput of the executed program is displayed in the terminal. How does Bash/terminal ...
1
vote
2answers
62 views

Map reading/writing a file to getting output from/sending input to a script?

Suppose I have two scripts, script1.sh and script2.sh. I am wondering if there is a way to make a filesystem interface such that, for example, I can go vim file and then have my system run script1.sh ...
2
votes
1answer
57 views

Capturing data from a Fluke 1620a via Bash /dev/tcp file descriptor

I am currently trying to develop a script that will connect to a Fluke DewK 1620a Themo-Hygrometer and pull the current temperature and humidity readings from the device. While these devices have a ...
2
votes
2answers
182 views

How to print total number of file descriptors with index 24

How can I print the total number of file descriptors with index 24 for all the running processes on Linux? I tried $ ls /proc/*/fd 2> errors.txt > stdout.txt | grep "^24" stdout.txt | wc -l ...
3
votes
2answers
209 views

How to read user input when using script in pipe

General problem I want to write a script that interacts with the user even though it is in the middle of a chain of pipes. Concrete example Concretely, it takes a file or stdin, displays lines ...
2
votes
1answer
26 views

Forcing write(2) to return 0

I am writing unit tests and would like to test some code's handling of the case where a call to write(2) returns zero. As ever, it would be nice to keep the test as authentic as possible. I can use a ...
1
vote
0answers
38 views

open() sys call [closed]

Process A wants to open a file fd1 = open("/var/file1", O_RDONLY); In the process of opening file1 which table is referred first system wide file table or file descriptor table or in-core inode ...
3
votes
5answers
2k views

Stripping directory paths to get file names

What I am doing is converting mp3's with LAME. I already have an old script I wrote that works but I want to add to it this ability--to no longer delete the file but instead save it in a new root ...
47
votes
2answers
2k views

Is >&- more efficient than >/dev/null?

Yesterday I read this SO comment which says that in the shell (at least bash) >&- "has the same result as" >/dev/null. That comment actually refers to the ABS guide as the source of its ...
0
votes
1answer
62 views

How to find file/device given the resource ID on Linux?

On a gentoo Linux system I am printing the value of a file descriptor from within a PHP script yielding the following output: Resource id #1234 How to find the corresponding file/device given this ...
1
vote
2answers
284 views

Process substitution with input redirection

I am trying to understand input redirection in combination with process substituation. I am using bash 3 An example with tr is the following $ tr "o" "a" <(echo "Foo") tr: extra operand ...
6
votes
3answers
1k views

Why can't I `tail -f /proc/$pid/fd/1`?

I wrote a simple script which echo-es its PID: #/bin/bash while true; do echo $$; sleep 0.5; done I'm running said script (it says 3844 over and over) in one terminal and trying to tail ...
2
votes
1answer
75 views

Is there a system call to bind a file descriptor directly into another?

A read() & write() loop would probably be as good as what I'm looking for, but nevertheless is anything like that around or is it impossible because of an obstacle I didn't envisage ? I'm curious
7
votes
1answer
95 views

How can I send reads and writes on a single file descriptor to different places?

I have a Linux executable foo that reads input from fd 0 and writes output to fd 0 (not fd 1). This works just fine for interactive use in the terminal. From the shell command line, how can I execute ...
7
votes
3answers
666 views

How can a log program continue to log to a deleted file?

From the Unix Power Tools, 3rd Edition: Instead of Removing a File, Empty It section: If an active process has the file open (not uncommon for log files), removing the file and creating a new ...
5
votes
2answers
138 views

“command | less” vs “less <(command)”

A common "idiom" for viewing large amounts of command output is to pipe it to less, via command | less. However, it's also possible (perhaps only in bash, haven't tested in other shells) to use the ...
1
vote
2answers
69 views

What fd's are used when doing diff <(cat old) <(cat new)?

If I do: diff <(cat old) <(cat new) Hoe does that work internally for the diff program? And how I do the same trick with a bash script?
8
votes
1answer
875 views

stderr over ssh -t

This sends output to STDERR, but does not propagate Ctrl+C (i.e. Ctrl+C will kill ssh but not the remote sleep): $ ssh localhost 'sleep 100;echo foo ">&2"' This propagates Ctrl+C (i.e. ...
2
votes
0answers
18 views

Is it safe to create a regression test that relies on running out of file descriptor limit?

I'm writing a regression test for my modification of Nmap and I'm wondering if it's a good idea to create a test that would basically perform a large-scale connect() scan on 127.0.0.1/8, which would ...
2
votes
2answers
472 views

Any way in Bash to write to a file every X seconds without closing it?

The hardware watchdog on my system needs a 0 written to /dev/watchdog at less than 60 seconds interval or it will fire. The file handle must be kept open however or the watchdog is then disabled. ...
2
votes
1answer
607 views

closing the unwanted file descriptors

I am trying to understand how pipes work . This code snippet is from some website . So my understanding of the program goes like this : When we do pipe(fd) then fd[0]=3 and fd[0]=4 ( assuming only ...
3
votes
0answers
66 views

Some puzzles of how to close file descriptors on sh or bash [duplicate]

Question 1. What will happen if we close a input file descriptor as if it is a output file descriptor? (or vice versa). For Example, $ exec 5<&2 $ exec 5>&- In my linux box, ...
4
votes
2answers
1k views

Process substitution from curl to bash as root

I'm trying to run a script from URL as root with this command: sudo bash <(curl -s http://copy.com/gLVZIqUubzcS/popcorn) But I'm getting this error: bash: /dev/fd/63: Aucun fichier ou dossier ...
5
votes
2answers
985 views

Portability of file descriptor links

I've always wondered this but never took the time to find out, so I'll do so now - how portable is the usage shown here of either /proc/$$/fd/$N or /dev/fd/$N? I understand POSIX guarantees /dev/null, ...
2
votes
4answers
121 views

Can a running script identify the logging context?

Let us say I redirect the STDOUT, STDERR of a program to files. ./script.sh 1> output.log 2> error.log Can the running program find this out i.e. know the paths to these files?
3
votes
2answers
3k views

Close all file descriptors in bash

Is there a way to close all the open file descriptors, without having an explicit list of them beforehand?
2
votes
2answers
226 views

Redirecting only stderr to a pipe

This code snippet is from Advanced Bash Scripting Guide. # Redirecting only stderr to a pipe. exec 3>&1 # Save current "value" of stdout. ls -l 2>&1 ...
5
votes
1answer
1k views

What does exec 3<&1 do?

I understand exec can do I/O Redirection on current shell level. but I only sees usage like exec 6<&0 # Link file descriptor #6 with stdin. # Saves stdin. exec 6>&1 # ...
0
votes
2answers
398 views

How to reset the file descriptor

I was trying to use file descriptor for printing the lines in the file #!/usr/bin/ksh exec 3<&0 exec 0< file.txt while read LINE do echo “$LINE” done and I am done with my code ,it ...
4
votes
3answers
795 views

Please help explain this bash output redirection

This script was posted as answer to a Question. And I'm trying to work out what's going on. result=$( { { ssh host app-status >&3 3>&-; echo "$?" } | { until read ...
3
votes
1answer
81 views

What is the relationship between a program's output, standard output, and its file descriptors?

Let's assume that I am running wget which is downloading something. It's printing some text to my terminal and writes the downloaded data into a file. From a terminology point of view, what is the ...
1
vote
0answers
299 views

Which socket is gnome-shell reading from continuously?

strace shows that the GNOME shell is continuously trying to read from a socket, which is often not available (or doesn't contain any data to read from): strace -p gnome-shell-pid -e trace=recvfrom ...
11
votes
2answers
250 views

Output order with process substitution

This is what I usually do to run grep and wc on a file without having to scan it twice <file.txt tee >(grep LITERAL) >(wc -l) >/dev/null However, this yields EXEC LITERAL 32 ...
4
votes
2answers
767 views

Does OpenBSD have a limit to the number of file descriptors?

What's the maximum number of file descriptors that a process can get on OpenBSD successfully? For example, there is an openfiles limit in login.conf(5) in OpenBSD. If I want to have as many file ...
3
votes
3answers
125 views

Print a message from the left-hand side of a pipe

I want to use a variable from the main shell in a sub shell. I did: export mysql_root_password="test" ( echo $mysql_root_password ) | dialog --gauge "Working hard..." 6 50 There is no output. ...
5
votes
1answer
464 views

Why doesn't the process substitution <() work with ssh -F

I have some vagrant virtual machines. To log into them I issue the vagrant ssh command. I want to log into them using regular ssh command. The vagrant ssh-config outputs the suitable config file $ ...
2
votes
2answers
662 views

AIX open() : file descriptor is zero (!?)

A daemon process, on detecting the presence of a text file in a particular directory then launches an executable file which opens the aforementioned text file in order to read and then transmit its ...
5
votes
1answer
123 views

Replacing stdout with stderr

Is it possible to redirect the output from a command to replace the text sent to stdout with the text from stderr? $ ./script this line is redirected to stdout this line is redirected to stderr $ ...
3
votes
2answers
2k views

File descriptor and fork

When a child is forked then it inherits parent's file descriptors, if child closes the file descriptor what will happen ? If child starts writing what shall happen to the file at the parent's end ? ...
6
votes
1answer
5k views

How to circumvent “Too many open files” in debian

If I want to see all relevant log files of my apache2 server at once, I use tail -f /var/kunden/logs/*log /var/kunden/logs/*log /var/log/apache2/*log |grep -v robots|grep -v favicon But since those ...