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50
votes
2answers
3k 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 ...
32
votes
5answers
10k views

When would you use an additional file descriptor?

I know you can create a file descriptor and redirect output to it. e.g. exec 3<> /tmp/foo # open fd 3. echo a >&3 # write to it exec 3>&- # close fd 3. But you can do the same ...
24
votes
2answers
32k views

Difference between 2>&-, 2>/dev/null, |&, &>/dev/null and >/dev/null 2>&1

Just looking for the difference between 2>&- 2>/dev/null |& &>/dev/null >/dev/null 2>&1 and their portability with non-Bourne shells like tcsh, mksh, etc.
20
votes
3answers
2k views

Portability of “> /dev/stdout”

Occasionally I need to specify a "path-equivalent" of one of the standard IO streams (stdin, stdout, stderr). Since 99% of the time I work with Linux, I just prepend /dev/ to get /dev/stdin, etc., ...
17
votes
3answers
8k views

Understanding /dev and its subdirs and files

$ ls -l /dev/stdin /dev/fd/0 lrwx------ 1 tim tim 64 2011-08-07 09:53 /dev/fd/0 -> /dev/pts/2 lrwxrwxrwx 1 root root 15 2011-08-06 08:14 /dev/stdin -> /proc/self/fd/0 $ ls -l /dev/pts/2 ...
13
votes
1answer
14k views

File descriptors & shell scripting

I am having a very hard time understanding how does one use file descriptors in shell scripts. I know the basics such as exec 5 > /tmp/foo So fd 5 is attached to foo for writing. exec 6 < ...
12
votes
3answers
1k views

Practical use for moving file descriptors

According to the bash man page: The redirection operator [n]<&digit- moves the file descriptor digit to file descriptor n, or the standard input (file descriptor 0) if n is ...
12
votes
2answers
4k views

What happens when I close() a file descriptor?

I am trying to get the whole picture with file descriptors. Say I have process1 which initially has these file descriptors: _process1_ | | | 0 stdin | | 1 stdout | | 2 stderr | ...
11
votes
4answers
5k views

How can same fd in different processes point to the same file?

Say I have process 1 and process 2. Both have a file descriptor corresponding to the integer 4. In each process however the file descriptor 4 points to a totally different file in the Open File Table ...
11
votes
3answers
740 views

Order of redirections

I don't quite understand how the computer reads this command. cat file1 file2 1> file.txt 2>&1 If I understand, 2>&1 simply redirect Standard Error to Standard Output. By that ...
11
votes
2answers
2k views

What's the purpose of the first argument to select system call?

From man select int select(int nfds, fd_set *readfds, fd_set *writefds, fd_set *exceptfds, struct timeval *timeout); nfds is the highest-numbered file descriptor in any of the three ...
11
votes
2answers
272 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 ...
11
votes
2answers
232 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, ...
10
votes
2answers
1k 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. ...
8
votes
1answer
100 views

OS X, bash: less works on open file descriptors, cat doesn't

In a bash script I'm working on (which has to run on Ubuntu and OS X), I need to redirect the output of hundreds of commands to a file. Rather than appending &>... to all of them, I simply do ...
7
votes
3answers
722 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 ...
7
votes
2answers
6k 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 ...
7
votes
2answers
8k views

Execute command in remote active terminal

Suppose you have a terminal emulator (T1) open with a PID of 6350. From another terminal, type this command (C1): echo "ls\n" > /proc/6350/fd/0 This writes ls and the new line in T1 but does ...
7
votes
1answer
143 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 ...
6
votes
3answers
2k 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 ...
6
votes
2answers
2k views

file descriptor vs. file name

I was wondering what differences and relations are between file descriptors and file names. Are they all used to access files? If yes, in the same way? For example, /dev/fd/0, /dev/stdin, and ...
6
votes
1answer
597 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 $ ...
6
votes
1answer
12k views

Limits on the number of file descriptors

I'm trying to install 389-ds, And it gives me this warning : WARNING: There are only 1024 file descriptors (hard limit) available, which limit the number of simultaneous connections. I understand ...
6
votes
1answer
704 views

Timestamp of socket in /proc/<pid>/fd

If I list /proc/<pid>/fd I see a number of entries for sockets. These entries have timestamps. At first I thought they were when the socket was created. But it doesn't always appear to be the ...
5
votes
1answer
5k views

What is the file descriptor 3 assigned by default?

$ ls -og /proc/self /proc/self/fd lrwxrwxrwx 1 64 Jun 18 11:12 /proc/self -> 32157 /proc/self/fd: total 0 lrwx------ 1 64 Jun 22 2012 0 -> /dev/tty1 lrwx------ 1 64 Jun 22 2012 1 -> ...
5
votes
2answers
1k 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, ...
5
votes
1answer
2k 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 # ...
5
votes
1answer
131 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 $ ...
5
votes
2answers
162 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 ...
5
votes
1answer
570 views

Why does reading from two connected pty's cause an infinite loop?

I want to fake a gsm modem for testing a program. I want the program to send AT-codes to me and that I can answer it back, kind of a VirtualSerialPort. But for some reason the data written from the ...
5
votes
0answers
105 views

How can different file descriptors point to the same file in open file table? [duplicate]

Possible Duplicate: How can same fd in different processes point to the same file? I have a hard time grasping the two ideas of File Descriptor Table and Open File Table. Open File Table ...
4
votes
3answers
2k views

Something's special about /dev/fd/3

I've been trying to learn about file descriptors. When I type "ls -l /dev/fd/" I get lrwx------ 1 me users 64 May 2 16:02 0 -> /dev/pts/5 l-wx------ 1 me users 64 May 2 16:02 1 -> ...
4
votes
5answers
4k 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 ...
4
votes
5answers
400 views

Testing if a file descriptor is valid

I'd like to make a bash script output additional information to file descriptors (FDs) greater than or equal to 3, when they are open. To test whether an FD is open, I devised the following trick: if ...
4
votes
2answers
2k 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 ...
4
votes
2answers
1k views

What is the referent of a file descriptor?

My understanding is that a file descriptor is an integer which is a key in the kernel's per-process mapping to objects such as open()ed files, pipes, sockets, etc. Is there a proper, short, and ...
4
votes
2answers
868 views

Cross-process dup on Linux

I'd like to dup a file descriptor running in an unrelated process on Linux. I know about sendmsg(2) and SCM_RIGHTS (e.g. ...
4
votes
2answers
373 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 ...
4
votes
3answers
877 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 ...
4
votes
1answer
5k views

How to find speed of wlan interface?

I'm trying to find speed of network interface using file-descriptor. It's easy to do it for ethX, just calling cat /sys/class/net/eth0/speed. Unfortunately this method doesn't work with wireless ...
4
votes
1answer
426 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 ...
4
votes
2answers
595 views

How does file descriptor re-assigment work?

In a way this question is a extension of another question. In bash you can do pretty neat stuff with file descriptors. Like { command > /dev/null } 2>&1 | grep filter; to grep on stderr ...
4
votes
2answers
902 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
5answers
455 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
2answers
3k 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 ? ...
3
votes
2answers
567 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 ...
3
votes
3answers
147 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. ...
3
votes
2answers
1k views

What sets a child's STDERR, STDOUT, and STDIN?

If a program runs fork() what sets standard streams STDOUT, STDIN and STDERR?
3
votes
2answers
5k views

What are the dangers of setting a high limit to max File Descriptors per process?

I'm working on an old legacy application, and I commonly come across certain settings that no one around cam explain. Apparently at some point, some processes in the application were hitting the max ...
3
votes
2answers
4k 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?