The file-descriptors tag has no wiki summary.
15
votes
3answers
1k 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 ...
11
votes
2answers
514 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., ...
8
votes
1answer
212 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 ...
8
votes
3answers
241 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 ...
7
votes
2answers
917 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 |
...
6
votes
1answer
3k 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 < ...
5
votes
4answers
1k 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 ...
5
votes
3answers
1k 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 ...
5
votes
2answers
904 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 ...
5
votes
1answer
500 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
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 ...
5
votes
1answer
257 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
88 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
2answers
854 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
457 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 ...
4
votes
2answers
387 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 ...
3
votes
3answers
152 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 -> ...
3
votes
2answers
60 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. ...
3
votes
1answer
158 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 ...
2
votes
2answers
347 views
stop pipe() opening stdin
I've currently got code that forks two processes. The first reads a http streaming radio and pushes the data down a pipe (opened with pipe() ) for the second process to read, decode and output to the ...
2
votes
2answers
670 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 ...
2
votes
1answer
52 views
Why is it possible to refer to a closed stderr under bash?
I'd like to know why bash doesn't display an error message when we refer to a closed stderr. When it comes to other file descriptors, they have to be opened if we want to, for example, duplicate them. ...
2
votes
1answer
159 views
Why doesnt ctrl+d work with this?
I wanted a simple way to process text in my clipboard without having to create a file. I tried using the following line:
awk '{print $1}' <(cat)
but I couldn't send cat the EOF character using ...
2
votes
1answer
58 views
open() console for default file descriptors
I'm reading a shell program implementation in C ( the xv6 shell from MIT's 6.828 Operating System Engineering course ).
The main() function for this shell starts with the following code:
//Assumes ...
2
votes
1answer
98 views
Significance of arrows symbols in duplicating/closing file descriptors under bash
I'm reading a book about Linux command line where author doesn't seem to follow the conventions in bash manual regarding arrows symbols used in redirection operations. Namely, he always uses left ...
2
votes
1answer
54 views
How is the inherited file descritor vulnerability dealt with in Unices?
Ulrich Drepper describes a vulnerability in programs that have open file descriptors then they fork() and execve() without lots of very careful locking in place. He also talks about new kernel ...
2
votes
1answer
76 views
Max Open Files, clarification needed
On my machine,
ulimit -n returns 2560
Given that -n returns
The maximum number of open file descriptors.
Does it mean that system won't allow more then 2560 open files to be out there at any ...
2
votes
1answer
267 views
Parameretrize file descriptor number to open a tcp socket in shell script
I'm tried to parameretrize in a variable the file descriptor number to open a tcp socket using exec command but it failed. Only work correctly when file descriptor number is a constant. In the next ...
1
vote
3answers
124 views
How to check which process is using a given file descriptor?
Somewhere in the middle of my application, the framework I'm using (ROOT) gives me the following error:
*** Break *** write on a pipe with no one to read it
SysError in ...
1
vote
2answers
191 views
What sets a child's STDERR, STDOUT, and STDIN?
If a program runs fork() what sets standard streams STDOUT, STDIN and STDERR?
1
vote
2answers
135 views
Where to place a Bash shell redirection for a command? [duplicate]
Possible Duplicate:
Order of redirections
Apart from the standalone exec >&2 to redirect the current shell's input and output are there any behavioral differences in the following ...
1
vote
1answer
44 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 ...
1
vote
2answers
119 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.
0
votes
2answers
111 views
Why doesn't Bash accept `&>&3`, i.e. redirecting stdout and stderr to file descriptor 3?
Given the preamble, foobar function and invocations of it:
exec 3>/dev/null
function foobar { echo foo; echo bar >&2; }
foobar >/dev/null
foobar 2>/dev/null
foobar ...
0
votes
1answer
65 views
Understand Fd`s and nodejs
I'm having a bit of a confused time understanding what a file descriptor is and if I even need one! I am trying to spawn a process in nodejs and have its out put be written directly to an out put ...
