Programming :: Redirecting Stdin / Stdout And Stderr For Program Run As Fork()/execl()?

Nov 24, 2010

I'm trying to write a program that will fork a series of FTP sessions. For each session, there should be separate input and output files associated with stdin and stdout/stderr.
I keep reading how I should be able to do that with dup2() in the child process before the execl(), but it's not working for me. Could someone please explain what I've done wrong? The program also has a 30-second sniper alarm for testing and killing of FTPs that go dormant for too long.

The code: (ftpmon.c)
#include <stdio.h>
#include <stdlib.h>
#include <string.h>


The output:

$ ftpmon
Connected to (
220 (vsFTPd 2.0.1)
ftp> waitpid(): Interrupted system call

Why am I getting the ftp> prompt? If the dup2() works, shouldn't it be taking input from my script and not my terminal? In stead, it does nothing, and winds up getting killed after 30 seconds. The log file is created, but it's empty after the run.

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Programming :: Redirecting Stdin / Stdout And Stderr And Finding Files Name And Other Stats?

Aug 8, 2010

I'm working on an application used for backup/archiving. That can be archiving contents on block devices, tapes, as well as regular files. The application stores data in hard packed low redundancy heaps with multiple indexes pointing out uniquely stored, (shared), fractions in the heap.

And the application supports taking and reverting to snapshot of total storage on several computers running different OS, as well as simply taking on archiving of single files. It uses hamming code diversity to defeat the disk rot, instead of using raid arrays which has proven to become pretty much useless when the arrays climb over some terabytes in size. It is intended to be a distributed CMS (content management system) for a diversity of platforms, with focus on secure storage/archiving. i have a unix shell tool that acts like gzip, cat, dd etc in being able to pipe data between applications.


dd if=/dev/sda bs=1b | gzip -cq > my.sda.raw.gz

the tool can handle different files in a struct array, like:


enum FilesOpenStatusValue {
FileIsClosed = 0,


Is there a better way of getting the file name of the redirected file, (respecting the fact that there may not always exist such a thing as a file name for a redirection pipe).
Should i work with inodes instead, and then take a completely different approach when porting to non-unix platforms? Why isn't there a system call like get_filename(stdin); ?

If you have any input on this, or some questions, then please don't hesitate to post in this thread. To add some offtopic to the thread - Here is a performance tip: When doing data shuffling on streams one should avoid just using some arbitrary record length, (like 512 bytes). Use stat() to get the recommended block size in stat.st_blksize and use copy buffers of that size to get optimal throughput in your programs.

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Programming :: Redirecting Stdout To File And Terminal And Stderr To File?

Apr 20, 2010

I have a script where I want to redirect stdout to the terminal and also to a log file aswell as redirecting stderr to the same log file but not the terminal.I have the following code which I found on the net which redirects both stderr and stdout to a file and the logfile,

Code: if [ -p $PIPE1 ]
rm $PIPE1


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General :: Redirecting STDOUT Of Previous Command To STDIN Of Next Command

Nov 20, 2009

i'm trying to redirect the output of a command to the input of the next command. not sure if i'm going about this the right way. an easy method would be just to store the output of the previous command in a file and redirect input to read that file, but i'm curious to see if this can be done without writing to any files.

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Software :: Get Stderr And Stdout Of A Program Into Separate Env Variables?

May 19, 2010


MY_STDERR=`my-command >&2`

That is, i want to have to run my-command only once and get the same result. I've tried this:


YYY=$(XXX=`{ echo -n 111; echo 222 >&2; }` 2>&1); echo $XXX $YYY

where "{ echo -n 111; echo 222 >&2; }" is my-command. But it gives this output:



instead of "111 222". What's wrong in my script?

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Programming :: Cannot Redirect All Stdout And Stderr Into File

May 12, 2009

I have a little complex Makefile system. A parent Makefile call dozens of Makefiles in subdirctories. And the subdirctory Makefile calles shell script to do real building. I want to grab all output this Makefile system generate. So, i employ "make 2>&1 > make.log". but not all output messages are filed into make.log. The message generated by sub-makefile called shell script cannot be recorded into make.log. And another curiouse thing is, if i launch "make 2>&1 > make.log" in a perl script, all output do be sent into make.log.

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Programming :: Libxml2: Removing Output On Stdout/stderr - Element CA Does Not Carry Attribute Maxlength

Jul 26, 2010

I'm using libxml2 to handle/manipulate some XML files. In order to check the consistency of a XML file, I have a DTD and I'm using the xmlValidateDtd method to compute the check.

However, when an error occures during the check (for example an attribute is missing in a XML tag), then libxml2 writes the error on the stdout/stderr. For exemple:


/home/XML/FreeFour.xml:18: element CA: validity error : Element CA does not carry attribute maxlength

The method return the right result (true or false depending on the check result), but occurring errors are written on the stdout/stderr, and I actually don't want that.

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Programming :: STDIN, STDOUT, STDERROR Redirection For Scripting - Bash: Log.txt: No Such File Or Directory

Sep 7, 2010

I'm writing a script to execute bash commands in the PHP CLI. I would like to suppress errors from bash and write my own error message if an error occurs. So far I have this (assuming log.txt doesn't exist!):


tac log.txt 2>/dev/null

Which works as expected, tac kicks up an error but the error is suppressed, but when I use this:


tac < log.txt 2>/dev/null

I get:


bash: log.txt: No such file or directory

The tac error is suppressed but bash still gives me a dirty error.

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General :: Getting Back To "stdin" After Redirecting A File To A Particular Program?

Nov 22, 2010

I need to achieve a particular effect using bash's redirection facilities.I know that I can redirect a file to some program's standard input:[user@host]$ application < file.txtThe thing is, I'd like to know can I regain control of this program's input after the file content's have been passed to it. In other words, I'd like to run a command similar to the above, and then, instead of the termination of the application, I'd want it to wait for further commands from standard input (keyboard).

As I write this question, it occured to me that I could probably write another application (or a script), that would at first write some data to standard output and then act as echo, like:[user@host]$ | applicationWould it work, and is there any better way to do so? There are a bunch of Googleable tutorials covering this issue, but they all amount to one advice - "reopen the stdin after the file contents have been read".

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General :: Redirect Stdin And Stderr To Another Terminal?

Jun 28, 2010

Is it possible to redirect stdout and stderr from one terminal say /dev/pts/2 to another /dev/pts/3?

I tried the following:

/dev/pts/2 2>&1 /dev/pts/3&

Then when I run a command the process stops.

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Software :: Bash Output Stderr To 2 Files And Stdout?

Apr 5, 2011

I want to output the stdout and stderr in a logfile,moreover i do want to log stderr also to a separate logfile, and print str to the screen I searched arround and tried:


$ command 2>&1 > log | tee -a log log.err

But then in log first the stdout appears, and then stderr.

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General :: Redirecting Output From Stderr?

Dec 1, 2010

I have seen a post where someone was explaining the virtuality of stdout and stderr and that it can be redirected with e.g. 2>file.txt but this apparently is not working for me!
I have a CUPS filter with fprintf(stderr,...)

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General :: Script To Output Both The Stderr And Stdout To The Same Text File?

Aug 5, 2011

Am having issues getting the output from a script to be logged in a file. I need the script to output both the stderr and stdout to the same text file.

At present I have the following script:

echo TR3_1 > printers.txt
snmpget -v 1 -c public 10.168.**.* SNMPv2-SMI::mib- &>> printers.txt


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General :: Redirect Stdout/stderr Of Multiple Commands In One Shot?

Jun 3, 2011

I have several commands in a bash script, and in the middle of the script there are several commands whose output and error streams I want to redirect to a file. I think I could simply add '>> myfile.txt' to the end of every command, but is there a way to set it before that block of commands, then reset the streams to their original state at the end of that block?

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Ubuntu Servers :: Limit Size Of Log File When Redirecting Stdout?

Jun 7, 2010

I have a command line server that logs to stdout, which I start along the lines of ./server > log.txt

What I want to do is limit the size of log.txt, without modifying the server.

I am assuming there must be some kind of tool already that lets me do this, something like where I can pass in my server, the output file and a size limit? If so, can anyone enlighten me?

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Programming :: Why Fork Is Executing The Program Twice From Start

May 25, 2011

I am learning about OS and I wrote this simple forking program... Here's the code..

#include <sys/types.h>
#include <unistd.h>
#include <stdio.h>


How can it print the first statement, that is BEFORE the fork() statement twice?

I am running Natty 11.04 btw..

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Ubuntu :: Redirecting Iwconfig - Eth0 - Has No Wireless Extension Are Still Outputted To The Terminal (stdout)

Mar 11, 2011

When iwconfig is redirected thus Code: iwconfig >> wireless.txt things like Code: eth0 - Has no wireles extension are still outputted to the terminal (stdout)

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Programming :: C/c++ Getting Serials \ Used An Execl Call To Dmidecode And Parsed It Using Sed To Get Serial Numbers?

Nov 23, 2010

I used an execl call to dmidecode and parsed it using sed to get serial numbersso a business friend could make a (mostly) unique serial when his cross platform software is installed on different machines.I know this is falling outside the spirit of Linux being a profit thing, but the request has been to now do the same thing without using Linux commands. Which sounds like a chore since I would be re-inventing something that already exist namely dmidecode. Is the source available for dmidecode?I have no idea where to start to look for the info like dmidecode provides.

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Software :: Qt Creater 4 - C++ & Stderr - Use QDebug() To Print Out Stderr Messages?

Jul 27, 2010

Can I use qDebug() to print out stderr messages? If I just use qDebug() << stderr; I get hex output.

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Ubuntu :: Redirecting Output From Printf Statement In C Program

May 22, 2010

I've got a C program that I've added some 'printf' statements to monitor a couple of variables. When I run this program manually or from a script, the output is displayed on screen. However, I need to change various variables in the 'test.c' file, run 'make clean' and 'make' a few hundred thousand times. I'm using a script to read the variables in and then using sed to do in-place edits of the file. Unfortunately, with this amount of iteration, it is getting rather tired!

Anyway, I've created a script that is working as long I respond to prompts. I've tried the following to no avail: Code: /path/to/script > /tmp/output /path/to/script > /tmp/output 2>&1 /path/to/script | tee (no output even after adding the -a option) In my C program, I have the following 'printf' statement: Code: printf ("variable1: $s variable2: $s",var1,var2); What am I missing? I've worked with redirection before and it's always worked out fine, but this one plain stumps!

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Programming :: Using Stdout For More Than One Task?

Jun 30, 2009

I want to parse my mail log file and reuse the results but I'm having a hard time structuring the syntax. Something like:


grep hostname /var/log/mail.log |
grep NOQUEUE: |
sed -e 's/hostname postfix/smtpd/[[0-9]*]: //g'

at this point I want to redirect what I have in hand to a file but also ... fork? or split? whatever the term, to continue onward so that I can pipe the results further into wc -l or sort or programX. without having to re-loop through that huge log file.

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Programming :: Bash: How To Discard Unwanted Stdin

Jun 22, 2010

Often in bash we read lines from stdin in a loop and implicitly discard the remaining stdin by terminating the loop. Is it possible to discard it without terminating the loop? It could lead to smaller code.

Here's an example which uses two loops and below is the same algorithm assuming unwanted stdin can be discarded

while read destination gateway _
[[ $destination = default ]] && found=yes && break
done <<< "$( route )"


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Programming :: Clearing Stdin Obtained From Pipe

Mar 14, 2010

I have a small program that reads stdin from a pipe using fgets. Now fgets blocks for the first line but after that it will not block.

The code, my_echo.c -
int main(int argc, char **argv) { char buf [2000] ;
char* pc ; printf("hello ") ; while (1)
{ buf[0] = (char) 0 ;
pc = fgets(buf, sizeof(buf), stdin);
if (pc != NULL) printf("%s ",buf); } return 0; }

How its called
* In terminal window 1: ./my_echo < my_fifo
* In terminal window 2: echo "1234" > my_fifo
* In terminal window 1: prints hello then 1234.
* Checking with ksysguard or top shows that my_echo is consuming 40% of CPU time.

Adding a few printf's shows that the gets is not blocking and returns a null pointer.
* In terminal window 2: echo "qwerty" > my_fifo
* In terminal window 1 qwerty prints.
I want a read function that does in fact block so my program does not tie up CPU time, read does not block.

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Programming :: Write A Script Which Will Get A Number From STDIN?

Dec 15, 2008

I need to write a script which will get a number from STDIN and then with that number echo a set number of questions (its for a firewall config). Heres what I want the user to receive.

How many ports do you want open? 3

1. specify port: 80
2. specify port: 21
3. specify port: 23

In the background this will echo a command out to a text file which will read as follows:

open port 80
open port 21
open port 23

I've done this before but I've completely forgotten how

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Programming ::make AWK Do *something* Without Giving It Any Stdin Nor Inputfile?

Oct 10, 2010


I've given the output from `df` on AWK's stdin. But what I wonder is if there's a way to get AWK to run `df` itself, produce the same output, and exit? Doesn't seem to be that simple. Here's some examples: Works for some reason, and I think only in Bash (nothing is required in the $()


does AWK absolutely need *something* on stdin, before it begins to process the data? Can it be made to open a file or stream internally, act upon that as though it were the stdin, and exit?

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Programming :: How To Redirect Stdin For Child Process

Aug 18, 2010

I'm trying to write a shell script that do ftp and download file periodically, this script should be called by a daemon running in the background.

the shell script "" is as follows:

yafc ftp://test:test@ < commands
and the "commands" files is

d Root/md5* /

if I run it will work just fine. But when the daemon software calls the "", the script will send ftp login request to the ftp server, but will not even answer the username or anything.

I believe it is something about child process redirection, but I don't know how to deal with it.

This problem is not only with yafc, it is the same with any ftp client or any application like telnet and so.

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Programming :: Read From Stdin In Perl Do Not Wait For EOF?

Apr 28, 2010

I am trying to read one line at a time in perl and not wait for stdin eof to start the action:

<do something>

if i pipe ls into it, it waits for ls to be done before doing the action.

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Programming :: Stuffing A Key Into Another Virtual Terminal's Stdin?

Oct 20, 2010

I want to put a keystroke into another virtual terminal's stdin.A simple 'echo p > /dev/tty7' causes a p to appear on the console of tty7 but not the app running in tty7 to respond as though a p key had been pressed. Per the instructions of a fellow in the software forum I tried using an ioctl, but that returned a permissions error, even when I made the target VT's permissions 777 (and it had the same owner).

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Programming :: Blacklist End Up In File And $a End Up In Stdout?

Jan 22, 2010

In this example, why does blacklist end up in the file blacklist and $a end up in stdout?


The desired result is to have a file containing the results of lsmod which had the first word on the line beginning with snd_ copied into another file preceded by the word blacklist.

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Programming :: How To Use Globals When Using Fork

Sep 16, 2010

As part of my ongoing project I need to put together a messaging server of sorts. So far I have coded a server which accepts multiple connections, which then runs a function that will perform a task, see below.On this sever I have a global structure which is populated using the data found in a read() buffer. However with each fork() that runs the below function each process is handed a blank structure. How could I go about allowing each fork() child process access the same structure?

My code should hopefully show what I am trying to do?
Code: void listen_for_client(int sock)


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