io redirection is the process of changing where the output and input of a program originate

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How to read from two input files using while loop

I wanted to know if there is any way of reading from two input files in a nested while loop one line at a time. For example, lets say I have two files FileA and FileB. FileA: [jaypal:~/Temp] cat ...
11
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3answers
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How can I pipe output to another process, but retain the error state of the first process? [duplicate]

Possible Duplicate: Get exit code of process that's piped to another I am using the following command line (in a makefile) to pipe the verbose error messages from my compiler through a ...
19
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1answer
816 views

Grep alias - line numbers unless it's in a pipeline

I want to create a bash alias for grep that adds line numbers: alias grep='grep -n' But that, of course, adds line numbers to pipelines as well. Most of the time (and no exceptions come to mind) I ...
1
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3answers
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No output to log from tee when starting mplayer as an arg to gnome-terminal

I want to view mplayer's normal terminal output, and also write it to a log. That works fine when I run mplayer from the commandline, eg: mplayer "$HOME/gdr01.wav" 2>&1 |tee $HOME/junk.log ...
14
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3answers
562 views

How to log whatever I enter in terminal to file

I am a novice just learning Fedora 13. I just wanted to know if its possible to redirect whatever I type in the terminal to a file, so that I would be able to see from the file, that a particular ...
12
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3answers
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Piping STDERR vs. STDOUT

According to "Linux: The Complete Reference 6th Edition" (pg. 44), you can pipe only STDERR using the |& redirection symbols. I've written a pretty simple script to test this: #!/bin/bash echo ...
30
votes
4answers
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Reading lines from a file with bash: for vs. while

I'm trying to read a text file and do something with each line, using a bash script. So, I have a list that looks like this: server1 server2 server3 server4 I thought I could loop over this using ...
24
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1answer
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Why is redirecting STDERR to /dev/null done this way?

This does not make sense to me. wibble > /dev/null 2>&1 I think it would make more sense if it was something like this: wibble 2>&1 > /dev/null In other words Commands ...
14
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3answers
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Non-blocking buffered named pipe?

I'm looking for something I suspect doesn't exist: A non-blocking buffered named pipe (fifo) for use from the command line. Is there such a thing? Here's the use case: Suppose I have a process that's ...
27
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2answers
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What does a “< <(…)” redirection mean?

I've seen that rvm (ruby version manager) is installed using the following command: bash < <(curl -s https://raw.github.com/wayneeseguin/rvm/master/binscripts/rvm-installer ) So as I ...
14
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6answers
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xargs with stdin/stdout redirection

I would like to run: ./a.out < x.dat > x.ans for each *.dat file in the directory A. Sure, it could be done by bash/python/whatsoever script, but I like to write sexy one-liner. All I could ...
4
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3answers
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Create a temporary file from a stdout redirect or pipe

Some commands only output to stdout. Some tools only work on files. Is there a command that can glue those together? Contrived simple example: diff $(echo 1 | stdout-to-temp-file) $(echo 2 | ...
8
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4answers
547 views

ls command operating differently depending on recipient

How does commands like ls know what its stdout is? It seems ls is operating different depending on what the target stdout is. For example if I do: ls /home/matt/tmp the result is: a.txt b.txt ...
2
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2answers
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Difference between > and | with /dev/tty

When I run this command: echo "1" > /dev/tty # runs successfully but when I run this: echo "1" | /dev/tty #permission denied What is the difference between these two operators > and | and ...
6
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2answers
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How to send input to a C program using a shell script

I have a C program in which I use scanf to get user input. Now, I would like to make a .sh script that automatically test this program with pre-defined user input, but I'm not sure how I can send the ...
4
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2answers
178 views

Unable to capture program exit

I started a program written using Node.js and started it using launch pipe into logfile and send it to background. Half way running the program exited w/o any error both on the console nor the log, I ...
16
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5answers
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Difference between 2>&1 > output.log and 2>&1 | tee output.log

I wanted to know the difference between the following two commands 2>&1 > output.log and 2>&1 | tee output.log I saw one of my colleague use second option to redirect. I know ...
52
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2answers
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View stdout/stderr of systemd service

I have created a simple systemd service file for a custom application. The application works well when I run it manually, but my CPU gets maxed out when I run it with systemd. I'm trying do track ...
3
votes
1answer
356 views

Removing redirection operator does not change output. Why?

I saw this usage of redirection somewhere, and thought it was a typo: grep root < /etc/passwd But after I run it, I saw that it gives the same output with grep root /etc/passwd: $ grep root ...
5
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1answer
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shorthand for feeding contents of multiple files to the stdin of a script

Hi good people of this community, Say I have a script called script, that reads from stdin and spits out some results to the screen. If I wanted to feed it contents of one file, I would have typed: ...
9
votes
1answer
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Why can't sudo redirect stdout to /etc/file, but sudo 'nano' or 'cp' can? [duplicate]

Why does redirection, using sudo give me an error for the following commands? $ sudo printf "foo" >/etc/file bash: /etc/file: Permission denied $ sudo printf "foo" ~/file; cat ~file ...
6
votes
1answer
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All about ssh ProxyCommand

I am looking for an in-depth explanation of the following ProxyCommand, down to the nuts and bolts of its operation, please. Can you kindly completely dissect it for me and improve on it if you can? ...
1
vote
1answer
1k views

Why can I append to a file but not overwrite it?

If I do this: iptables -nvL > output.txt output.txt ends up empty. If I do: iptables -nvL >> output.txt It works fine. Appending is working, but overwriting is not. Why?
39
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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 ...
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4answers
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Bash Reuse Process Substitution File

I have a big script which takes a file as input and does various stuff with it. Here is a test version: echo "cat: $1" cat $1 echo "grep: $1" grep hello $1 echo "sed: $1" sed 's/hello/world/g' $1 I ...
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1answer
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Taking output of /dev/tty into a variable

I created a two backup scripts - one for files (filebackup.sh) and another for database (dbbackup.sh). filebackup.sh calls dbbackup.sh and stores the output in a variable. The output returned by ...
2
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5answers
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How can I run a process in a terminal window and then hide that window?

I want to run python ~/program/proxy.py in a terminal when I boot up. And then hide this terminal window (because it is just a proxy program, and I don't want to see the debug info). So how can I get ...
9
votes
1answer
2k views

stdin/stdout names in BASH?

I have a program that takes a -o log_file option, which writes extra data to log_file. Is there any way for me to redirect that to stdout? i.e. is there a "special name" for stdout that can be used ...
20
votes
4answers
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What happens to the output of a process that has been disowned and lost its terminal?

If I close the virtual terminal, where some process was started, does the output just go straight to /dev/null, or can it pollute memory somehow? Can I anyhow grab the output to continue reading it at ...
11
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6answers
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How to limit log file size using >>

How can I limit the size of a log file written with >> to 200MB? $ run_program >> myprogram.log
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2answers
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Performance difference between stdin and command line argument

For some commands, it is possible to specify certain input as either stdin or a command line argument. Specifically, suppose command can take stdin input and a filename as command line argument, and ...
32
votes
4answers
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Process substitution and pipe

I was wondering how to understand the following: Piping the stdout of a command into the stdin of another is a powerful technique. But, what if you need to pipe the stdout of multiple ...
4
votes
3answers
685 views

Output exceeding terminal window height

I had run a program that resulted in many more lines of output than my current terminal window width. As a result, I could only see partial results...is there any way to get the full output? I am ...
7
votes
1answer
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Why does redirection (>) not work sometimes but appending (>>) does?

While studying for the RHCE, I came across a situation where stdin redirection does not work in bash: # file /tmp/users.txt /tmp/users.txt: cannot open `/tmp/users.txt' (No such file or directory) ...
2
votes
4answers
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append string where it belongs in sorted file

Suppose I have a sorted file, named sorted.txt, like this: beautiful easy fast functional handy Now, doing echo fine-grained >> sorted.txt will put "fine-grained" at the end of the file, ...
0
votes
1answer
270 views

Is 'some_program <some_file' faster than 'cat some_file | some_program'? [duplicate]

Possible Duplicate: Should I care about unnecessary cats? Both would be functionally equivalent, but when using cat wouldn't it be slower because you have a process first reading the file ...
3
votes
2answers
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Why is piped input okay to zenity, but <file malfunctions?

My question is as simple as what the zenity --text args say in the example... but what is causing this 100% CPU grab by the redirection? ...(and by the way, is this particular usage of < actually ...
6
votes
3answers
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How to get file descriptor other than stdin stdout and stderr (to do something like $ program 1>file_1 3>file_2)?

When an executable produces an output I prefer to let the executable print the output to stdout in order to redirect them in a file as well to pipe them to another command. Let suppose that an ...
-1
votes
2answers
466 views

Copying a few lines from stdout to stderr - but only if stderr is different from stdout

I have a bash script "backup.sh", which prints out everything to stdout, including all the files that are transferred during backup. I'd like to make it possible to keep that full output, but when ...
6
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2answers
1k views

redirection and pipeline

I was wondering about differences and relation between redirection and pipeline. Is pipeline only used to connect stdout output of a command to stdin input of another command? Is redirection only ...
6
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2answers
180 views

understand a sequence of redirections

If several redirections are used together, does changing their order make difference? How shall one understand the meaning of their order? Is the chained channel of redirections built as reading the ...
4
votes
4answers
169 views

Make mistakenly thinks rule succeeded because of 0-sized file generated by output redirect

In a makefile, I have several rules that look like this: out.txt: foo.sh input.txt ./foo.sh -i input.txt > out.txt If foo.sh fails, then out.txt will be created as a 0-sized file. If I run ...
21
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2answers
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How can I assign the output of a command to a shell variable?

I want to assign the result of an expression to a variable and concatenate it with a string, then echo it. Here's what I've got: #!/bin/bash cd ~/Desktop; thefile= ls -t -U | grep -m 1 "Screen Shot"; ...
6
votes
2answers
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Howto create a permanent client connection with netcat?

I'm writing a bash script that constantly read a folder in a loop to send data to a server at a defined time interval. I'm using netcat as the tool to connect to the server and send the data. My ...
8
votes
6answers
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IO redirection and the head command

I was trying to quickly edit an .hgignore file from the Cygwin bash shell today, and I added a line that was a mistake. I'm not sure if this was the best way to do it, but I quickly thought of using ...
9
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2answers
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See the STDOUT redirect of a running process

If I start an app with this command: /path/to/my/command >> /var/log/command.log And the command doesn't return, is there a way, from another prompt, to see what the STDOUT redirect is set ...
7
votes
2answers
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How to detect if input is from argument, file or terminal

In bash, is there a way to know if a given script has been invoked with: $ myscript.sh myfile or: $ myscript.sh < myfile Inside some scripts I always have accessed the contents of myfile with ...
19
votes
5answers
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view stdout for another pts [duplicate]

Here is the situation. I left my pc at home doing an rsync from a 2TB hard drive to another 2TB hard drive (it's going to take a while since they are both USB 2.0). I am now at work and I have ssh-ed ...
4
votes
3answers
594 views

script to use the output of a command (ls) and use it in another command

I have a collection of pdf files I would like to convert to epub. I know Calibre's command ebook-convert will do the trick (nicely might I add). However I have more than one file. I'd like to be able ...
1
vote
1answer
382 views

Which process/program create/write the file which I/O is redirected to?

When learning SELinux, I executed semanage fcontext -l > fcontext.txt under /root directory to dump massive fcontext information to a file. The funny result is: fcontext.txt was created, but the ...