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

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0
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6answers
47 views

What are some practical uses of STDIN redirection? [on hold]

I am having a very difficult time trying to think of a single practical use for STDIN redirection. cat < /etc/passwd is an example of STDIN redirection, but it is silly in the same way as a useless ...
7
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4answers
235 views

Can I specify a redirected input before a compound command?

Bash lets you specify a redirected input before a command: $ <lines sed 's/^/line: /g' line: foo line: bar Bash also lets you redirect input to a compound command like a while loop: $ while ...
1
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2answers
58 views

How to grab output from command in shell script [duplicate]

I'm trying to write a shell script in RHEL which will execute grub-md5-crypt and the user will type their password. Now the problem is how can I grab the encrypted md5 hash displayed to the user in ...
2
votes
2answers
50 views

Should I use piping or redirection for input to 'sort'

I want to do awk -F "," '{print $1 }' inputfile1.txt | sort -u > distinctOutput.txt Is this good performance-wise ? Or should I redirect / use a temp file since I think it is better to make sort ...
1
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1answer
24 views

fuser output looks ambiguous

I am using fuser the output is like this: $ fuser / /: 1354rc 1362r 1370rc 1371rc 1395rc 1399rc 1403rc 1410rc 1414rc 1428r 1451rc 1458rc 1470rc 1472rc 1477rc 1480rc ...
-4
votes
1answer
59 views

UNIX homework question regarding piping output, I don't understand what it's asking?

Basically, I need to come up with a command that sends the contents of a file xxx into a program yyy, then the output is piped to the input of another program zzz, where the output is appended to a ...
11
votes
2answers
180 views

Why does cat x >> x loop?

The following bash commands go into an infinte loop: $ echo hi > x $ cat x >> x I can guess that cat continues to read from x after it has started writing to stdout. What is confusing, ...
1
vote
1answer
41 views

How do I Nohup an interactive shell-script?

My Script is having 2 command-line arguments and then just couple of questions, after these questions the script will run for itself, I'm able to pass the command-line argument by just doing, ...
1
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2answers
80 views

Process substitution with input redirection

I am trying to understand input redirection in combination with process substituation. I am using bash 3 An example with tr is the following $ tr "o" "a" <(echo "Foo") tr: extra operand ...
1
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1answer
23 views

Cannot redirect output from sed

I'm piping output from clock through sed to remove leading zeroes from numbers. It looks like this: clock -sf 'S%A, %B %d. %I:%M %P' | sed 's/\b0\+\([0-9]\+\)/\1/g' That works fine and produces the ...
0
votes
1answer
39 views

Bash STDERR still appearing after redirection [closed]

I am trying to parse the output of yum check-update in Bash 4 (Red Hat) and want to skip the first line that states: 'This system is not registered to Red Hat Subscription Management. You can use ...
5
votes
2answers
445 views

Is there any way to read lines from command output?

I have a pre process command to output a file ./preprocess.sh > preprocessed_file and the preprocessed_file will be used like this while read line do ./research.sh $line & done < ...
-2
votes
0answers
16 views

Explain a tsums command line

In putty, I am blindly writing codes as below, cat >! lot tsums @lot -format=lot,machine_id > ! MCID what is the meaning of these 2 lines?
1
vote
3answers
54 views

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 ...
3
votes
4answers
166 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?
1
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3answers
32 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 ...
1
vote
1answer
63 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 ...
4
votes
2answers
53 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 ...
4
votes
4answers
276 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 ...
4
votes
3answers
355 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 ...
0
votes
3answers
97 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 ...
1
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1answer
46 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 ...
1
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1answer
40 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 ...
1
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2answers
39 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} ...
0
votes
0answers
47 views

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:-}" = ...
4
votes
1answer
57 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, ...
7
votes
1answer
55 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 ...
-1
votes
3answers
92 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 ...
3
votes
1answer
53 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 ...
3
votes
2answers
65 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 > ...
4
votes
2answers
89 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 ...
2
votes
2answers
125 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 ...
8
votes
2answers
218 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 - ...
3
votes
1answer
60 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 ...
5
votes
3answers
162 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, ...
4
votes
1answer
108 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 ...
1
vote
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?
1
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2answers
148 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 ...
12
votes
7answers
359 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
2
votes
1answer
494 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 ...
5
votes
1answer
89 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 ...
1
vote
2answers
52 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?
4
votes
3answers
194 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 ...
1
vote
1answer
115 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 ...
3
votes
1answer
81 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 ...
9
votes
2answers
322 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 ...
10
votes
5answers
1k 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 > ...
3
votes
0answers
63 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, ...
3
votes
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
votes
3answers
465 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 ...