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

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Why cant I overwrite a file with processed value?

This question prompted a thought I was not sure I understood well. I know that it is not possible or correct to use pipelines like cat myfile | grep -v mypattern > myfile due to how file handles ...
5
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2answers
30 views

What exactly happens if input and output file is the same?

Let's say that I execute following command: tr a-z A-Z < file > file There are two redirections: < file and > file. Both are processed before tr command and as far as I know, if there ...
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1answer
31 views

unzip without replacing original file and report to log

I want to write a bash script to decompress a file without overwriting the original one and redirect the operation result into a log file. I found an answer to the first part of my question at: How ...
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4answers
541 views

Bash program not executed if redirection would fail

In bash, I notice that if a command using redirection would fail, any programs which run prior to that are not run. For example, this program opens the file "a" and writes 50 bytes to file "a". ...
2
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2answers
283 views

How can I redirect the output of a child process?

I am running the following git clone command through sudo and bash and I want to redirect STDOUT to a log file: % sudo -u test_user bash -c "git clone https://github.com/scrooloose/nerdtree.git ...
2
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1answer
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Unable to redirect stdout/stderr to log file [duplicate]

I have a script tmp.sh #!/bin/bash echo hello sleep 3 echo 3 I want to log the script running time (as well as script stdout) to a log file. user$ time ./tmp.sh >& log.tmp real ...
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0answers
29 views

Deleted file being written to by running process, can I recreate it? [duplicate]

I've started a long running process with nohup and redirected it's output to a log file Example: nohup ./bin/zkServer.sh start-foreground > /usr/local/myuser/logs/zkout.log 2>&1 & I ...
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1answer
31 views

How to generate a text report in Linux?

I have a command I'm using to find all of the *.xml files on our server. I'm using: find -type f -exec grep -il "xml" {} \; This is great, but there are maybe a thousand files like this on the ...
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How is redirection implemented?

Is redirection a concept of the shell, not of the OS kernel? How is redirection implemented in terms of system calls? Is redirection implemented similarly to pipe?
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1answer
27 views

Change a file while redirecting it as stdin input to a command

In Bash, while running mycommand < myfile, I modify myfile's content. What content of myfile does mycommand get? Does mycommand get the original content of myfile when i started running it, or ...
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2answers
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Why is this script looping on the first line of the txt file?

#!/bin/bash usernameFile="/home/netadmin/username_list.txt" logFile="/var/log/netvpn-mag-archive/netvpn-mag-20160" while read -r line < $usernameFile; do if [[ "$line" != " " ]]; then ...
2
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1answer
22 views

Connect via telnet and doing certain operations

I'm trying to code a script where I can connect via telnet to another server and execute certain operations, but I'm having problems with the result of this process. The objective is to connect to a ...
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0answers
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Emptying a log file getting updated with 2>&1 output redirection [duplicate]

I have a server which constantly writes outputs and errors to stdout and stderr. The command line I use for running my server is: Server options-list > log.txt 2>&1 & Now the problem ...
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2answers
41 views

Piping sed/grep output does not work

If I run the commands bellow, only out1 has output, the out2 and out3 are empty. # this is just to generate a self-signed certificate openssl genrsa -out /tmp/ssl.key 2048 openssl req -sha256 -new ...
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2answers
63 views

How to redirect STDIN of background process?

In my Ubuntu, i run java application in background. I use bash script to run it and now it looks like: nohup java -jar app.jar & exit 0 The problem that i want to be able to write an input ...
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1answer
804 views

Shell variable $_ not behaving as expected

What the reason the number of lines differs? $ head -n 100000 ./access.log > ./data/log.sample $ cat $_ | wc -l 1933424
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1answer
20 views

Archlinux'es pacman + tee == unclear console IO [closed]

I have a small issue with capturing the pacman's IO with the tee utility. I try this command line: pacman -S group-1 group-2 | tee pacman.log And there is an issue: when the pacman query which ...
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1answer
38 views

“time > time.txt” refuses to work propely

I just started learning Linux commands. I was experimenting with the > command, that, as far as I understand, makes the command before it write it's output to the file after the sign. This ...
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1answer
30 views

Redirection Error in Function [closed]

I'm trying to figure out why the code below doesn't work, and gives me an error Bad file descriptor. It is sort of a follow-up to this question as applies to the script I'm currently working on. ...
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2answers
81 views

Returning a String and Understanding Redirection and Output

I'm trying to figure out how redirects could be used to selectively output a string from a bash function which needs to print multiple lines to the terminal. I found an answer on this question at ...
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2answers
47 views

Difference between 1> and >

Is there any difference between (# comments taken from documentation) command > filename # Docs: Redirect stdout to a file. and command 1> filename # Docs: Redirect stdout to file ...
2
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1answer
29 views

Sending `time` command into text file [duplicate]

I want to get the result of time command into a text file but it's not working it only put blank space in the text file. I already tried this commands, A- $ x=`time` $ echo $x > log.txt $ cat ...
4
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1answer
63 views

File descriptors and redirect in bash

Can someone explain why this command works: gpg --enable-special-filenames --verify --batch \ <(curl -s https://www.gnupg.org/ftp/gcrypt/gnupg/gnupg-2.0.29.tar.bz2.sig) \ <(curl ...
4
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3answers
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Trying to make sense of bash redirection syntaxes and their outputs

I'm new to Linux and trying to understand how redirections work. I have been testing various syntaxes for redirecting stdout and stderr to the same file, which don't all produce the same results. ...
0
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2answers
59 views

How can I clear/empty a file being piped to?

Say I have some process which pipes to a file: yes > foo I know want to clear/empty the file foo, but still want the process to write to the file, i.e. start filling the file again after ...
0
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1answer
44 views

avahi-daemon fails to start with “Failed to chroot(): Permission denied”. But only without stderr-IO-redirection

After a recent upgrate, avahi-daemon is not starting up anymore. Furthermore, the behaviour differes weather if I redirect stderr to a file or not. If I start avahi-daemon --debug with debugging ...
2
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1answer
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redirect to a named pipe

I know the difference between > and >> when redirecting to a plain file. But is there a difference when redirecting to a named pipe (fifo)? Assume the namedpipe is a named pipe. Are the ...
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3answers
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redirecting output of a command as if it was file input

I find myself always having to arrow up and going backwards through the command line to change one part of an earlier command that is pipped to a later grep or head or whatever command. For a crude ...
2
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1answer
33 views

Possible redirection bug in zsh 5x

[This post's title needs some work; suggestions welcome.] The following command produces different outputs in zsh v. 4.x and 5.x: { ( printf "X\nY\n" printf "WARNING: foo\nWARNING: ...
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2answers
62 views

shell stderr to /dev/null

I have PID file and I want to get PID number so this is what I am doing. [root@linux ~]# PID=`cat /usr/local/foo/var/run/foo.pid` [root@linux ~]# echo $PID 3414 sometimes this file doesn't ...
0
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2answers
182 views

Why we need to have 2>&1 in /dev/null 2>&1?

I saw in linux script there was a command argument > /dev/null 2>&1, I know it is to redirect the output to null, means silencing it. I also know about the numbering for 0,1,2 (STDIN, ...
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0answers
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Redirect stderr to stdout and tee to file and check return value [duplicate]

I know how to redirect stderr and stdout to a file: curl -sSo file http://foo.bar >> log 2>&1 I know how to get stdout to appear on the console and in a file: curl -sSo file ...
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2answers
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Output pings with time greater than 100ms to file with a timestamp appended

I want to save in a file every ping with a time greater than 100ms and append a timestamp to every line. Searching I have managed to get this: ping HOST | grep --line-buffered "time=[0-9][0-9][0-9]" ...
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3answers
657 views

Does the shell fork when I use built-in commands?

When I run the command type type, this gives me a result to standard output. So, now I try this command: type type > abc.txt It redirects the standard output to abc.txt. So, if this command ...
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1answer
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using command line arguement with redirecting

#include <fcntl.h> #include <stdio.h> #include <stdlib.h> int main(int argc, char** argv) { int val; if(argc!=2) { printf("Usage: %s ...
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Is the delimiter in here document seen and used by the program which takes it as input?

The format of here-documents is: <<[−]word here-document delimiter Is the delimiter seen and used by the program which takes the here document as stdin input? Or is the delimiter used by the ...
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2answers
110 views

Order between `nohup`, `&` and redirection?

What is the correct order between nohup, & and redirection, when using together? Why are other orders not correct? For example, $ nohup firefox& &> /dev/null $ nohup firefox ...
3
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1answer
70 views

Stdin input to a command with nohup

from http://www.gnu.org/software/coreutils/manual/html_node/nohup-invocation.html nohup runs the given command with hangup signals ignored, so that the command can continue running in the ...
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bash: tee eats newlines after exec

To output both stdout and stderr from a script to the console and to a log file, I’ve found a beautiful solution with tee: exec &> >(tee log.file) The problem is, that sometimes tee seems ...
2
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3answers
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Script output depending on parameter

I have script that prints out few lines of text. This text is used as configuration by other process. When I normally run program I redirect standard output to file which is fine: ...
7
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1answer
258 views

How can I pipe only stderr in zsh?

In bash: $ echo hi 2>&1 1>/dev/null | cat $ While in zsh: $ echo hi 2>&1 1>/dev/null | cat hi $ Is there a way to pipe only standard error while redirecting standard out?
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1answer
31 views

trap echo does print not write to console when writing to file

When I terminate "CTRL+C" the following script #!/bin/bash function waa(){ for i in {1..10} do echo "start.sh: sleeping $i" sleep 1s done } trap "echo 'GAGA'" TERM waa ...
2
votes
1answer
41 views

Two input pipes through file descriptor shuffling and /dev/fd

I want to pipe two programs into one. If my shell supports it, I can use Process substitution. For example, to list the common lines of two files in indifferent order, I can use comm -12 <(sort a) ...
0
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2answers
169 views

Bash - Redirect output to a variable or file descriptor, then read from the variable or file descriptor

Here's an example of a bash script that redirects all output to a file (and shows output on the screen too): # writing to it exec > >(tee --ignore-interrupts ./log) exec 2>&1 echo "here ...
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1answer
76 views

patch: “bad file descriptor” in combination with pipe and LD_PRELOAD/ libtrash

Very short summary: cat "$file" | patch produces a Bad file descriptor-error, but patch < "$file" works, when libtrash is preloaded with LD_PRELOAD, and only on bigger projects. Description: For ...
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1answer
35 views

How to get file descriptor number of a output and how to redirect all of them at once

Lets take this command for example $ time ssh ec2 ls www apps real 0m0.554s user 0m0.004s sys 0m0.000s $ time ssh ec2 ls 1>/dev/null real 0m0.554s user 0m0.004s sys 0m0.000s ...
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2answers
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Trying to direct variable contents into command gives Ambigious Redirect

I'm trying to write a script which tests the http response from various Node servers I have running and which then emails me if any respond with a 502 error from Nginx (meaning one or more server has ...
1
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3answers
34 views

How do I organize and cut this portion of a directory into a file?

I'm new to unix and I'm not sure how to name my title but my main objective is to copy the contents of a directory and send it to a file. Then cut out the directory location to just have the name. ...
4
votes
1answer
30 views

Prevent customized bash autocomplete from affecting io redirection (< and >)

I've used complete -W 'firstdatabase databasetwo etc' mysql to save a list of databases, so that I can autocomplete them using the mysql command. However, in many cases I would type mysql db < ...
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2answers
79 views

Unable to run a process on background

I need help with a Solaris system. I have used the following command to run the cp.sh script in the background. nohup sh cp.sh < /dev/null & > /dev/null & output: Invalid null ...