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

learn more… | top users | synonyms (1)

61
votes
5answers
195k views

How to append multiple lines to a file with bash

I am writing a bash script to look for a file if it doesn't exist then create it and append this to it: Host localhost ForwardAgent yes So "line then new line 'tab' then text" I think its a ...
0
votes
1answer
28 views

How to redirect the warning message from command line to a log file?

I have a below loop that is used to check the active namenode in Hadoop cluster and if the first node is inactive it picks up the second. for (( i=0;i<$(($(wc -l < ...
0
votes
1answer
12 views

Cache not filling error when using mplayer to read from stdin

I have a program that's generating audio and I can't get mplayer to play it. I'm doing ./myprogram | mplayer - -cache 1024 -cache-min 30 -noconsolecontrols And I get the message Cache fill: X% (Y ...
2
votes
1answer
41 views

redirect split output

I split a gz file using gunzip piped to split: time gunzip -c file.gz | split -l 500 -d -a 4 - pref_ Which generates the following files: pref_0000 pref_0001 I'd like to pipe those files to zip ...
-1
votes
0answers
27 views

ngrep output redirection with -q option

when using ngrep in bash with -q option the '####' characters are gone and only the matches are shown but redirecting this to a text file results only in a empty file. sudo ngrep -q >> log.txt ...
0
votes
2answers
40 views

Can sed save its output to a file?

> can do it. echo "text" > file tee can do it. echo "test" | tee file Can sed do it without using either of the above? Is it possible to save the output of a sed command to a file without ...
2
votes
2answers
48 views

What does the < (left chevron / triangle bracket) do? [duplicate]

I am having a difficult time wrapping my head around this. I understand that the > (right chevron) is for redirecting the STDOUT of a program to a file, as in echo 'polo' > marco.txt will ...
1
vote
0answers
15 views

Redirect all output, including input prompts [duplicate]

I know that doing command &> /dev/null suppresses all output file descriptors. But I notice that it does not work for commands that request input from the user, most notably ssh: > ssh ...
4
votes
1answer
2k views

Bash, remove lines containing string with grep

This is currently deleting the whole contents of the file in question: #!/bin/bash rpc_port="9201"; dir="/etc/apache2/ports.conf" cat $dir | grep -v "Listen $rpc_port" > $dir The file contents: ...
0
votes
1answer
32 views

Redirect output of: yum repolist all

If you run the command: yum repolist all you will see there are basically four columns of data. repo id, repo name, status, install count. If however your run something like: yum repolist all > ...
4
votes
2answers
310 views

Linux init script what does 0<&- &> achieve

I looking into writing my own init.d scripts to control several services running on my Linux server. I came across an example online which contained: nohup $EXEC_SCRIPT 0<&- &> ...
0
votes
2answers
25 views

nohup with at the end <&- [duplicate]

nohup grunt server:debug >grunt.out 2>&1 <&- & I have a nohup command that ends with <&- . What does this direction do? Can some explain with pointers. I did search ...
3
votes
2answers
504 views

Redirecting/grep'ing an existing shell's STDOUT

I run a lot of long running processes (simulations) that print progress to STDOUT. I occasionally forget to redirect to STDOUT to a file I can grep, and it's usually too far along to restart. ...
20
votes
4answers
17k views

combine output from two commands in bash

Is it possible to combine output from these two commands? node ~/projects/trunk/index.js python ~/projects/trunk/run.py run Neither command exits so I'm not sure how to do this.
0
votes
2answers
28 views

operate on different sections of stdin without intermediate file

Is there a simple way to do this with redirection or pipes without creating FILE1? I want to apply process2 to the body of the output of process1, without touching the first and last few lines. ...
8
votes
5answers
459 views

Is there a tool to dynamically redirect output to a new file on request

I'm currently redirecting the output of a monitoring tool to a file, however what I'd like to do, is to redirect this output to a new file on my request (using a keybinding), without stopping the said ...
0
votes
2answers
1k views

How to redirect output of a 'xterm -hold -e' command

I know you can use tee to log output from a command in a terminal, e.g.: df -h | tee ~/log.txt However, how can I do this with a more complex command like this: xterm -hold -e program arg1 arg2 ...
0
votes
2answers
38 views

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 ...
4
votes
1answer
833 views

KSH styling text based menu using STDERR

Is it possible to format the STDERR in order to have a better looking menu using the select command? I have a simple select select oChoice in $(<tempMenu.menu) ; do case "$oChoice" in ...
5
votes
2answers
31 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 ...
0
votes
1answer
36 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 ...
8
votes
4answers
561 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
votes
1answer
5k views

Correct syntax to avoid bash 'ambiguous redirect' message

I am trying to execute a command which accumulates stdout into an existing file and sends the error messages to another using the command below. commmand >> /home/user/accumulate_output.log ...
0
votes
1answer
268 views

Ambigous redirect done >$2

I have finished my bash script but I am still getting two errors. I've also searched for related issues but none worked: #!/bin/bash avem_element () { local e for e in "${@:2}"; do [[ "$e" == ...
2
votes
2answers
243 views

Error message on terminal despite using 2> symbol

Why does this command show error message despite using 2> symbol? cat < foo 2>bar bash : cannot open foo : No such file or directory
6
votes
1answer
116 views

Extra output on input redirection

I have a folder with three files: $ ls a b c If I pipe the output of ls to wc, I get the right result: $ ls | wc -l 3 However, when I specify the input to wc as the output of ls, I get extra ...
7
votes
2answers
280 views

Redirecting standard error output to bash variable

Here's a C code snippet that will cause segfault: // segfault.c #include <string.h> int main() { memset((char *)0x0, 1, 100); return 1; } Compile it with: gcc segfault.c -o ...
2
votes
2answers
315 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
votes
1answer
28 views

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 ...
40
votes
5answers
15k views

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 ...
1
vote
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 ...
2
votes
1answer
25 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 ...
4
votes
4answers
3k views

Getting stdin from a named pipe

What I am trying to do is run python in a terminal window and redirect it's stdin from a named pipe. Then I write to the named pipe in another terminal and have that command execute on python. ...
0
votes
1answer
34 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 ...
1
vote
1answer
158 views

Sed regex printing into a file.txt

I'm connected to a MQTT stream, and I filter out the data by entering : mosquitto_sub -d -t +/# 2>&1 | sed -n "/PUBLISH/{s|.*\('.*',\).*|\1$(date),|;N;s/\n//;p}" into the Ubuntu terminal. ...
1
vote
2answers
80 views

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 ...
1
vote
5answers
3k views

Why does the command shuf file > file leave an empty file?

I know this is sort of a duplicate of another question (Why this sort command gives me an empty file?) but I wanted to expand on the question in response to the answers given. The command shuf ...
35
votes
5answers
22k views

How can I make iconv replace the input file with the converted output?

I have a bash script which enumerates through every *.php file in a directory and applies iconv to it. This gets output in STDOUT. Since adding the -o parameter ( in my experience ) actually writes ...
2
votes
2answers
252 views

xargs and < and > to the same files

I want to run git stripspace on all my source code files, on each build (or precommit or just manually), to remove extraneous whitespace, and later maybe to format the code in a standard way, So ...
0
votes
6answers
109 views

Is there a way to do a “full file buffering” inside a chain of piped commands?

Very often I do simple operations on a file, for example: cat file1.txt|sed -r 's/^ *//' to remove spaces at the beginning of the line. If I want to overwrite the file, the only way I know is: cat ...
8
votes
6answers
1k views

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 ...
36
votes
5answers
48k views

How to view the output of a running process in another bash session?

I have left a script running on a remote machine from when I was locally working at it. I can connect over SSH to the machine as the same user and see the script running in ps. $ ps aux | grep ...
19
votes
5answers
18k views

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 ...
3
votes
2answers
2k views

How do I redirect only stderr?

I am having a bit of trouble doing this. I am required to run a compiled .java file and redirect only stderr to a file called error. So the .java file is named javaProgram.java. This is what I have ...
0
votes
2answers
50 views

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?
0
votes
1answer
29 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 ...
0
votes
0answers
21 views

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 ...
0
votes
2answers
51 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 ...
1
vote
2answers
87 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 ...
39
votes
3answers
8k views

What are the shell's control and redirection operators?

I often see tutorials online that connect various commands with different symbols. For example: command1 | command2 command1 & command2 command1 || command2 command1 && command2 ...