Questions tagged [io-redirection]
I/O redirection is used in shell scripting. This feature enables one to take the output of a command and redirect it as an input for another command, or a command can take the input from a file instead of from the standard input stream or from the terminal. The whole process helps connect data streams in various ways.
1,395
questions
0
votes
2
answers
45
views
Is it possible to execute a command before creating the file in which I want to redirect its output
I want to execute the following command:
dune exec -- ocaml-print-intf file.ml
And redirect its output to file.mli
The issue is that I can not write
dune exec -- ocaml-print-intf file.ml > file....
- 103
0
votes
3
answers
35
views
Is there an equivalent to /dev/stdout for file descriptors 3-255?
I know that one can often trick command line utilities that expect a file to pipe into stdout out by passing /dev/stdout as the file name.
I'm wondering whether it's possible to make a tool pipe to a ...
- 180
0
votes
1
answer
23
views
Is it possible to create a ncat loop?
If you had two computers, wanting to control one from another using only ncat, without using telnet, ssh or other networking tools and only two shells, how could i go about that?
I have successfully ...
3
votes
1
answer
98
views
"Ambigous Redirect" for redirecting text to a variable
I'm making a shell script to limit the amount of time someone spends on a program but it gives me these errors:
./time_limit: line 26: $Log: ambiguous redirect
./time_limit: line 16: [: root: unary ...
- 48
2
votes
1
answer
553
views
Why is using "|" or "&" provide different sizes of the result file?
Here is my explanation:
strings *.bin > bin.txt | sort -n bin.txt > logs1.txt
that gives me logs1.txt, but it has 621 KB.
strings *.bin > bin.txt & sort -n bin.txt > logs1.txt
that ...
- 29
0
votes
1
answer
32
views
How to get color in terminal but no color in output file [duplicate]
To show output in both terminal and to a file, tee can be used like below
ls --color=auto | tee opfile
But now, there is no color in the terminal. If I force colors, opfile also has colors which ...
- 4,049
-1
votes
1
answer
20
views
Using a pipe with extra arguments at the end. Correctly directing the pipe instruction with multiple arguments
General description:
I had a couple of times the same issue, that I solved by doing an step by step script. But I know that with pipe, one-liners would make my life way easier.
Simply:
command1 -flag ...
- 385
4
votes
1
answer
77
views
Bash redirection weird behavior
I was messing around with trapping exit codes and redirecting stdout and stderr of a case statement when I ran into an interesting outcome that I am hoping someone can shine some light into.
I am ...
- 339
4
votes
1
answer
165
views
feed output of command to another command as file
I can make config file of tor with cat <<EOF command as follows :
cat <<EOF
SocksPort 9050
ControlPort 9150
DataDirectory /usr/var/lib/tor/torrc
EOF
but it can save output with redirect &...
- 43
0
votes
1
answer
23
views
How to redirect while ignoring some of the text
Some potential duplicates may be the following:
How to ignore specific lines from being redirected
BUT I can't extrapolate from that to solve my problem. So, I am asking here instead.
So, here is what ...
4
votes
1
answer
186
views
Why I need to type press Enter key to finish this command? [duplicate]
I'm a new Linux user and I was doing some experiments and trying to understand Process Substitution. I believe I already have a basic understanding of it. But here is a case that I don't know why. I'm ...
- 855
0
votes
1
answer
89
views
How to keep the output order when redirecting both stdout and stderr?
To describe the issue, I created a simple C program.
root@u2004:~# cat test.c
#include <stdio.h>
int main(){
printf("output 1\n");
fprintf(stderr, "error 1\n");
...
- 855
0
votes
1
answer
31
views
stdin/stderr redirection, piping and color
I've seen a zillion permutations on this question but I think none is an exact duplicate.
Can I copy only stderr (or eventually stderr and stdin) to a file, while keeping the colorized output on the ...
1
vote
1
answer
34
views
How does Input/Output Redirection work in Linux/Unix?
Example:
cat < test.txt
Is the content of the file test.txt written/passed to stdin of the cat, and then the cat reads its stdin?
OR
Does the file test.txt itself become the stdin of the cat? ...
- 13
0
votes
0
answers
24
views
pass output as grep pattern [duplicate]
I wish to pass each line of a file as a grep pattern to be searched in data.txt file.
Thus, i need cat allsearchstrings.txt | grep <should be lines of allsearchstrings.txt> data.txt
cat ...
- 307
-1
votes
1
answer
40
views
What actually happens when I enter the following prompt on the CLI: cat demo.txt >> cat sample.txt?
Let say we have a text file demo.txt that only contains the word 'demo' and a text file sample.txt that only contains the word 'sample'.
I understand that cat demo.txt >> sample.txt would append ...
2
votes
0
answers
22
views
bash: debug (xtrace) target of redirections
Is there a way to show redirections with bash -x?
$ bash -exc 'for word in foo bat baz; do echo $word >$word; done'
+ for word in foo bat baz
+ echo foo
+ for word in foo bat baz
+ echo bat
+ for ...
- 21
0
votes
1
answer
126
views
Can redirecting stderr and stdout to one file result in an error?
I'm writing a script that takes files (filename...) as arguments, and redirects both the stdout and stderr to a file of name filename.output. In the files are commands, and I'd only like to bash once (...
- 3
0
votes
0
answers
52
views
"> $logfile" does not truncate, file size goes to 0 and a second later, is back to full size [duplicate]
I have a script that writes to a logfile like this:
$ nohup myscript.sh > myscript.out 2>&1 &
when the log file gets very large, I need to truncate it like this:
> myscript.out
I ...
- 1
0
votes
2
answers
27
views
ps aux --headers problem on openSUSE Leap 15.3 when redirect output
The following works ok on openSUSE Leap 15.3. As in the header line is repeated for each screen of output when I scroll the screen back with the scroll bar on the terminal:
ps aux --headers
But when ...
1
vote
0
answers
26
views
Why does redirection cause output to be duplicated? [duplicate]
In zsh, why does
echo hi >&2 | cat
produce the output
hi
hi
?
My understanding is
>&2 causes dup2(STDERR_FILENO, STDOUT_FILENO) to be called.
| cat causes dup2(fd, STDOUT_FILENO) to ...
- 356
1
vote
1
answer
50
views
Redirect Output Back to File [duplicate]
I issued the following command, thinking it would take the contents of foo.txt, process it, and write it back to the original file. Instead when I opened the file, it was empty.
cat foo.txt | tr "...
- 115
0
votes
2
answers
41
views
Why do these two commands to write text-processing results back to the input file behave so differently?
I have a file authorized_keys and want to deduplicate the content, i.e. remove duplicate entries.
I found two possible solutions to achieve this:
Use cat and uniq, then redirect the output to the ...
- 1
0
votes
0
answers
57
views
How do I capture binary output from a process different from the console output?
I'm currently able to launch a process from within Java's ProcessBuilder, which continually appends to a (binary) file output.WAV, and is continually dumping lines of console updates. That works ...
- 153
0
votes
2
answers
36
views
Redirect bash script output of application to file keeping the application interactive
I'm running a bash script expecting occasional user input to continue processing the script. Is it possible to redirect the script output to a file while keeping the terminal output and keeping the ...
- 269
0
votes
0
answers
22
views
File I/O redirection help in Bash for running badblocks into gzip and monitor with zcat [duplicate]
I am running this on a 10 TB bad drive, so the log grows out of proportions in an hour or so.
This was my idea, not working until I kill the badblocks:
badblocks -v -b 4096 -c 98304 -w -s /dev/disk/by-...
- 23.7k
2
votes
0
answers
78
views
What is the purpose of a pipe after stdout redirect?
I was reading the documentation for Kaniko and saw a shell redirect-then-pipe construct that I haven't seen before:
echo -e 'FROM alpine \nRUN echo "created from standard input"' > ...
- 121
0
votes
1
answer
91
views
grep screen output differs from output to file
When using grep (and sed also) the results I get in the terminal are different than what I get if I direct the output to file. My starting file is the captured output from a darknet yolo mAP run and ...
- 1
0
votes
1
answer
165
views
echo "Hello World" > /dev/stdout prints nothing
Scenario:
$ echo "Hello World" > /dev/stderr
Hello World
$ echo "Hello World" > /dev/stdout
$ uname -a
CYGWIN_NT-10.0 xxx 3.3.4(0.341/5/3) 2022-01-31 19:35 x86_64 Cygwin
...
- 397
2
votes
1
answer
76
views
POSIX Shell: Output to log and prepend lines with timestamps using a function
I would like a shell script to have all it's output for stdout and stderr logged to an external file with timestamps prepended to each line (plus a few extra items), without changing how the script is ...
- 73
0
votes
1
answer
145
views
CRON 2>&1 >/dev/null vs >/dev/null 2>&1 [duplicate]
I just created a tiny shell script to be run every minute.
What happened was that it started flooding my mail.
$ crontab -l
* * * * * /home/user/bin/test 2>&1 >/dev/null
I changed the ...
- 23.7k
0
votes
1
answer
52
views
redirect TTY input to output without serial cable
I'm using simulation software for unix (ucsim) that runs two simulated programs that supposed to connect to each other via its own simulated serial ports.
The software provides me with these methods ...
- 1
1
vote
0
answers
52
views
SQL output piped to `kubectl exec` crashes, if not replacing TABS with SPACES
The context:
I'm generating SQL from a backup dump, inside a subshell, to then pipe it to mysql for execution:
(
echo "SET FOREIGN_KEY_CHECKS=0;";
tar --to-stdout -xvjf archive.tar;
echo &...
- 203
0
votes
1
answer
34
views
How might I save the the output of "find .... -execdir ..." to a log?
I cannot get the following script:
#!/usr/bin/bash
/usr/bin/find ~/CS -iname "*.html.pmd" -type f -execdir /usr/bin/raco pollen render {} \; > ~/rendering
to redirect the output to the ...
- 353
0
votes
0
answers
42
views
Why doesn't creating file with echo work for remote directories in this script?
I aliased the following script:
#!/usr/bin/bash
if [[ -d "template.html" ]]; then
:
else
cp -n /home/$(whoami)/CS/Software/Pollen/template.html .
fi
/usr/bin/echo "#lang pollen
[...
- 353
0
votes
0
answers
99
views
Listening to stdout of a remote machine
I'm looking for a way to get a local terminal tail log from an stdout execution on a remote machine without directly executing with ssh.
The general procedure is:
The local client machine runs a ...
- 101
2
votes
1
answer
94
views
Create parent directories when using shell redirect
In Zsh, the following fails:
$ echo hi > /tmp/this/path/does/not/exist/out.txt
zsh: no such file or directory: /tmp/this/path/does/not/exist/out.txt
Obviously the problem is that > cannot ...
- 684
1
vote
1
answer
76
views
Redirect stdout from a runing script
In C, you can redirect stdout somewhere when a program is running, for example:
int fd = open("some_file", O_RDWR);
dup2(fd, STDOUT_FILENO);
printf("write to some_file\n");
Can I ...
- 205
3
votes
2
answers
420
views
How can I pipe a command to grep and get the exit status while still sending command output to stdout and errors to stderr?
Concrete use case:
I'm trying to curl a URL with -v, output the verbose debug information to stderr as normal, output the response body to stdout as normal, but also check for the presence of a regexp ...
- 386
1
vote
1
answer
81
views
Piping output from GitHub clone using tee misses output
I thought I would log the output of cloning a GitHub repo to a file. If I run
git clone git@github.com:my_repo/my_repo.git | tee -a log
I get an empty log file, but the following output to the ...
- 113
8
votes
4
answers
2k
views
How does "cat << EOF | grep" work in bash?
In this answer to How does "cat << EOF" work in bash? on Stack Overflow, I get the first two points. But I don't get the third point Pass multi-line string to a pipe in Bash
Pass ...
- 411
1
vote
2
answers
145
views
Avoid creating an empty file when using redirection [duplicate]
foo | bar > out.txt would normally save the output to out.txt.
However, if foo fails, then clearly bar cannot have any output, so it makes no sense to create out.txt. However, usually the result is ...
- 684
0
votes
1
answer
61
views
What's the "printing difference" between `git tag` and `git describe --tags`?
git tag seems to print in a separate process (window? app?) whereas git describe --tags is printed "in place". I can achieve the same printing behavior with git tag as git describe --tags by ...
- 37
0
votes
0
answers
178
views
How to pipe an input into the cat command
I was reading an answer regarding sending large files from one Linux system to another by quick methods, and I was trying this solution
The answer is saying that if you don't want to use the tar ...
- 259
0
votes
3
answers
82
views
Using `1>&- 2>&- &' as an alias after a shell command
To run a program with bash in background, with no text output, it is possible to use: 1>&- 2>&- &. I would like to define an alias for this. However, if I write in .bashrc
alias ...
- 4,503
1
vote
1
answer
49
views
Temporarily showing stdout while a command is running
I'm scp'ing several large files to a few servers in succession. I'd like to see the progress (so I don't want to use scp --quiet), but I don't need to see the stats when the copy is complete.
Is ...
- 2,319
0
votes
1
answer
117
views
How might I direct the output of "wget -qO- whatever.tar.gz" to both tar and sha256sum?
The following creates csfile with a hash in it, unpacks go1.17.4.linux-amd64.tar.gz downloaded from Golang's official website,
prints out the checksum to stdout and then removes the no longer needed ...
- 353
2
votes
0
answers
105
views
Possible reasons for redirection to /dev/null suddenly breaking
I am using a Bash loop to monitor a process that I spawned, and after some time (hours/days) it sometimes produces an "operation not permitted" error trying to redirect output to /dev/null:
/...
- 43
0
votes
1
answer
89
views
Pipe output of two commands into another script as arguments
ntopng can handle multiple network interfaces and I've a script that takes the name of the interface and outputs it in to stdout. I can pipe this output into ntopng
my_packet_tracer.sh | ntopng -i -
...
- 145
0
votes
2
answers
60
views
exec vscode not redirecting output to /dev/null
I have the following bash script:
exec '/mnt/c/Users/WindowsUserName/AppData/Local/Programs/Microsoft VS Code/Code.exe' $@ &>/dev/null &
When I run the script I get
➜ ~ ./code foo.js
➜ ~
...