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How to redirect output to a file and stdout
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Closed 4 years ago.
I am trying to throw command output to file plus console also. This is because i want to keep record of output in file. I am doing following and it appending to file but not printing ls output on terminal.
$ls 2>&1 > /tmp/ls.txt
Yes, if you redirect the output, it won't appear on the console. Use tee.
ls 2>&1 | tee /tmp/ls.txt
It is worth mentioning that 2>&1 means that standard error will be redirected too, together with standard output. So
someCommand | tee someFile
gives you just the standard output in the file, but not the standard error: standard error will appear in console only. To get standard error in the file too, you can use
someCommand 2>&1 | tee someFile
(source: In the shell, what is " 2>&1 "? ). Finally, both the above commands will truncate the file and start clear. If you use a sequence of commands, you may want to get output&error of all of them, one after another. In this case you can use -a flag to "tee" command:
someCommand 2>&1 | tee -a someFile
In case somebody needs to append the output and not overriding, it is possible to use "-a" or "--append" option of "tee" command :
ls 2>&1 | tee -a /tmp/ls.txt
ls 2>&1 | tee --append /tmp/ls.txt
Related
I know this much:
$ command 2>> error
$ command 1>> output
Is there any way I can output the stderr to the error file and output stdout to the output file in the same line of bash?
Just add them in one line command 2>> error 1>> output
However, note that >> is for appending if the file already has data. Whereas, > will overwrite any existing data in the file.
So, command 2> error 1> output if you do not want to append.
Just for completion's sake, you can write 1> as just > since the default file descriptor is the output. so 1> and > is the same thing.
So, command 2> error 1> output becomes, command 2> error > output
Try this:
your_command 2>stderr.log 1>stdout.log
More information
The numerals 0 through 9 are file descriptors in bash.
0 stands for standard input, 1 stands for standard output, 2 stands for standard error. 3 through 9 are spare for any other temporary usage.
Any file descriptor can be redirected to a file or to another file descriptor using the operator >. You can instead use the operator >> to appends to a file instead of creating an empty one.
Usage:
file_descriptor > filename
file_descriptor > &file_descriptor
Please refer to Advanced Bash-Scripting Guide: Chapter 20. I/O Redirection.
Like that:
$ command >>output 2>>error
Or if you like to mix outputs (stdout & stderr) in one single file you may want to use:
command > merged-output.txt 2>&1
Multiple commands' output can be redirected. This works for either the command line or most usefully in a bash script. The -s directs the password prompt to the screen.
Hereblock cmds stdout/stderr are sent to seperate files and nothing to display.
sudo -s -u username <<'EOF' 2>err 1>out
ls; pwd;
EOF
Hereblock cmds stdout/stderr are sent to a single file and display.
sudo -s -u username <<'EOF' 2>&1 | tee out
ls; pwd;
EOF
Hereblock cmds stdout/stderr are sent to separate files and stdout to display.
sudo -s -u username <<'EOF' 2>err | tee out
ls; pwd;
EOF
Depending on who you are(whoami) and username a password may or may not be required.
I need to run a script and output the full contents to the terminal.
I then want to get the last 10 lines from the terminal output and put them in a log file.
I have tried using ./script.sh 2>&1 | tail -10 > log.log
but this stops the output to the terminal.
Leverage process substitution of bash with tee:
./script.sh |& tee >(tail -10 >file.txt)
|& is a shortcut for sending both STDOUT and STDERR over the pipe.
tee redirects it's STDIN to both STDOUT and to the file(s) given as argument(s) -- we have used process substitution to get a file descriptor and used tail -10 >file.txt inside process substitution to save the desired content.
For that you have to use the tee command. Then you can pipe to a file and your console.
ls -a | tee output.file
I have a script which run this command successfully. I am using this command in another script which gives me error on this line (.md5: Permission denied).
I am running the previous script with sudo.
for i in ${NAME}*
do
sudo md5sum $i | sed -e "s/$i/${NAME}/" > ${NAME}.md5${i/#${NAME}/}
done
So you want to redirect output as root. It doesn't matter that you executed the command with sudo, because redirection is not part of the execution, so it's not performed by the executing user of the command, but by your current user.
The common trick is to use tee:
for i in ${NAME}*
do
md5sum $i | sed -e "s/$i/${NAME}/" | sudo tee ${NAME}.md5${i/#${NAME}/}
done
Note: I dropped the sudo from md5sum, as probably you don't need it.
Note: tee outputs in two directions: the specified file and stdout. If you want to suppress the output on stdout, redirect it to /dev/null.
You take the output of sudo md5sum $i and pipe it to a sed which is not running as root. sudo doesn't even know this sed exists.
But that's not the problem, because the sed does not need root permissions. The problem is > ${NAME}.... This redirects the output of sed to the file with this name. But the redirection is actually executed by your shell which is running as your user. And because > is a shell built-in operator, you can not prefix it with sudo.
The simple solution is to use tee. tee is a program (so you can run it with sudo) which writes it's input to the standard output and also to a file (like a T-Pipe, hence the name).
So you can just:
for i in ${NAME}*
do
md5sum $i | sed -e "s/$i/${NAME}/" | sudo tee ${NAME}.md5${i/#${NAME}/}
done
Note this will also dump all hashes to your standard output.
New to bash programming. I am not sure what is meant by 'output to stdout'. Does it mean print out to the command line?
If I have a simple bash script:
#!/bin/bash
wget -q http://192.168.0.1/test -O - | grep -m 1 'Hello'
it outputs a string to the terminal. Does this mean it's 'outputting to stdout' ?
Thanks
Yes, stdout is the terminal (unless it's redirected to a file using the > operator or into the stdin of another process using |)
In your specific example, you're actually redirecting using | grep ... through grep then to the terminal.
Every process on a Linux system (and most others) has at least 3 open file descriptors:
stdin (0)
stdout (1)
stderr (2)
Regualary every of this file descriptors will point to the terminal from where the process was started. Like this:
cat file.txt # all file descriptors are pointing to the terminal where you type the command
However, bash allows to modify this behaviour using input / output redirection:
cat < file.txt # will use file.txt as stdin
cat file.txt > output.txt # redirects stdout to a file (will not appear on terminal anymore)
cat file.txt 2> /dev/null # redirects stderr to /dev/null (will not appear on terminal anymore
The same is happening when you are using the pipe symbol like:
wget -q http://192.168.0.1/test -O - | grep -m 1 'Hello'
What is actually happening is that the stdout of the wget process (the process before the | ) is redirected to the stdin of the grep process. So wget's stdout isn't a terminal anymore while grep's output is the current terminal. If you want to redirect grep's output to a file for example, then use this:
wget -q http://192.168.0.1/test -O - | grep -m 1 'Hello' > output.txt
Unless redirected, standard output is the text terminal which initiated the program.
Here's a wikipedia article: http://en.wikipedia.org/wiki/Standard_streams#Standard_output_.28stdout.29
I have some scripts where I need to see the output and log the result to a file, with the simplest example being:
$ update-client > my.log
I want to be able to see the output of the command while it's running, but also have it logged to the file. I also log stderr, so I would want to be able to log the error stream while seeing it as well.
update-client 2>&1 | tee my.log
2>&1 redirects standard error to standard output, and tee sends its standard input to standard output and the file.
Just use tail to watch the file as it's updated. Background your original process by adding & after your above command After you execute the command above just use
$ tail -f my.log
It will continuously update. (note it won't tell you when the file has finished running so you can output something to the log to tell you it finished. Ctrl-c to exit tail)
You can use the tee command for that:
command | tee /path/to/logfile
The equivelent without writing to the shell would be:
command > /path/to/logfile
If you want to append (>>) and show the output in the shell, use the -a option:
command | tee -a /path/to/logfile
Please note that the pipe will catch stdout only, errors to stderr are not processed by the pipe with tee. If you want to log errors (from stderr), use:
command 2>&1 | tee /path/to/logfile
This means: run command and redirect the stderr stream (2) to stdout (1). That will be passed to the pipe with the tee application.
Learn about this at askubuntu site
another option is to use block based output capture from within the script (not sure if that is the correct technical term).
Example
#!/bin/bash
{
echo "I will be sent to screen and file"
ls ~
} 2>&1 | tee -a /tmp/logfile.log
echo "I will be sent to just terminal"
I like to have more control and flexibility - so I prefer this way.