Redirecting STDERR to file and screen, STDOUT only to file
I have to redirect STDERR messages both to screen and also capture the same in a file but STDOUT only to the same file.
I have searched in this formum for a solution, but something like
doesn't work for me...
Don't know if I understand your question correct, but this works I think:
If no error, result is only in file "errs", if error result is both on screen and in file "errs".
I have to redirect STDERR messages both to screen and also capture the same in a file but STDOUT only to the same file.
doesn't work for me...
Lets start with some general information to make the problem understandable:
A Unix process is like a (y-shaped) water hose: you fill something in (via <stdin>) and something comes out (via <stdout>) the one way and something else the other way (<stderr>). You can - to continue the analogy - put a bucket under each of the outlets, even the same bucket under several outlets, but they will still remain different outlets of data.
By default, when a process is born, its 3 default I/O-channels are directed to:
stdin: keyboard
stdout: display
stderr: display
As <stdout> and <stderr> are both pointing to display, why is it that
picks up the output from <stdout> but not from <stderr>? The answer is that "|" is a special form of connector, not just another bucket like ">". "|" means: redirect <stdout> of process_1 to <stdin> of process_2.
Now there is another redirection device, which is:
This means: redirect output channel 2 (=<stderr>) to where output channel 1 (=<stdout>) points to right now. All redirections are read and carried out from left to right.
Having understood this let us try to solve your problem:
will redirect <stderr> to <stdout>, so the next "|" will catch <stderr> output now too. Closer!
This will pick up everything coming out of <stderr> and <stdout> of script and display it as well as appending it to "/some/file". Closer again, but <stdout> should not be displayed, so we will have to direct it away before the pipe picks up its input:
This finally does what we want: output to <stdout> is put into "/some/file", output to <stderr> is being displayed before being appended to /some/file" too.
The only uncertainty left is that i am not sure if the exact sequence of the messages will be preserved, especially if there is high load and many messages. You will have to try that. I'll be tankful if you could post a follow-up telling us this.
The y-shaped hose and bucket analogy paints a memorable image. Nicely done. The only thing I'd add, explicitly (it's implied in your explanation), is that pipe redirection occurs before other, left-to-right redirections.
Quote:
Originally Posted by bakunin
The only uncertainty left is that i am not sure if the exact sequence of the messages will be preserved...
Even without the pipe and redirections, you can't depend on your average script/executable to emit messages in the exact order that they're generated, since typically a mix of unbuffered (stderr) and buffered (stdout) streams are used.
Of much greater importance is that script 2>&1 1>/some/file | tee -a /some/file involves multiple processes writing to the same file without any form of communication. Writes from script will clobber writes from tee, or vice versa. If they at least shared a file descriptor, while one message could still split another in two, there would never be any overwriting.
The y-shaped hose and bucket analogy paints a memorable image. Nicely done.
Thank you for this, as well as the additional info. I was not aware that <stderr> is unbuffered while <stdout> is not, so i have learned more here than i have explained. Nice gain. ;-))
It is probably a good idea to do like i have always done (out of luck - you have given me a reason after all) in my scripts: prepend standard and error output with respective prefixes:
Bakunin, it appears that I was editing my previous post just as you were responding. I apologize for that incovenience.
Your suggestion won't work at all. As I mentioned in my post (probably after you read it but before you posted), script and tee will clobber each other since they're using file descriptors backed by independent file descriptions each with their own offset.
Quote:
Originally Posted by 244an
Don't know if I understand your question correct, but this works I think:
If no error, result is only in file "errs", if error result is both on screen and in file "errs".
That will not work for the same reason as bakunin's suggestion.
Quote:
Originally Posted by thuranga
I have to redirect STDERR messages both to screen and also capture the same in a file but STDOUT only to the same file.
I have searched in this formum for a solution, but something like
doesn't work for me...
Has anyone an idea???
That approach cannot be made to work because, from the tee-side of the pipe, the distinction between stdout and stderr has been lost; they've been merged into one stream.
The following should work:
That may look like a useless use of cat (Hi, Corona ), but writing to a pipe guarantees atomicity for writes up to PIPE_BUF bytes inclusive.
If you rather not have that second pipe, you can simply delete highlighted text, leaving only the logfile redirection. However, if you do that, you may see interleaving of messages even for small writes.
Dear all,
redirecting STDOUT & STDERR to file is quite simple, I'm currently using:
Code:
exec 1>>/tmp/tmp.log; exec 2>>/tmp/tmp.log
But during script execution I would like the output come back again to screen, how to do that?
Thanks
Luc
edit by bakunin: please use CODE-tags like the... (6 Replies)
Hi folks
I need/want to redirect output (stdout, stderr) from an exec call to separate files. One for stderr only and two(!) different (!) ones for the combined output of stderr and stdout.
After some research and testing i got this so far :
(( exec ${command} ${command_parameters} 3>&1... (6 Replies)
Dear all,
redirecting STDOUT & STDERR to file is quite simple, I'm currently using:
exec 1>>/tmp/tmp.log; exec 2>>/tmp/tmp.logBut during script execution I would like the output come back again to screen, how to do that?
Thanks
Lucas (4 Replies)
I originally wrote my script using the korn shell and had to port it to bash on a another server. My script is working find for backing up but noticed that now after the move, I am not getting any output to my log files.
Using Korn shell, this worked for me for some odd reason. This was sending... (2 Replies)
Hi,
I already searched through the forum and tried to find a answer for my problem but I didn't found a full working solution, thats way I start this new thread and hope, some can help out.
I wonder that I'm not able to find a working solution for the following scenario:
Working in bash I... (8 Replies)
Hi
I am not if this is possible: is it possible in bach (or another shell) to redirect GLOBALLY the stdout/stderr channels to a file.
So, if I have a script
script.sh
cmd1
cmd2
cmd3
I want all stdout/stderr goes to a file. I know I can do:
./script.sh 1>file 2>&1
OR
... (2 Replies)
working on a c sell script
I think I understand the concept of it, which is:
filename >> file.txt (to appaend)
or filename | tee -a file.txt (to append)
The problem is that my shell script is used with several parameters, and these commands don't seem to work with just filename. They... (2 Replies)
Hi friends
I am facing one problem while redirecting the out of the stderr and stdout to a file
let example my problem with a simple example
I have a file (say test.sh)in which i run 2 command in the background
ps -ef &
ls &
and now i am run this file and redirect the output to a file... (8 Replies)
Friends
I have to redirect STDERR messages both to screen and also capture the same in a file.
2 > &1 | tee file works but it also displays the non error messages to file, while i only need error messages.
Can anyone help?? (10 Replies)
In bash, I need to send the STDOUT and STDERR from a command to one file, and then just STDERR to another file. Doing one or the other using redirects is easy, but trying to do both at once is a bit tricky. Anyone have any ideas? (9 Replies)