Sheel Script redirection tip - Tilted Forum Project Discussion Community

Silvy · 03-01-2004, 06:12 AM

Hi,

Note: this tip is about redirecting output to a file and the screen at the same time. The original 'hack' has been replaced by a much better solution. See the second post!

I'm leaving my 'hack' as an odd example of the many ways problems can be solved.

I justed wanted to share this neat 'hack' I figured out:

To redirect output to a file, but still see it on screen at the same time, use this construct:

Code:

#!/bin/bash
OUPUTFILE=~/output.txt
some_long_command >$OUTPUTFILE &
tail -f $OUTPUTFILE

The idea being:
You want to see the output of a command, but you also want to keep it as reference.

The code below would do this as well, but if the 'command' took a long time you'd be staring at an empty line during execution.

Code:

# this works but is not usable for commands
# that have a long execution time.
command > output_file
cat output_file

I run Gentoo Linux, and emerge's take a relatively long time, so I want to see the output to wether all is well. But in the future I might also want to read back the output to see what warnings etc it might have given.

I'm planning on writing a mother-script around 'emerge' that appears the same as the regular 'emerge' (with it's screen output), but also keeps rotating logs of the results. This construction lets me do that.

NOTES:
- I couldn't find an "output splitter" that redirects output to two places (ie screen and file) or that would've been better.
- Over slow network connections Tail seriously lags behind the actual running of the script, and is very 'jumpy' in its output.
- The above code is not executable, as I'm not presenting a specific situation in it. Simple examples could be easily made however.
- I hope this tip helps someone and/or interests them in scripting for themselves!

mgcloud · 03-05-2004, 12:06 AM

Quote:

Originally posted by Silvy

NOTES:
- I couldn't find an "output splitter" that redirects output to two places (ie screen and file) or that would've been better.

Very cool stuff

Was doing some digging around on this, and foudn (but haven't tested) a command called tee. 'man tee' says:

Quote:

tee - read from standard input and write to standard output and files

Silvy · 03-05-2004, 01:00 AM

Thanks, I'll have a look-see!

Edited the spelling of the subject line too, too bad it doesn't update the Board thread listing

EDIT: yes, `tee` works fine!

Thanks a lot , it works a lot better than my 'ugly' solution...

In case anyone is interested: this is the script I tested it with:
replace "some_long_command" by a command that takes a long time to execute and produces output during execution. (I used un-tar on a large file)

Code:

#!/bin/bash
OUTPUTFILE=output.txt
COMMAND="some_long_command |tee $OUTPUTFILE" 
echo $COMMAND  
eval $COMMAND

03-01-2004, 06:12 AM	#1 (permalink)
Silvy paranoid Location: The Netherlands	Shell Script redirection tip Hi, Note: this tip is about redirecting output to a file and the screen at the same time. The original 'hack' has been replaced by a much better solution. See the second post! I'm leaving my 'hack' as an odd example of the many ways problems can be solved. I justed wanted to share this neat 'hack' I figured out: To redirect output to a file, but still see it on screen at the same time, use this construct: Code: #!/bin/bash OUPUTFILE=~/output.txt some_long_command >$OUTPUTFILE & tail -f $OUTPUTFILE The idea being: You want to see the output of a command, but you also want to keep it as reference. The code below would do this as well, but if the 'command' took a long time you'd be staring at an empty line during execution. Code: # this works but is not usable for commands # that have a long execution time. command > output_file cat output_file I run Gentoo Linux, and emerge's take a relatively long time, so I want to see the output to wether all is well. But in the future I might also want to read back the output to see what warnings etc it might have given. I'm planning on writing a mother-script around 'emerge' that appears the same as the regular 'emerge' (with it's screen output), but also keeps rotating logs of the results. This construction lets me do that. NOTES: - I couldn't find an "output splitter" that redirects output to two places (ie screen and file) or that would've been better. - Over slow network connections Tail seriously lags behind the actual running of the script, and is very 'jumpy' in its output. - The above code is not executable, as I'm not presenting a specific situation in it. Simple examples could be easily made however. - I hope this tip helps someone and/or interests them in scripting for themselves! __________________ "Do not kill. Do not rape. Do not steal. These are principles which every man of every faith can embrace. " - Murphy MacManus (Boondock Saints) Last edited by Silvy; 03-05-2004 at 01:54 AM..

03-05-2004, 01:00 AM	#3 (permalink)
Silvy paranoid Location: The Netherlands	Thanks, I'll have a look-see! Edited the spelling of the subject line too, too bad it doesn't update the Board thread listing EDIT: yes, `tee` works fine! Thanks a lot , it works a lot better than my 'ugly' solution... In case anyone is interested: this is the script I tested it with: replace "some_long_command" by a command that takes a long time to execute and produces output during execution. (I used un-tar on a large file) Code: #!/bin/bash OUTPUTFILE=output.txt COMMAND="some_long_command \|tee $OUTPUTFILE" echo $COMMAND eval $COMMAND __________________ "Do not kill. Do not rape. Do not steal. These are principles which every man of every faith can embrace. " - Murphy MacManus (Boondock Saints) Last edited by Silvy; 03-05-2004 at 01:51 AM..