exec redirection plus tee? possible?

[Trever Furnish]

Using the handy functionality of exec to redirect filehandles for an entire script has really cleaned up our scripts:

exec 1>log 2>&1

...but now I'm wondering if there's any way to use that in combination with tee.

The end result that I want is:
- Only specify redirection once, at the beginning of the script.

- Always have stdout go to a log file.

- If the script was run on an interactive terminal, then also have stdout go to the terminal (and still to the logfile).

A simple function to both send a message to a log and to the terminal isn't enough - I need to send stdout both places.

I guess I could buffer all output into variables and then echo it, but that gets ugly.

I think what I really need is some way to attach stdout to two file descriptors and then redirect those file descriptors separately.

Possible? Suggestions?

[Steve Steel]

Hi

Have you ever looked at the script command

It will do what you ask.

You get normal i/o to the screen and a copy in a log.


Steve Steel

[Trever Furnish]

Script - Hmmm... That's an approach I hadn't thought of. I don't really want to launch a separate sub-shell for every process though and I'm not sure what I'd do with the output on stdout when the script is run non-interactively. That would also complicate quoting quite a bit.

[Robin Wakefield]

Hi,

==========================
#!/usr/bin/ksh

exec 1>/tmp/ls.log 2>&1
ls |tee /dev/tty
==========================

this should log to the screen and to the file.

To see if you're in interactive mode, you can use the syntax:

if [ -t 0 ] ; then
...
...
fi


rgds, Robin

[John Palmer]

Neither script or tee will do what you want because having used exec, all the output is going to your log file only.

You could experiment with 'tail -f' to display the logfile as it is written to your terminal.

Simple way:

tail -f
You have to ctrl-C the tail command when the script finishes.

Possible scripted way, you may only want to do this if a certain flag is supplied to the script:

CHILD=""
exec 1>log 2>&1
if [[ -t 1 ]]; # controlling terminal exists
then tail -f &
CHILD=${!}
fi
... body of script ...

Yourscript exit code to kill the tail:
if [[ -n ${CHILD} ]];
then kill ${CHILD}
fi
exit

Regards,
John

[Trever Furnish]

Robin, thanks, but that method doesn't work - the pipe into tee overrides the redirection from exec for that command.

And if it did work, then I'd be back at the beginning, appending something to every command.

[Trever Furnish]

John,

Can't eyeball that one and tell if it'll work, but it's going down the only path I've been able to imagine so far - forking so that one process does the actual work and another displays the output on an interactive terminal. I was really hoping there was a simpler way. :-)

[curt larson_1]

well here is one way

#!/usr/bin/ksh

logfile=
tty=$(tty)
exec 3> $tty
if [ -t = 0 ] ;then
/usr/local/bin/outputredirect $tty $logfile |&
exec >&p
else
exec 1>$logfile 2>&1
fi
rest of your script

file outputredirect
#!/usr/bin/ksh
tty=$1
logfile=$2

while read line
do
print $line >> $logfile
print $line > $tty
done

give that a try and see how it works for you.

[Jordan Bean]

At the moment I don't have access to either a UNIX or Linux system, so I'll have to wing it. I'm thinking of co-processes... Try this in the POSIX shell:

logfile=$0.log
if [ -t 0 ]
then
tee $logfile |&
exec 1>&p 2>&p
else
exec 1>$logfile 2>&1
fi

[Trever Furnish]

Jordan, yours was close but didn't quite work. Curt, yours worked.

BTW, the bits about co-processes in the manual pages never made sense until now - thanks for being great teachers! :-)

Curt, when I first tested your method, I didn't think it worked - not sure how I goofed, but it does work perfectly, so if you want the rest of the points, just post again and I'll assign them.

Thanks for your help, everyone.

[Curtis Larson_2 · Accepted solution]

a couple of additions,

you probably caught that i left out redirection of stderr to the coprocess.

/usr/local/bin/outputredirect $tty $logfile |&
exec >&p

should have been
/usr/local/bin/outputredirect $tty $logfile |&
exec >&p 2>&p

the file outputredirect works using the tee command also:
#!/usr/bin/ksh
tee -a $2 >$1

but I can not tell you why it works in a seperate file but not as part of the shell script

[Curtis Larson_2]

but you don't need the outputredirect file if you use
/usr/bin/ksh -c tee -a $logfile >$tty |&

that way everything can just be in the one shell script.

[Jordan Bean]

Oh, I'm being stupid. stdout from the co-process returns to the parent script. This means that, in this case, tee is writing to the log file, but also back to the script.

[Jordan Bean]

Got it! Simple fix. You must also tell tee to send stdout to stderr (aka tty) when becoming a co-proc so it doesn't map back to the returning pipe.

logfile=$0.log
if [ -t 0 ]
then
tee $logfile >&2 |&
exec >&p 2>&1
else
exec >$logfile 2>&1
fi

[Jordan Bean]

Curtis' method accomplishes the same task.