Question

UNIX Shell script stream output buffering with "tee"

Asked by: father-o-seamus

Hi - I have the following script, it appears though that when I pipe the output from do_countdown through any kind of stream filter - sed, grep, tr ..etc, and then through tee that the output is being buffered until do_countdown completes.

Does anyone know why this is, or if there's a workaround? Thanks!


script
=======

#!/bin/sh

COUNT_OUTPUT=./count_out

do_countdown()
{
    COUNT=5
    while [ $COUNT -gt 0 ];
    do
        echo "count $COUNT"
        sleep 1
        COUNT=`expr $COUNT - 1`      
    done
}


do_countdown | tee $COUNT_OUTPUT                # unbuffered ?
do_countdown | tr "[a-z]" "[A-Z]" | tee $COUNT_OUTPUT        # buffered ?

output
=======

count 5
count 4
count 3
count 2
count 1
COUNT 5
COUNT 4
COUNT 3
COUNT 2
COUNT 1

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Asked On
2007-08-31 at 01:12:53ID22799421
Tags

shell

,

tee

,

unix

,

script

Topics

Bourne Shell (sh)

,

Unix Operating Systems

,

Shell Scripting

Participating Experts
3
Points
0
Comments
6

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Answers

 

by: ozoPosted on 2007-08-31 at 01:41:33ID: 19806696

do_countdown | perl -pe 'BEGIN{open T,">",shift or die $!;$"=1}tr[a-z][A-Z]; print T' $COUNT_OUTPUT

 

by: dgraingerPosted on 2007-10-19 at 17:48:49ID: 20113351

Commands like sed, grep, tr, etc are buffered. This is just how they are written. If you check the man pages for them, you might find they have an unbuffered mode. I'm not on a Solaris box at the moment but I know that the Linux implementation of sed has a -u option for unbuffered processing. I have never seen an unbuffered version of tr. For grep, look for --line-buffered. This is less likely to be on the native Solaris grep but then anything you can do with grep you can do with sed.

 

by: father-o-seamusPosted on 2007-10-22 at 01:11:35ID: 20121475

Apologies for the slow response on this one

the solution is more simple than I thought - if you write your own tee function in shell it's unbuffered

# Function:       tee_unbuffered()
#            behaves as per original UNIX tee command
#
# Parameters:      1) output file
#
tee_unbuffered()
{      
      if [ $# -eq 2 ] && [ "$1" = "-a" ]; then
            APPEND=YES
            PARAM_OUTPUT_FILE=$2
      else
            APPEND=NO
            PARAM_OUTPUT_FILE=$1
      fi
      
      if [ "$APPEND" = "NO" ]; then
      
            # remove output file if it already exists
            if [ -f $PARAM_OUTPUT_FILE ]; then            
                  rm $PARAM_OUTPUT_FILE
            fi
      fi
      
      while read LINE;
      do            
            echo "$LINE"            
            echo "$LINE" >> $PARAM_OUTPUT_FILE
      done
}

 

by: ozoPosted on 2008-01-10 at 12:53:06ID: 20631087

an answer was given

 

by: Computer101Posted on 2008-01-14 at 18:23:39ID: 20659580

PAQed with points refunded (185)

Computer101
EE Admin

20120131-EE-VQP-002

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