Using pipes within variables Post: 302439000

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Top Forums Shell Programming and Scripting Using pipes within variables Post 302439000 by stv_t on Wednesday 21st of July 2010 10:33:17 AM

07-21-2010

Registered User

Using pipes within variables

Hi,

I'm trying avoid having a large number of 'if' statements in my script by setting up a variable within 'ksh' which can be expanded depending on parameters passed to the script. The output from the 'cat' command should be able pipe its output into any additional commands on the command line depending on the contents of the variable.

ie The code below should be able to list the complete file, only the executable lines, only the comment lines or be able to count the number of each type of line if required

Code:

extra_cmd=""

set -- $( getopt :ecl "$*" ) 

while [[ $1 != "--" ]] ; do
    case arg in
        "-e")  # Remove comments
                extra_cmd=" | grep -v \"^#\" " 
                ;;

        "-c") # Remove non comment lines
                extra_cmd=" | grep \"^#\" "
                ;;

         "-l")  # count lines
                extra_cmd="${extra_cmd} | wc -l "
                ;;

           *)  # Default
                extra_cmd=""           
                ;;
    esac
done
...
...
cat file ${extra_cmd}

However when this runs it displays the file on the screen rather than piping the output to 'grep' or 'wc' depending of the parameter passed to the script and then compains about the contents of the variable, which it takes as the name of another file to list

Quote:

cat: | grep -v "^#" : No such file or directory

I have tried a variety of different ways of quoting the contents or escaping the '|' when setting up the variable but none are having the desired effect of piping the output from 'cat' into 'grep' or 'wc'.

Any help would be greatly appreciated

Stv T

stv_t

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LEARN ABOUT SUSE

unbuffer

UNBUFFER(1)						      General Commands Manual						       UNBUFFER(1)

NAME

       unbuffer - unbuffer output

SYNOPSIS

       unbuffer program [ args ]

INTRODUCTION

       unbuffer  disables  the output buffering that occurs when program output is redirected from non-interactive programs.  For example, suppose
       you are watching the output from a fifo by running it through od and then more.

	    od -c /tmp/fifo | more

       You will not see anything until a full page of output has been produced.

       You can disable this automatic buffering as follows:

	    unbuffer od -c /tmp/fifo | more

       Normally, unbuffer does not read from stdin.  This simplifies use of unbuffer in some situations.  To use unbuffer in a pipeline,  use  the
       -p flag.  Example:

	       process1 | unbuffer -p process2 | process3

CAVEATS

       unbuffer -p may appear to work incorrectly if a process feeding input to unbuffer exits.  Consider:
	       process1 | unbuffer -p process2 | process3

       If  process1  exits,  process2 may not yet have finished.  It is impossible for unbuffer to know long to wait for process2 and process2 may
       not ever finish, for example, if it is a filter.  For expediency, unbuffer simply exits when it encounters an EOF from either its input	or
       process2.

       In  order  to  have a version of unbuffer that worked in all situations, an oracle would be necessary.  If you want an application-specific
       solution, workarounds or hand-coded Expect may be more suitable.  For example, the following example shows how to allow grep to finish pro-
       cessing	when  the  cat before it finishes first.  Using cat to feed grep would never require unbuffer in real life.  It is merely a place-
       holder for some imaginary process that may or may not finish.  Similarly, the final cat at the end of the pipeline is  also  a  placeholder
       for another process.

       $ cat /tmp/abcdef.log | grep abc | cat
       abcdef
       xxxabc defxxx
       $ cat /tmp/abcdef.log | unbuffer grep abc | cat
       $ (cat /tmp/abcdef.log ; sleep 1) | unbuffer grep abc | cat
       abcdef
       xxxabc defxxx
       $

BUGS

       The man page is longer than the program.

SEE ALSO

       "Exploring Expect: A Tcl-Based Toolkit for Automating Interactive Programs" by Don Libes, O'Reilly and Associates, January 1995.

AUTHOR

       Don Libes, National Institute of Standards and Technology

								    1 June 1994 						       UNBUFFER(1)

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