Re: I think bash logic in a loop like : while [ condition] |read somevar is flawed.

[Linda Walsh]

rens wrote:

> this script:
> _______________________
> export cval=0
> echo entering while
>
> # listing ten files, to generate some output to count.
> ls -1 /usr/bin|head -10 |while read fname
> do
> cval=$(( cval +1 ))
> echo cval = $cval  file = $fname
> done
> # one would think cval is now 10. but it is not, contrary to any other 
> programming language........
>
> echo  " after while: cval = $cval"
> _______________________________
>
> does not set the value of cval after exiting the while loop.
>
> that makes no sense.


> Bash  should not work that way. no programming language handles logic 
> this way. Not pascal, korn shell, c shell, cobol, c, c++, lua , fortran 
> or any other language i ever used.
----
   Bash is a command-line interpreter.  It is not a language;

It accepts commands and has ways of sequencing commands and funneling
output from one command to the next.

Korn shell works the same way. csh has no read builtin that I can
find, so you can't write the program the same way.

Every time you use "|", you are ending the previous program and piping results
to another program.

The above is equal to the *first* 'while' output in the program below:

If you want your progam to work right, you need to have the count be in your
shell, and NOT have any "|" in the invocation line (i used < <(...))

The below produces the wrong output first (cval=0) and then runs it again with
the commands re-arranged to not use '|' -- which produces the right value.

Does this help you understand what's going on?
-------------------------------------------------
#!/bin/bash

shopt -s expand_aliases

alias program=function
alias int=declare\ -i
alias my=declare


program myhead {
 my inp=${1:-10}
 if [[ ${inp:0:1} == - ]]; then inp="${inp:1}"; fi
 int lines=$inp
 while ((lines-- > 0)); do
   my line
   read line
   echo "$line"
 done
}

declare -xi cval=0
echo entering while

program print_exported_cval {
 while [[ ! $(typeset -p cval |sed -r 's/ +/\t/g'|cut -f2) =~ x ]];do
   unset cval
 done
 echo -n "exported_cval=$cval, "
}

program  number {
 #int cval=$cval
 while read fname; do
   cval=$cval+1
   echo -n "local_cval=$cval, "
   (print_exported_cval)
   echo "file = $fname"
 done
}


#number< <(ls -1 | myhead -10 )
ls -1 | myhead -10 |number

echo " after 1st while: cval = $cval"

number< <(ls -1 | myhead -10 )

echo " after 1st while: cval = $cval"

#end script   -----------------------------------------