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

From: Linda Walsh
Subject: Re: I think bash logic in a loop like : while [ condition] |read somevar is flawed.
Date: Tue, 24 Dec 2013 16:57:55 -0800
User-agent: Thunderbird

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
cval=$(( cval +1 ))
echo cval = $cval  file = $fname
# 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?

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"

declare -xi cval=0
echo entering while

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

program  number {
 #int cval=$cval
 while read fname; do
   echo -n "local_cval=$cval, "
   echo "file = $fname"

#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   -----------------------------------------

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