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Arithmetic Evaluation and Pipes in BASH

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Join Date: Mar 2005

Posts: 1

Reputation: chrissi is an unknown quantity at this point

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chrissi

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Newbie Poster

Arithmetic Evaluation and Pipes in BASH

Mar 13th, 2005

Hi,

I had some irritation with a "while"- loop, which turned out to be endless.
Anyway, i was able to find the problem and reduce it to be shown on the commandline:

I "typeset" (and initialize) an array and an integer variable:

 Help with Code Tags 
 Shell Scripting Syntax (Toggle Plain Text) 
$ typeset -a arr; arr=(bourne again shell)
$ typeset -i ind=0
$ typeset -a arr; arr=(bourne again shell)
$ typeset -i ind=0

Everything works as expected when doing:

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 Shell Scripting Syntax (Toggle Plain Text) 
$ echo ${arr[((ind++))]}
bourne
$ echo ${arr[((ind++))]}
again
$ echo ${arr[((ind++))]}
shell
$ echo ${arr[((ind++))]}
bourne
$ echo ${arr[((ind++))]}
again
$ echo ${arr[((ind++))]}
shell

But when i use the "echo" command in a pipe, (like:)

 Help with Code Tags 
 Shell Scripting Syntax (Toggle Plain Text) 
$ typeset -i ind=0
$ echo ${arr[((ind++))]} | grep e
bourne
$ typeset -i ind=0
$ echo ${arr[((ind++))]} | grep e
bourne

the postincrement of "ind" is not evaluated, so the next time i also get:

 Help with Code Tags 
 Shell Scripting Syntax (Toggle Plain Text) 
$ echo ${arr[((ind++))]} | grep e
bourne
$ echo ${arr[((ind++))]} | grep e
bourne
$ echo ${arr[((ind++))]} | grep e
bourne
$ echo ${arr[((ind++))]} | grep e
bourne

Well, this is not really a problem for i can do it in two lines, but i wonder, if it's possible to make it work in one line.

Anywone who can give a comment?

Thanks for reading and trying to help!
Bye,
Christoph

•

Join Date: Jun 2009

Posts: 1

Reputation: atucsprof is an unknown quantity at this point

Solved Threads: 0

atucsprof

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Newbie Poster

Re: Arithmetic Evaluation and Pipes in BASH

Jun 26th, 2009

Christoph,

The answer is that every member of a pipeline is turned into a separate process with its own environment. So changing the variable in a pipeline changes it in a subshell and not the current shell.

You'll have to change the value of the variable after invoking the pipe.

•

Originally Posted by chrissi

Hi,

I had some irritation with a "while"- loop, which turned out to be endless.
Anyway, i was able to find the problem and reduce it to be shown on the commandline:

I "typeset" (and initialize) an array and an integer variable:
Help with Code Tags

Shell Scripting Syntax (Toggle Plain Text)
$ typeset -a arr; arr=(bourne again shell)
$ typeset -i ind=0
$ typeset -a arr; arr=(bourne again shell) $ typeset -i ind=0
Everything works as expected when doing:
Help with Code Tags

Shell Scripting Syntax (Toggle Plain Text)
$ echo ${arr[((ind++))]}
bourne
$ echo ${arr[((ind++))]}
again
$ echo ${arr[((ind++))]}
shell
$ echo ${arr[((ind++))]} bourne $ echo ${arr[((ind++))]} again $ echo ${arr[((ind++))]} shell
But when i use the "echo" command in a pipe, (like
Help with Code Tags

Shell Scripting Syntax (Toggle Plain Text)
$ typeset -i ind=0
$ echo ${arr[((ind++))]} | grep e
bourne
$ typeset -i ind=0 $ echo ${arr[((ind++))]} | grep e bourne
the postincrement of "ind" is not evaluated, so the next time i also get:
Help with Code Tags

Shell Scripting Syntax (Toggle Plain Text)
$ echo ${arr[((ind++))]} | grep e
bourne
$ echo ${arr[((ind++))]} | grep e
bourne
$ echo ${arr[((ind++))]} | grep e bourne $ echo ${arr[((ind++))]} | grep e bourne
Well, this is not really a problem for i can do it in two lines, but i wonder, if it's possible to make it work in one line.

Anywone who can give a comment?

Thanks for reading and trying to help!
Bye,
Christoph