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Top Forums Shell Programming and Scripting Array Variable being Assigned Values in Loop, But Gone when Loop Completes??? Post 302717889 by Corona688 on Thursday 18th of October 2012 05:08:29 PM
Old 10-18-2012
Code:
echo -n "$MY_STR" | while read ...

The main shell forks off a copy of itself to run while read independently of the echo so they can both run while connected by a pipe. The copy, or "sub-shell", runs successfully, setting all your variables and such as you please, until the pipe runs out of information and returns EOF. Then the loop quits.

Then the subshell dies. Control is returned to the main shell, which remains unchanged. You set all your variables in the subshell, not the main one.

In short: It's the pipe that does it. Your shell and everything behind the pipe are different processes and don't share variables.

Pipes are overkill if you have substring operators anyway. This way of getting a single character ought to work identically in bash and ksh:

Code:
N=0

while [ "$N" -lt "${#STRING}" ]
do
        echo "${STRING:$N:1}"
        let N=N+1
done

This User Gave Thanks to Corona688 For This Post:
 

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PIPE(2) 							System Calls Manual							   PIPE(2)

NAME
pipe - create an interprocess channel SYNOPSIS
#include <u.h> #include <libc.h> int pipe(int fd[2]) DESCRIPTION
Pipe creates a buffered channel for interprocess I/O communication. Two file descriptors are returned in fd. Data written to fd[1] is available for reading from fd[0] and data written to fd[0] is available for reading from fd[1]. After the pipe has been established, cooperating processes created by subsequent fork(2) calls may pass data through the pipe with read and write calls. The bytes placed on a pipe by one write are contiguous even if many processes are writing. Write boundaries are preserved: each read terminates when the read buffer is full or after reading the last byte of a write, whichever comes first. The number of bytes available to a read(2) is reported in the Length field returned by fstat or dirfstat on a pipe (see stat(2)). When all the data has been read from a pipe and the writer has closed the pipe or exited, read(2) will return 0 bytes. Writes to a pipe with no reader will generate a note sys: write on closed pipe. SOURCE
/sys/src/libc/9syscall SEE ALSO
intro(2), read(2), pipe(3) DIAGNOSTICS
Sets errstr. BUGS
If a read or a write of a pipe is interrupted, some unknown number of bytes may have been transferred. When a read from a pipe returns 0 bytes, it usually means end of file but is indistinguishable from reading the result of an explicit write of zero bytes. PIPE(2)
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