Bash variable scope, pipe and cat

Here is a nice little bit of bash snagging for you. You need to grab lines from a file, process each in turn, and then at the very end report whether or not there was a problem for any of them. Simple ? Not quite.

The following scripts are mangled to show how this works, so don't expect them to operate as-is.

#!/bin/bash


PROBLEM=0
cat $CONF |  grep backup | grep @ |

while IFS=$TAB read var1 var2 var3 var4

 do

    PROBLEM=1

    echo PROBLEM:$PROBLEM

 done

 echo PROBLEM: $PROBLEM

No matter what happens - PROBLEM ends up as 0. This is because of the pipe that creates a subprocess for the while loop. The while loop and the rest of th script are actually independent.

It turnsd out there is another syntax:
while read LINE do blah blah done <$FILE

instead of :
cat $FILE |while read LINE do blah blah done

However, this assumes that you have a file to pipe to read. If you need to parse your input, things don't quite work. Turns out, you can use parenthesis "()" to group commands together. The following syntax works - just don't add any unnecessary spaces,especially after the "<".

while read LINE do blah blah done <($FILE | grep test)

Subjects:
bash

Linux

programming