Bash: what happens to my variables?

elinenbe · 01-30-2008, 08:26 AM

Here's my code:

Code:

counter=0
head -n500 esn.txt | while read line
do
	let "counter += 1"
	if [ "$counter" -eq 1 ]
	then
		email=${line:55}
		echo $email
	elif [ "$counter" -eq 4 ]
	then
		ESN=$line
		echo $ESN
	fi
done
echo $ESN
echo $email

why at the end when I echo $ESN and $email are they blank? I'm missing something...

Thanks,
Eric

acid_kewpie · 01-30-2008, 08:42 AM

as you create them inside of the loop, they are destroyed due to scoping once you leave it. if you put "email=whatever" at the start and then *change* it in the loop then it'll persist.

rupertwh · 01-30-2008, 08:50 AM

Try changing your loop to

Code:

counter=0
while read line
do
	...
done < <(head -n500 esn.txt)
echo $ESN
echo $email

Matir · 01-30-2008, 09:04 AM

It's all about scope: as acid_kewpie points out, variables created inside a loop are only local to that loop. You must create them first, then use them within (and possibly outside) the loop.

elinenbe · 01-30-2008, 09:11 AM

rupertwh: Thanks... I did that suggestion and everything worked as it should. I also now understand where I went wrong. Thanks everyone.

colucix · 01-30-2008, 02:32 PM

Actually it's not the loop which retains local variables: it is the pipe redirection which opens a subshell that uses local variables

Code:

head -n500 esn.txt | while ...
do
      #
      # this is the subshell scope due to pipe redirection
      #
done

When using "process substitution" (as posted by rupertwh) a new subshell is not called and the modified variables are retained outside the block of code. Another way to do the same is to link a file descriptor to the input file (or part of it if using the command head with process substitution). The option -u of read let you take input from the specified descriptor:

Code:

#!/bin/bash
counter=0
exec 3< esn.txt
while read -u 3 line
do
	let "counter += 1"
	if [ "$counter" -eq 1 ]
	then
		email=${line:55}
		echo $email
	elif [ "$counter" -eq 4 ]
	then
		ESN=$line
		echo $ESN
	fi
done
echo $ESN
echo $email
exec 3<&-