bash: cant reach internal var from outside!

kalleanka · 02-13-2011, 07:23 PM

Hi,

any idees how I get mystr: unix to become mystr: linux?

#!/bin/bash

mystr="unix"
echo "linux" | while read myline
do
mystr="$myline"
echo "str: "$mystr
done
echo "mystr: "$mystr

# $ bash test
# str: linux
# mystr: unix

grail · 02-13-2011, 08:28 PM

Because you are piping the information into the while loop it is creating its own shell and hence anything set within the loop is lost once
the shell is closed (ie when the loop ends).
You can check this out for the reasons and solutions.

Nominal Animal · 02-13-2011, 10:22 PM

Hey, grail, your BashFAQ/024 is great, but lacks one useful method: using a temporary descriptor to pass the result values. This works in all POSIX shells, like bash and dash, it might even work in Bourne shell.

Code:

exec 3>&1
result=`some-command some-parameters ... >&3 ; echo new-result`
exec 3>&-

For longer stuff and multiple variables, this can be extended to

Code:

exec 3>&1
result=(`exec 4>&1 1>&3

         # do some bash stuff, stdout and stderr work normally

         echo newval1 newval2 newval3 >&4
       `)
exec 3>&-
val1="${result[0]}"
val2="${result[1]}"
val3="${result[2]}"

No descriptors are left open; this is quite clean. Although $(...) would nest much better, older shells may only support `...` so I used the latter just in case here.

Nominal Animal

konsolebox · 02-14-2011, 12:24 AM

for bash, perhaps simply:

Code:

mystr="unix"

while read myline; do
    mystr="$myline"
    echo "str: $mystr"
done < <(echo "linux")

echo "mystr: $mystr"

noting:

Code:

command1 | while ...

puts 'while' into another subshell

kalleanka · 02-14-2011, 03:24 AM

thanks all to all of you. really helpful.

i normally program in c and do not know bash that well.

This is for automatic mounting of samba(with root since the S option do not work any more) and nfs command. The plan is to also make thunar plugins.

Where do i publicize these commands later? Any idees?