[bash] re-evaluate variable inside variable

muzzol · 11-30-2010, 05:41 AM

hi,

i need to re-evaluate a variable that is included into another variable but declared after.

little example:

Code:

#!/bin/bash

VAR1="simple text - $VAR2 - here"

VAR2="ZZZ"

echo "$VAR1"

i've been playing with indirect variables without success.

any hints?

catkin · 11-30-2010, 06:06 AM

It is generally better to tell us what you want to achieve rather than asking us to fix a solution that may not be optimal.

BTW, the scrippet posted does not include any declarations unless you include implicit declaration by assignment.

muzzol · 11-30-2010, 06:28 AM

sorry for not using correct terms, but the example i've provided is very similar to final script.

what i want is a script with few variables at the beggining (what i call configuration block) that will be edited by operators, and then the main code of the script. some of the variables of the configuration block can include variables that are used later.

i dont want operators mess with the code, just configure somethings. this is a little elaborated example:

Code:

#!/bin/bash

# configuration block
FLOG="$DIRLOG/register-in.log"

#############################################
## please dont touch nothing from this point
#############################################

DIRLOG="/usr/local/appregister/logs"

tail -f "$FLOG"

catkin · 11-30-2010, 06:38 AM

Quote:

Originally Posted by muzzol

i dont want operators mess with the code, just configure somethings.

That makes sense.

The tidiest solution would be to use an external file for the configuration data. Most simply this can be sourced into the master script. It's easier to illustrate than describe. Here's the config file, let's call it /etc/local/my.conf:

Code:

FLOG="register-in.log"

and here the script:

Code:

#!/bin/bash

source /etc/local/my.conf

DIRLOG="/usr/local/appregister/logs"
FLOG=$DIRLOG/$FLOG

tail -f "$FLOG"

This has the advantage that the script is not changed to configure anything which makes support a lot easier, including knowing which version is in use.

muzzol · 12-01-2010, 03:10 AM

that's not really what i want.

anyway i've ended doing it like this:

Code:

#!/bin/bash

# configuration block
FLOG="<dirlog>/register-in.log"

#############################################
## please dont touch nothing from this point
#############################################

DIRLOG="/usr/local/appregister/logs"

FLOG=`echo "$FLOG" | sed "s|<dirlog>|$DIRLOG|"`

tail -f "$FLOG"

thx anyway

catkin · 12-01-2010, 03:14 AM

Glad you found a solution

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grail · 12-01-2010, 03:25 AM

Well personally I cannot see how that is more maintainable, but good to see you found a solution.

rupertwh · 12-01-2010, 03:31 AM

BTW, your original idea is quite possible, you're just missing

a backslash which will keep bash from interpreting $VAR2 right away while it's still empty
an 'eval' which tells bash to reevaluate its argument(s), this time replacing '$VAR2' with its content

E.g.:

Code:

#!/bin/bash

VAR1="simple text - \$VAR2 - here"

VAR2="ZZZ"

echo $VAR1
eval echo $VAR1

catkin · 12-01-2010, 06:22 AM

Quote:

Originally Posted by grail

Well personally I cannot see how that is more maintainable, but good to see you found a solution.

There are many possible solutions to the requirement with various pros and cons, each of which has more or less weight depending on context; it is hard to imagine a context in which the chosen solution is optimal.

tuxdev · 12-01-2010, 11:31 AM

For configuration, IMO catkin's solution of sourcing a config file is the most elegant way to go about it, and the method I use whenever I need something like that. eval is bad here, you need to be doing something *really* special to justify all the headaches it brings.