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setting variable value to dynamic sed match - escaping hell

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# 1  
Old 11-04-2010
Question setting variable value to dynamic sed match - escaping hell
Hello All,

I'm trying to write a script that will perform a dynamic match (of a dynamic variable) and set a variable to have the resulting (match) value.

The idea is that the environment variable to check ($1) and the regular expression to use ($2) are given as parameters.

For example, to print the user's username only if it begins with an 'a', we would make the call:
Code:
check-var.sh USER "^a.*$"

I've tried doing this:
Code:
VAR_NAME=$1
 
VAR_REGEXP=$2
 
VAR_MATCH=`eval "echo \$${VAR_NAME} | sed -n \"/$VAR_REGEXP/p\""
 
echo $VAR_MATCH

But there seems to be a problem with by quoting and escaping because if I execute the eval directly like this:
Code:
eval "echo \$${VAR_NAME} | sed -n \"/$VAR_REGEXP/p\""

the expected result is printed.

Anyone out there have an idea of how I can capture the result in the VAR_MATCH variable? I ask because I actually need to work with the matched value later on in the script and not simply send it to stdout.

Thanks,
- Andy
Last edited by Scott; 11-04-2010 at 07:20 AM.. Reason: Please use code tags
# 2  
Old 11-04-2010
If you start the script with:
Code:
check-var.sh USER "^a.*$"

you can get the result in your script with:
Code:
VAR_MATCH=$(echo $1 | grep $2)

echo $VAR_MATCH

Or am I missing something?
This User Gave Thanks to Franklin52 For This Post:
aedgar
# 3  
Old 11-04-2010
Quote:
Originally Posted by aedgar
Hello All,
Code:
VAR_MATCH=`eval "echo \$${VAR_NAME} | sed -n \"/$VAR_REGEXP/p\""

But there seems to be a problem with by quoting and escaping

Thanks,
- Andy
Looks like you missed the closing "`".
# 4  
Old 11-04-2010
Further to majormark.

We need more backslash characters to properly escape the eval statement when run from within a shell script.
Also I had to export $USER before invoking the script.

Code:
VAR_MATCH=`eval "echo \\$\${VAR_NAME} | sed -n \"/${VAR_REGEXP}/p\""`

# 5  
Old 11-04-2010
Quote:
Originally Posted by Franklin52
If you start the script with:
Code:
check-var.sh USER "^a.*$"

you can get the result in your script with:
Code:
VAR_MATCH=$(echo $1 | grep $2)
 
echo $VAR_MATCH

Or am I missing something?
Thanks for quick answer; you put me on the right track.

What I am actually trying to do to check the *value* for the variable named in $1 and not the variable name itself.

What did the trick was:
Code:
VAR_MATCH=$(eval echo \$${VAR_NAME} | grep -e "$VAR_REGEXP")

Thanks for your help,
- Andy
# 6  
Old 11-04-2010
Just for interest, we can do this without "eval".
Code:
VAR_MATCH=$(set | grep \^"${VAR_NAME}=${VAR_REGEXP}")

We must not have a "^" at the start of $2 for this to work.
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