shell scripting string replace using shell variables

Question

This is my very basic script:

  temp=hello 
  while read line;
  do
     echo ${line}
  done

However, ${line} will itself consist of "value of temp = ${temp}"

I want my script to echo "value of temp is hello". I've tried doing many things, even

echo `echo ${line}`

but each time it always just prints "value of temp is ${temp}"

I hope this question is clear. THanks!

Mosty Mostacho · Accepted Answer · 2011-10-02 17:16:24Z

1

What about this?

temp=hello 
while read line; do
    echo "Value of temp = `eval echo ${line}`"
done

Then in the console just type:

${temp}

answered Oct 2, 2011 at 17:16

Mosty Mostacho

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1 Comment

Bill Over a year ago

Thanks mate. That seems to work, what I did was replace the middle line with simply: eval echo "${line}"... Thank! (I'd upvote but don't have the karma apparently)

ДМИТРИЙ МАЛИКОВ · Accepted Answer · 2011-10-02 17:17:35Z

1

Well, I have solutions with eval, but using eval is mauvais ton, actually.

$> cat 7627845.sh 
#!/bin/bash

temp=hello 
cat file_with_values.log | while read line;
do
    eval echo "${line}"
done


$> cat file_with_values.log 
value of temp = ${temp}

$> ./7627845.sh 
value of temp = hello

answered Oct 2, 2011 at 17:17

ДМИТРИЙ МАЛИКОВ

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3 Comments

glenn jackman Over a year ago

useless use of cat: while read line; do eval echo "$line"; done < file_with_values.log

ДМИТРИЙ МАЛИКОВ Over a year ago

Frankly speaking there is no huge difference between those methods.

glenn jackman Over a year ago

depends what you're doing in the while body. If you cat file | while ... then the while command runs in a subshell, and any variables you set in the while body disappear when the subshell ends. So, potentially, there could be a huge difference. Obviously, that's not the case here, but this is a teaching site, so it's better to teach good principles.

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