eval in shell script - purpose of \${value}-CodePudding

I have inherited a shell script which has following code

while IFS='=' read -r key value
do
  # echo "key ${key}"
  if [[ ${key} =~ ^# ]]; then
    # echo "comment line ${key},  skipping....."
    continue
  fi
  eval ${key}=\${value}
  echo "key: ${key}, value: ${value}"
done <kafka-parameters.txt

where kafka-parameters.txt file contains entries like below

#Schema Registry
DEV_SR_URLS=http://sr1-dev:8081, http://sr2-dev:8081
QA_SR_URLS=http://sr1-qa:8081, http://sr2-qa:8081
STAGE_SR_URLS=http://sr1-qa:8081, http://sr2-qa:8081
PROD_SR_URLS=http://sr1:8081, http://sr2:8081
#
#Bootstrap Servers
DEV_BOOTSTRAP_SERVERS=broker1-dev:9092,broker2-dev:9092,broker3-dev:9092
QA_BOOTSTRAP_SERVERS=broker1-qa:9092,broker2-qa:9092,broker3-qa:9092
STAGE_BOOTSTRAP_SERVERS=broker1-qa:9092,broker2-qa:9092,broker3-qa:9092
PROD_BOOTSTRAP_SERVERS=broker1:9092,broker2:9092,broker3:9092
#

In the above snippet, what is the purpose of "\" before ${value} ?

From my understanding eval ${key}=${value} creates a variable of name ${key} and assigns value of ${value} to it and eval makes that assignment available after the loop.

Is this correct? Still do not understand the purpose of \${value}

or what is difference between

eval ${key}=\${value}

and

eval ${key}=${value}

Thank you

CodePudding user response：

Consider the following assignment:

value="foo bar"
key=$value

This will work, because the right-hand side doesn't undergo word-splitting; the name key will get the value foo bar.

Now consider

key=foo
$key=$value

This is just an error, because it's not an assignment; it's an attempt, after parameter expansion and word-splitting, to run a command name foo=foo with an argument bar.

Using eval, we choose to let some expansions happen befor eval runs, and some to happen after. With

eval $key=\${value}

eval will see the string foo=${value}, which it will execute by expanding $value} and assigning the entire result to foo, similar to the first example above.

However, if you let both parameters expand first,

eval $key=${value}

then after parameter expansion eval gets two arguments, key=foo and bar, which results in a command named bar being executed with key=foo in its environment.