how to create a variable for evaluation in Bash [duplicate] - bash

Question

This question already has answers here:
Dynamic variable names in Bash
(19 answers)
Lookup shell variables by name, indirectly [duplicate]
(5 answers)
Closed 12 months ago.
I have a CSV that says this:
Source,Target,Database,Table,Is_deletable,Action
DBServer1,DBServer2,DB,TBL1,true,Add
DBServer2,DBServer1,DB,TBL2,true,Add
I have shell script that does this:
while IFS=, read -r source target database table is_deletable action; do
echo "Building pipeline for ${database}.${table}";
total=$((total+1))
Shost="server1.myorganization.com"
Thost="server2.myorganization.com"
I need Shost to look like this:
Shost = ${'the value of the source column'}
Thost = ${'the value of the target column'}
How do I set this to evaluate dynamically the variable I need based on the value of the column data.
For example:
Shost=${DBServer1}
Thost=${DBServer2}
then on the next loop:
Shost=${DBServer2}
Thost=${DBServer1}
Thanks!

Answer 1

Something like this should work for you:
DBServer1='server1.myorganization.com'
DBServer2='server2.myorganization.com'
while IFS=, read -r source target database table is_deletable action; do
[[ $source = "Source" ]] && continue
((total++))
Shost="${!source}"
Thost="${!target}"
# check variables Shost Thost total
declare -p Shost Thost total
done < file.csv
declare -- Shost="server1.myorganization.com"
declare -- Thost="server2.myorganization.com"
declare -- total="1"
declare -- Shost="server2.myorganization.com"
declare -- Thost="server1.myorganization.com"
declare -- total="2"

Answer 2

We can also use an associative array for this problem:
declare -A servers=(
[DBServer1]='server1.myorganization.com'
[DBServer2]='server2.myorganization.com'
)
{
read header
while IFS=, read -r source target database table is_deletable action; do
Shost=${servers[$source]}
Thost=${servers[$target]}
...
done
} < file.csv

Answer 3

In Bash -- but not necessarily in other shells -- you can reference a variable indirectly, exactly as you seem to want to do:
$ DBServer1=db1.my.com
$ source=DBServer1
$ echo ${source}
DBServer1
$ echo ${!source}
db1.my.com
As the Bash manual puts it (ref):
If [in a parameter expansion of the
form ${parameter}] the first character of parameter is an exclamation point (!), and parameter is
not a nameref, it introduces a level of variable indirection. Bash
uses the value of the variable formed from the rest of parameter as
the name of the variable; this variable is then expanded and that
value is used in the rest of the substitution, rather than the
value of parameter itself.
Applying that to your sample code and data, we get
DBServer1=server1.myorganization.com
DBServer2=server2.myorganization.com
# ...
while IFS=, read -r source target database table is_deletable action; do
echo "Building pipeline for ${database}.${table}";
total=$((total+1))
Shost="${!source}"
Thost="${!target}"