Extensible variables in a shell script

Question

I have a query that I need for a loop.

query="select '$dbserver' as server;"

while read dbserver username password dbname type
do
mysql -h$dbserver -u$username -p$password $dbname -Be"$query" >> /home/develop/myreport.csv
done < $dblist

The next line expands correctly.

mysql -h$dbserver -u$username -p$password $dbname -Be"select '$dbserver' as server;" >> /home/develop/myreport.csv

But when I take out the request and save it in a variable, as shown above, it stops working as expected and gives an empty value for "dbserver". The line above in the loop does not work. How to fix this problem?

Answer 1

If you want the '$dbserver'query to expand inside the loop, you probably write:

while read dbserver username password dbname type
do
    query="select '$dbserver' as server;"
    mysql -h$dbserver -u$username -p$password $dbname -Be"$query"
done < $dblist  > /home/develop/myreport.csv

As originally written, the query string is evaluated before any value is assigned $dbserver, so you got an empty string in the output.

, - done, ( , ).

---

, , -, , eval. , $dbserver , . ( SQL), eval:

query='select \"$dbserver\" as server;'
echo "$query"
while read dbserver username password dbname type
do
    echo 1: "$query"
    eval echo 2: "$query"
    qval=$(eval echo "$query")
    echo mysql -h$dbserver -u$username -p$password $dbname -Be"$qval"
done

, """ "'\''":

query='select \'\''$dbserver\'\'' as server;'
echo "$query"
while read dbserver username password dbname type
do
    echo 1: "$query"
    eval echo 2: "$query"
    qval=$(eval echo "$query")
    echo mysql -h$dbserver -u$username -p$password $dbname -Be"$qval"
done

, , , - , ! [... ...] !

:

• .
• escape-.
• , - .
• 4 - '\''.
• .
• , .
• .
• , .
• $dbserver - .
• , .
• .

eval . ; $dbserver. .

eval , . MySQL, , . , eval. , , :

query='select \'\''$dbserver\'\'' as server, \`ls\` as column;'
echo "$query"
while read dbserver username password dbname type
do
    echo 1: "$query"
    eval echo 2: "$query"
    qval=$(eval echo "$query")
    echo mysql -h$dbserver -u$username -p$password $dbname -Be"$qval"
done

, .

Answer 2

. (.. )

Answer 3

while read -r dbserver username password dbname type
do
 query="select '$dbserver' as server;"
 mysql -h${dbserver} -u${username} -p${password} ${dbname} -Be"${query}" >> /home/develop/myreport.csv
done < $dblist