Subshell MySQL-query in bash - mysql

Question

I'm trying to set variable in the cycle in Ubuntu bash, which is getting recordset from database, but this variable is setting to its previous value.
Here is a code:
#!/bin/bash
PREV_FILE_PATH="127"
while true
do
echo "$PREV_FILE_PATH"
mysql -h$DB_HOST -u $DB_USER -p$DB_PASS $DB_NAME --skip-column-names --default-character-set=UTF8 -e "here is a query" | while read "here is getting variables from recordset";
do
PREV_FILE_PATH="777"
done
done
And this code prints every time:
127
127
127
But whe I replaced this block-:
mysql -h$DB_HOST -u $DB_USER -p$DB_PASS $DB_NAME --skip-column-names --default-character-set=UTF8 -e "here is a query" | while read "here is getting variables from recordset";
with just while true and break at the end of cycle it works fine and prints:
127
777
777
777
Script creates some subshell and running that MySQL query in that subshell. So what should I do to make script change that variable?

Answer 1

As you noted the issue is due to the creation of a subshell which is being caused by piping the output of the mysql command to the while loop. A simple example:
PREV_FILE_PATH=127
echo test | while read _; do
PREV_FILE_PATH=777
done
echo $PREV_FILE_PATH
# output: 127
Since you're using BASH you can move the mysql command from being a pipe to a substituted process fed to the while loop via STDIN redirection. Using the previous simple example:
PREV_FILE_PATH=127
while read _; do
PREV_FILE_PATH=777
done < <(echo test)
echo $PREV_FILE_PATH
# output: 777
So to fix your code, you will want to move your mysql command in the same fashion that I moved the echo command above:
while read "here is getting variables from recordset"
do
PREV_FILE_PATH="777"
done < <(mysql -h$DB_HOST -u $DB_USER [..remaining options..])
Note that process substitution via <() is a BASH-ism and isn't POSIX compliant.