The variable inside while loop is not remembered. Mysql bash script auto_increment

问题

I've working on this script for quite some time. But I started to see some errors when I tried to add a variable ($EMAIL_MSG) to store some strings of text to later be emailed. It seems that the variable inside while loop is not remembered. (A variable modified inside a while loop is not remembered)

Here is part of the script:

#!/bin/bash
#
# This is bash script checks when auto_increment column is reaching its limit
# To run Script $ ./auto_increment_check.sh [username] [password]

MYSQL_USER="$1"
MYSQL_PASSWD="$2"
MYSQLCONNECT="mysql -u$MYSQL_USER -p$MYSQL_PASSWD"
MAX_RATIO="0.8" # Max percentage of fullness of an auto_increment column   (ex. '0.8' means 80% full)
EMAIL_MSG=""
EMAIL_RCPNT="user@company.com"

QUERY="
SELECT table_schema,
       table_name,
       data_type,
       ( CASE data_type
           WHEN 'tinyint' THEN 255
           WHEN 'smallint' THEN 65535
           WHEN 'mediumint' THEN 16777215
           WHEN 'int' THEN 4294967295
           WHEN 'bigint' THEN 18446744073709551615
         end >> IF(Locate('unsigned', column_type) > 0, 0, 1) ) AS max_value
FROM   information_schema.columns
WHERE  table_schema NOT IN ( 'mysql', 'information_schema', 'performance_schema')
       AND data_type IN ('tinyint','smallint','mediumint','int','bigint')
       AND extra = 'auto_increment'"

$MYSQLCONNECT --batch -N -e "$QUERY" | while read DATABASE TABLE DATA_TYPE MAX_VALUE;

do
  NEXT_AUTO_INCREMENT=`$MYSQLCONNECT --batch -N -e "show create table $DATABASE.$TABLE" |  awk -F'AUTO_INCREMENT=' 'NF==1{print "0";next}{sub(/ .*/,"",$2);print $2}'`
  AUTO_INCREMENT_RATIO=$(awk 'BEGIN {printf "%3.2f\n", '$NEXT_AUTO_INCREMENT' / '$MAX_VALUE'}')

  if [[ $(awk 'BEGIN{print ('$AUTO_INCREMENT_RATIO'>='$MAX_RATIO')}') -eq 1 ]] ; then
    EMAIL_MSG="$EMAIL_MSG\n\nAuto Increment Warning on $(hostname) - $DATABASE.$TABLE - NEXT AUTO INCREMENT: $NEXT_AUTO_INCREMENT, MAX CAPACITY: $MAX_VALUE, RATIO: $AUTO_INCREMENT_RATIO."
  fi

done

if [ EMAIL_MSG != "" ]; then
    echo -e $EMAIL_MSG | mail -s "Auto Increment Warning on $(hostname) " $EMAIL_RCPNT
fi

The problem seems to be that the while loop is executed in a subshell. So any changes I do for the variable $EMAIL_MSG will not be available once the subshell exits.

I read that I should modify the while loop like this:

while read DATABASE TABLE DATA_TYPE MAX_VALUE;
do
  ...
  ...
  ...
done <<< '$MYSQLCONNECT --batch -N -e "$QUERY"'

if [ EMAIL_MSG != "" ]; then
    echo -e $EMAIL_MSG | mail -s "Auto Increment Warning on $(hostname) " $EMAIL_RCPNT
fi

But I'm getting errors:

[root@localhost /]# ./vagrant/auto.sh root root
Mon Jan 27 21:04:01 UTC 2014: Auto Increment Check starting.
./vagrant/auto.sh: line 53: syntax error near unexpected token `--batch'
./vagrant/auto.sh: line 53: `done <<< $MYSQLCONNECT --batch -N -e "$QUERY"'

Any ideas how to fix that?

回答1:

Try:

done < <($MYSQLCONNECT --batch -N -e "$QUERY")

The construct <(...) is called process substitution. It runs the command inside parentheses and makes it output available as if a FIFO had been created. The construct < <(...) connects the output of the FIFO to stdin on your while loop.

Process substitution requires both bash and an operating system, such as linux, that supports FIFOs. A mere bourne shell will not support it.

Alternatively, this could be done as a here string:

done <<<"$($MYSQLCONNECT --batch -N -e "$QUERY")"

This uses command substitution ($(...)) to capture the output of the $MYSQLCONNECT command and then uses a here-string (<<<) to provide that as stdin to your while read loop. Note the double-quotes outside the command substitution. They are necessary to preserve the newlines within the command output. Without those double-quotes, the output of $MYSQLCONNECT would appear to the while read loop as a single long line.

If single-quotes were used instead of double quotes, as in:

done <<<'$($MYSQLCONNECT --batch -N -e "$QUERY")' # Don't use this

then no command would be executed and the literal string $($MYSQLCONNECT --batch -N -e "$QUERY") would be provided to the while read loop.

回答2:

The following won't do what you think:

if [ EMAIL_MSG != "" ]; then

because it doesn't expand the variable $EMAIL_MSG, it just uses the literal string "EMAIL_MSG".

Use this:

if [ ! -z "$EMAIL_MSG" ]; then

There are other problems I can see in your script.

The issue of using <<< is immaterial. Go back to running a command and piping in to while like your original script.
Don't use single-quotes if you want to expand shell variables inside the string.
Your passwords will fail if they contain spaces, or semicolon, or ampersand, or hash, or several other special characters.

Actually, I wouldn't use the -u and -p options at all. Put the username and password into a config file and then pass --defaults-extra-file to load that config file. See http://dev.mysql.com/doc/refman/5.6/en/option-files.html

There might be more problems -- that's just what I noticed.

来源：https://stackoverflow.com/questions/21393700/the-variable-inside-while-loop-is-not-remembered-mysql-bash-script-auto-increme

标签

mysql

bash

shell

while-loop

auto-increment