How to understand special character for line reading in bash shell?


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# 1  
How to understand special character for line reading in bash shell?

I am still learning shell scripting. Recently I see a function for read configuration. But some of special character make me confused. I checked online to find answer. It was not successful. I post the code here to consult with expert or guru to get better understanding on these special characters such as:^[^#]*=, line%%=*, line#*=. how to understand the usage of these special character here. Thanks.

Code:
read_cfg()
{​
  while read line; do​
     if [[ "$line" =~ ^[^#]*= ]]; then​
        l_parameter=`echo ${line%%=*} | tr -d ' '` l_value=`echo ${line#*=} | tr -d ' '`​
​
        case "$l_parameter" in ​
           "EMAIL_LIST" ) ​      #email list provided in command line overwrites the one in configuration file​
               if [ -z "$g_bkp_email" ] ; then​
                  g_bkp_email=$l_value​
               fi​
               ;;​

# 2  
Not sure I'm an expert nor a guru, but this is what I'd do: refer to the respective man pages.
man bash:
Quote:
Parameter Expansion

${parameter#word}
${parameter##word}

Remove matching prefix pattern.


${parameter%word}
${parameter%%word}

Remove matching suffix pattern.



CONDITIONAL EXPRESSIONS.



An additional binary operator, =~, is available, with the same precedence as == and !=. When it is used, the string to the right of the operator is considered a POSIX extended regular expression and matched accordingly (as in regex(3))
Of course you need to learn the difference between shell's pattern matching and regex matching.

man regex:
Quote:
An atom is a regular expression ... '^' (matching the null string at the beginning of a line)

A bracket expression is a list of characters enclosed in "[]". It normally matches any single character from the list (but see below). If the list begins with '^', it matches any single character (but see below) not from the rest of the list.
Admittedly the ambiguous use of the caret is something you need to accustom to.
These 2 Users Gave Thanks to RudiC For This Post:
# 3  
^[^#]*= is an extended regular expression and it means 0 or more non-# characters starting at the beginning of the line followed by an equal sign..

${line%%=*} is parameter expansion and it returns the characters to the left of the first = sign
${line#*=} is parameter expansion and it returns the characters to the right of the rightmost = sign
These 3 Users Gave Thanks to Scrutinizer For This Post:
# 4  
RudiC, Scrutinizer:

Thanks a lot to both of you. The explanation is clear and make me better understanding on these special characters. I will learn more.
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