How do I use sed to change my configuration files, with flexible keys and values?

Solution 1:

sed -i -e '/central\.database =/ s/= .*/= new_value/' /path/to/file

Explanation:

• -i tells sed to save the results to the input file. Without it sed will print the results to stdout.
• /central\.database =/ matches lines that contain the string between slashes: central.database =. The . is escaped since it's a special character in regex.
• The s/OLD/NEW/ part performs a substitution. The OLD string is a regular expression to match and the NEW part is the string to substitute in.
• In regular expressions, .* means "match anything". So = .* matches an equal sign, space, and then anything else afterward.

Solution 2:

Here's an example expression:

sed -i 's/^\(central\.database\s*=\s*\).*$/\1SQLTEST/' file.cfg

If you want to match stuff with / in it, you can use another delimiter:

sed -i 's#^\(cent/ral\.data/base\s*=\s*\).*$#\1SQL/TEST#' file.cfg

Or with variable expansion:

VAL="SQLTEST"
sed -i "s/^\(central\.database\s*=\s*\).*\$/\1$VAL/" file.cfg

In your example:

sshRetValue=`sed -i "s/^\(\1$CENTRAL_DB_NAME\s*=\s*\).*\$/\1$CENTRAL_DB_VALUE/" /home/testing.txt`;

There's a \1 before $CENTRAL_DB_NAME that's invalid. Also, sed doesn't print it's return value. This is the preferred way to check return values:

sed -i "s/^\($CENTRAL_DB_NAME\s*=\s*\).*\$/\1$CENTRAL_DB_VALUE/" /home/testing.txt;
sed_return_value=$?

And ultimately piping to ssh (not tested):

sed_return_value=$(ssh server <<EOF
    sed -i "s/^\($CENTRAL_DB_NAME\s*=\s*\).*\$/\1$CENTRAL_DB_VALUE/" /home/testing.txt;
    echo $?
EOF
)

The -i is for replacing data in the input file. Otherwise sed writes to stdout.

Regular expressions are a field of their own. It would be impossible to explain them in depth in a stackoverflow answer, unless there is some specific function that's eluding you.

Solution 3:

I know it is too late to add an answer to this question however, I thought to share my knowledge to you all. There is a very general approach which I have followed to solve a similar kind of problem. I have deleted the whole line which is matching the string and added the required values to that key. To your question here is the answer

replaceValue=SQLTEST
sed -i "/central.database =/d" /home/testing.txt
echo "central.database = $replaceValue"  >> /home/testing.txt

sed deletes the matching string line from the file and the immediate next line is inserting the required key and value to the file.

Solution 4:

I like using awk for this, since it is quite easy to understand what it is doing and takes care very well of the separator (=) and also the fact that it must be done to an uncommented line:

awk -v var="my_var" -v new_val="NEW VALUE" \  # set the vars
    'BEGIN{FS=OFS="="}                        # set separator to =
     match($1, "^\\s*" var "\\s*") {          # check if it matches
         $2=" " new_val                       # if so, replace the line
     }1' conf_file                            # print all lines

This uses match() to check if the pattern occurs in any given line. If it does, it performs the replacement with the given value.

For example:

$ cat conf
hello
my_var= SOME VALUE
#my_var = ANOTHER VALUE
bye

Let's change the value in my_var to NEW VALUE:

$ awk -v var="my_var" -v new_val="NEW VALUE" 'BEGIN{FS=OFS="="}match($1, "^\\s*" var "\\s*") {$2=" " new_val}1' conf
hello
my_var= NEW VALUE
#my_var = ANOTHER VALUE
bye

It is also possible to set the values in shell variables and then use them with -v:

$ var="my_var"
$ new_value="NEW VALUE"
$ awk -v var="$var" -v new_val="$new_value" 'BEGIN{FS=OFS="="}match($1, "^\\s*" var "\\s*") {$2=" " new_val}1' conf

And you can of course put all of this within a shell function that you then call normally:

#!/bin/bash

replace () {
   file=$1
   var=$2
   new_value=$3
   awk -v var="$var" -v new_val="$new_value" 'BEGIN{FS=OFS="="}match($1, "^\\s*" var "\\s*") {$2=" " new_val}1' "$file"
}

# Call the replace() function with the necessary parameters
replace "conf" "my_var" "NEW VALUE" 

Upon execution, this returns

hello
my_var= NEW VALUE
#my_var = ANOTHER VALUE
bye

While you can also make the script receive the parameters in a way like: ./script.sh "conf_file" "var_to_replace" "NEW VALUE" to then pass them to the function.