Can we replace one variable with multiple variables using sed

I have a config file where i need to replace the entire line with new values. It can be either a word or a URL. I am trying to write a script to replace only this particular parameter with new values.

I have tried using grep to extract the line using the parameter and divided the values separately and saved in two different variables. Now I am trying to replace the whole line with the parameter along with new value or url usind sed

jaram=`grep -i "$a" app.properties`;
param=`grep -i "$a" app.properties |sed 's/'$a'=*//'`;

sed -e 's~'$jaram'~'$a=''$changed_param'~g' app.properties

The config file contains:

abc1=http://howareyou:scema=olk
abc2=http://howareyou:scema=olk

Here I am trying to replace the url of only abc1. though both have same value. I need to replace the entire url with something different url or a word.

Here I am trying to find the line which contains abc1 and this line url after = is saved in a different variable.

I tried to replace the url with new one using sed:

sed -i 's~'$jaram'~'$a=''$param'~g' app.properties
sed: -e expression #1, char 0: no previous regular expression

Seems like I am doing wrong at some syntax when using sed

trying to replace something like

sed 's/jaram/'{$a=$param}/'

Expecting an output like

abc1=http://jalkek:kj/iuwerj
abc2=http://howareyou:scema=olk

Answer

The following script with comments:

# recrete the input config file
cat <<EOF >input
abc1=http://howareyou:scema=olk
abc2=http://howareyou:scema=olk
EOF

# some input variables
name="abc1"
newvalue='http://jalkek:kj/iuwerj'

# the sed script
sed -i 's'$'1''^'"$name"'=.*'$'1'"$name"'='"$newvalue"$'x01' input

# and the output
cat input

produces the following output:

abc1=http://jalkek:kj/iuwerj
abc2=http://howareyou:scema=olk

Notes:

• I used the 0x01 byte as the separator for s command inside sed. So it should work with all printable characters.
• Remember about quoting. All variables should be inside " double quotes, but all the rest would be best if inside ' single quotes.
• I match the name= with ^, so from the beginning of the line. So that for example blablaabc1= doesn’t match.
• I use ANSI-C Quoting from bash to generate the unredable 0x01 bytes to delimit sed command.

A little more readable version of the sed script could be this one, that just uses " for quoting and uses ~ as a separator:

 sed "s~^${name}=.*~${name}=${newvalue}~"