i am working on shell scripting and i want to change a text inside a file. I know i have to use sed command but there is a problem. When i run the script i am passing three arguments

$ ./program arg1 arg2 arg3
arg1 = file path to the directories
arg2= the string that it needs to find
arg3=the new string that replaces arg2

Nevertheless, inside the program file i have a line
sed 's/'$2'/'$3'/g' $x>out.txt

and here is the problem. when i run the script it says:
sed: -e expression #1, char 0: no previous regular expression

I know it has something to do with the quotes ('s/'$2'/'$3'/g') because when i change the variables ($2, $3) to regular strings it does the job. I think i tried all kinds of combinations on how to get this line to work but with no luck.

can somebody tell me the correct combination of quotes?

thanks,

Try this:
"s/$2/$3/g"

Two points:
I don't think you ever need to quote the value of a variable.
The single quote (') is called a strong quote and forces everything inside to be taken literally. Thus, your syntax caused sed to look for the literal string "$2"
OOPS--no: it was looking for "'$2'"

thanks pixellany but the problem persists. It still gives me the error.

sed: -e expression #1, char 0: no previous regular expression


any other ideas?

thanks,

finally !!!!!!

i have found the way:

's/'"$2"'/'"$3"'/g'


damn, what a pickle

thanks,

This may have worked for this situation, but creating code that convoluted will eventually get you in trouble. I am not even going to TRY to understand how triply-nested quotes somehow make things work.
If I could, I would really want to dig into why the simple structure I tested on my machine would not work on yours.....

This also works:

s/$2/$3/g (NO quotes)