So there’s actually a story behind this 
My conkyrc file has grown to a fairly large size and it’s a bit of a pain to go through it to change the colors. After all there’s lots of ${color} in it
So I needed a command that could do this for me. That’s when I found sed, a stream editor. sed can replace a word or set of words with a different word or set of words.
There’s a couple different ways to use sed. You can have it replace only the first instance of a word in a sentence or you can have it replace the word globally.
To replace only the first instance of a word in a sentence the command looks like this
sed -i ’s/original_word/new_word/’ file.txt
To change all instances of a word in a file the command is
sed -i ’s/original_word/new_word/g’ file.txt
the “g” at the end tells sed to perform the action globally.
You can also do more than one word, like a phrase. To do this correctly you need to use a “\” backslash before the space. So for example I wanted to replace all instances of “black” with “light blue” in my conkyrc. This is what the command looked like
sed -i ’s/black/light\ blue/g’ .conkyrc
That saved me about 5 minutes of editing my conkyrc.
So there you have it, a very useful time saving command.
Or you can use the Replace button in Gedit which somehow I managed to overlook while trying to figure out a command to do this! 
That’s a good one to remember.
I use a different technique in the .conkyrc though. Instead of using hex numbers like FF6600 in the color tags, I have color declarations just before the TEXT part.
E.g. I could have:
color1 447788
color2 110099
color3 334477
And then under the TEXT part, instead of using ${color 447788} to change to the color I want, I just use ${color1} instead (or you can leave out the brackets). Then, when you go through and want to change colors, you can just do it at the top.
Anyway that’s my way of doing it, but using sed seems just as good
Interesting. I never thought of doing it that way. That’s a good way to do it.
How do I replace only if the keyword is at the begining of a line?
Code:
—-
a = “This is a print statement”
print(a)
—-
What if I want to replace print by #print only in the second line i.e only if the line starts with that keyword.
Please help me out. I’m new to SED.
“s/^print/replacement/”
…
but you really need to get used to regexps
at least enough to understand what’s the difference between
“s/^print/replacement/”
and
“s/^ *print/replacement/”