Using 'sed' to find and replace
sed is the stream editor, in that you can use | (pipe) to send standard streams (STDIN and STDOUT specifically) through sed and alter them programmatically on the fly, making it a handy tool in the Unix philosophy tradition; but can edit files directly, too, using the -i parameter mentioned below.
Consider the following:
sed -i -e 's/few/asd/g' hello.txt
s/ is used to substitute the found expression few with asd:
The few, the brave.
The asd, the brave.
/g stands for "global", meaning to do this for the whole line. If you leave off the /g (with s/few/asd/, there always needs to be three slashes no matter what) and few appears twice on the same line, only the first few is changed to asd:
The few men, the few women, the brave.
The asd men, the few women, the brave.
This is useful in some circumstances, like altering special characters at the beginnings of lines (for instance, replacing the greater-than symbols some people use to quote previous material in email threads with a horizontal tab while leaving a quoted algebraic inequality later in the line untouched), but in your example where you specify that anywhere few occurs it should be replaced, make sure you have that /g.
The following two options (flags) are combined into one, -ie:
-i option is used to edit in place on the file hello.txt.
-e option indicates the expression/command to run, in this case s/.
Note: It's important that you use -i -e to search/replace. If you do -ie, you create a backup of every file with the letter 'e' appended.
sed -i 's/fea/asd/g' hello.txt
g: Global
s: substitute
-i : realtime works with file inplace