To replace a text in a file, you can invoke sed as in the following example :
% cat file.txt | sed -e 's/text/replacement/g' > result.txt
This will change all the occurences of “text” to “replacement” in “file.txt” and output the result in “result.txt”
Note : As suggested by Matthias from adminlife in the comments, if you wanted to do “in place” text replacement (that is modify the file without a temporary file in between), you can do the following :
sed -i ās/text/replacement/gā file.txt
For more complicated text manipulations you might consider moving to Perl, but sometimes you don’t need the sledge-hammer š