Write a command to change all and in file foo.html to and , respectively in unix. Solution sed -i \'s###g; s###g\' foo.html sed is used for search and replace in unix. -i option is for replacing inplace g option is for replacing globally all the occurences in the file instead of only first occurence \'s#findstring#replacestring#g\' means seach for \'findstring\' string and replace it with \"replacestring\" string globally.