sed (search and replace text)
This command allows you to search and replace strings in text files. Here is an example of its usage:
sed `s/bad/good/g' file.txt
This will output file.txt with every occurrence of the string bad replaced by good. The strings are interpreted as regular expressions (see Appendix A in the printed book). The s in the above invocation refers to sed's search and replace command. sed accepts many other commands, but s is the one that's most commonly used.
By default, s will replace only the first string on each input line. In most cases that's not what you want. The above example contains the modifier g (“global”), which widens the scope of the s command to the whole of every input line.
sed does not alter input files by default. In the above example, file.txt is not modified, as the search/replace operation only affects the output that you see on the terminal. You can redirect sed's output to another file to save the changes, for example:
sed `s/bad/good/g' file.txt > new.txt
You can also instruct sed to edit the input file directly, using the option -i (“in place”):
sed -i.bak `s/bad/good/g' file.txt
This will save the result of the search/replace operation in file.txt, overwriting the file's previous contents. A backup of the original file ending in .bak will be created. If you don't supply a suffix such as .bak to the -i option, sed won't create a backup. You should always specify a suffix to avoid butchering files by mistake.