Replacing strings in files based on certain search criteria is a very common task. How can I

• replace string foo with bar in all files in the current directory?
• do the same recursively for sub directories?
• replace only if the file name matches another string?
• replace only if the string is found in a certain context?
• replace if the string is on a certain line number?

Replacing strings in files based on certain search criteria is a very common task. How can I

• replace string foo with bar in all files in the current directory?
• do the same recursively for sub directories?
• replace only if the file name matches another string?
• replace only if the string is found in a certain context?
• replace if the string is on a certain line number?
• replace multiple strings with the same replacement
• replace multiple strings with different replacements