What command is used to search for a pattern in a file using extended regular expressions?

The command used to search for a pattern in a file using extended regular expressions is egrep. This command is an extended version of grep, specifically designed to support extended regular expressions more fully. Extended regular expressions allow for additional metacharacters and functionalities, such as the use of + for one or more occurrences, ? for zero or one occurrence, and | for logical OR, among others.

While grep does support basic regular expressions, egrep is more powerful and convenient when dealing with complex patterns due to its ability to interpret extended regular expressions without needing to escape certain characters that grep would require. This makes egrep particularly useful for users who need to perform sophisticated pattern matching in text processing tasks.

The other commands listed serve different purposes. For instance, fgrep is used for fixed-string searches and does not recognize any regular expressions at all, while find is a command for searching for files in a directory hierarchy rather than for searching within files. Thus, egrep stands out as the appropriate command for searching patterns in files using extended regular expressions.