Which encryption algorithm was replaced with the introduction of the /etc/shadow file in the Linux OS?

The original Unix password hashing used the crypt() function based on DES, and hashes were stored world-readable in /etc/passwd. The introduction of /etc/shadow moved password hashes to a root-readable-only file, replacing the DES-crypt scheme and enabling support for stronger algorithms such as MD5 and later SHA-256 and bcrypt. DES is therefore the algorithm that was in use prior to and replaced by the shadow file infrastructure.

MD5 was adopted as a password hashing option after /etc/shadow was introduced, not before it - it represents a successor algorithm enabled by the shadow file system, not the one it replaced.

SHA-256 is a modern hashing algorithm that was incorporated into Linux PAM and shadow utilities long after /etc/shadow already existed, making it a later improvement rather than a predecessor.

Blowfish-based bcrypt is a relatively modern password hashing scheme that became available as an option in shadow-compatible systems, not the legacy algorithm that shadow replaced.