UNIX can consist of four main files types:
• Normal files—Contain user data
• Directories—Containers that hold files
• Special files—Input and output devices, such as a disk drive, printer, or CD-ROM
• Links—Pointers to another file
UNIX stores files and important information in directories. The following are some common examples (might vary according to the UNIX version):
• /bin/—Executable system utilities, such as sh, cp, and rm.
• /etc/—System configuration files and databases.
• /lib/—Operating system and programming libraries.
• /tmp/—System scratch files (all users can write here).
• /lost+found/—Where the file system checker puts detached files.
• /usr/include/—Standard system header files.
• /usr/lib/—More programming and system call libraries.
• /usr/local/—Typically a place where local utilities go.
NOTE Certain system files created by UNIX store important details about the operational characteristics, such as the password lists for all users.
The file named shadow in the /etc directory is a read only, protected file referenced by the program login.
The file named passwd contains the passwords for all users.
The file named wtmp contains an account of all users that logged into the UNIX host.
The file named lastlog contains details of when a user logged out of a UNIX host.
The file .rhosts contains information permitting remote devices, such as routers, the capability to TFTP or Remote Copy Protocol (RCP) files to a UNIX host.
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