/etc/passwd

Contains user informations. The information fields are separated by a colon.

user-name:password:UID:GID:info:home-dir:shell

• user-name: The name that the user needs to log in.
• password: Usually the entry x, meaning that the password is saved in /etc/shadow.
• UID: The user-id (usually greater than 1000).
• GID: The group-id.
• info: Additional information such as the real name. Subfields separated by coma.
• home-dir: The home dir of the user.
• shell: The login-shell of the user (needs to be listed in /etc/shells).

/etc/shadow

Saves the user password and related information. The file can be read only by root.

name:password:DOC:minDay:maxDay:warn:deaktIn:deaktSince:unused

• name: User name (same as in /etc/passwd).
• password: Encrypted password.
• DOC: Day of last change (number of days since 1.1.1970).
• minDay: Earliest possible day to change password.
• maxDay: Latest possible day to change password.
• warn: Number of days before maxDay that a warning occures.
• deaktIn: Day when password expires.
• deaktSince: Number of days since password expired.
• unused: For feature use.

/etc/group

Stores informations about groups.

groupName:password:groupId:member1,memger2,...

• groupName: Name of the group.
• password: Usually x, meaning that the password is saved in /etc/passwd.
• groupId: Group ID (GID).
• member1,memger2,...: User names of the members of the group (comma separated).

Mounts

• /etc/fstab: Lists drives to mount at startup
• /etc/exports: Folders provided to clients over NFS

Network Files

• /etc/services: Services provided on the network, mapping of ports to processes (see getservbyname() and getservbyport()).
• etc/networks: Informations about connected networks (see getnetbyname() and getnetbyaddr())
• /etc/protocols: Information about network portocols (see getprotobyname() and getprotobynumber())
• /etc/hosts: Mapping from hostnames to IP’s, mainly replaced by DNS (see gethostbyname() and gethostbyaddr())

Logging

• /etc/syslog.conf: Configuration of applications that use syslog
• /etc/asl.conf: Additional config on macOS

cron Jobs

• The config file is usually: /var/spool/cron
• use crontab -e to edit the right file

Shell

Configurations:

• bash: use ~/.bashrc and source it in ~/.bash_profie
• zsh: use ~/.zshrc

References

Zsh/Bash startup files loading order