Today, I will share 50 important Linux configuration files that every operations engineer should be familiar with. How many do you know?

1 User and Permission Management Related

File Path	Function and Purpose
<span><span>/etc/passwd</span></span>	Stores basic information for all users, such as username, UID, GID, home directory, and default shell. It is the first step for the system to identify users.
<span>/etc/shadow</span>	Stores encrypted user passwords and password policies (validity period, minimum modification days, etc.). Readable only by root, enhancing account security.
<span>/etc/group</span>	Stores user group information for the system. Used for permission management, controlling user access to files and services.
<span>/etc/gshadow</span>	Stores encrypted user group password information. Used to enhance group access control security.
<span><span>/etc/sudoers</span></span>	Configures which users/groups can use sudo to execute privileged commands and their permissions. Must be edited using <span>visudo</span> to prevent configuration errors.
<span><span>/etc/login.defs</span></span>	Defines default policies for user accounts, such as password validity period, UID range, account expiration time, etc.
<span><span>/etc/skel/</span></span>	Stores template files that are copied by default for new users, such as <span>.bashrc</span>, used to initialize user environments.
<span><span>/etc/security/limits.conf</span></span>	Sets resource limits for each user or group, such as maximum number of processes and maximum number of open files. Commonly used to prevent resource abuse.
<span><span>/etc/pam.d/</span></span>	PAM configuration directory, controlling authentication logic for login, sudo, su, etc.
<span>/var/log/lastlog</span>	Binary format, records the last login time, IP, and terminal for all users. Viewable using <span>lastlog</span>.

2 Network Configuration and Access Control Related

File Path	Function and Purpose
<span><span>/etc/hosts</span></span>	Local DNS mapping, domain name resolution takes precedence over remote DNS. Commonly used for internal network resolution and testing domain names.
<span><span>/etc/resolv.conf</span></span>	DNS server configuration file, the system defines domain name resolution through the nameserver specified here.
<span><span>/etc/hostname</span></span>	Current hostname configuration, affects command line prompts and host identification.
<span><span>/etc/nsswitch.conf</span></span>	Controls the order of sources for resolving hosts, users, and service names (e.g., files, dns, ldap).
<span><span>/etc/hosts.allow</span></span>	TCP Wrapper whitelist, allows specific IPs to access certain services.
<span><span>/etc/hosts.deny</span></span>	TCP Wrapper blacklist, denies access requests from specific IPs to certain services.
<span><span>/etc/network/interfaces</span></span> (Ubuntu)	Legacy network interface configuration file, defines IP, mask, gateway, etc. for network cards.
<span><span>/etc/sysconfig/network</span></span> (Red Hat)	Global network configuration (e.g., hostname, default gateway), applicable to early RHEL systems.
<span><span>/etc/sysconfig/network-scripts/ifcfg-*</span></span>	Each network card has a configuration file defining parameters like IP, DNS, gateway, etc.
<span><span>/etc/netplan/*.yaml</span></span>	Default network configuration system for Ubuntu 18.04 and later, defines network structure using YAML format.

3 System Boot and Service Management Related

File Path	Function and Purpose
<span><span>/etc/fstab</span></span>	Sets disks, partitions, and network storage to be automatically mounted at system boot. Incorrect entries may prevent the system from booting.
<span>/etc/rc.local</span>	Startup script that executes automatically after the system boots. Suitable for executing one-time initialization commands.
<span><span>/etc/inittab</span></span>	SysV-style initialization configuration file, mostly replaced by systemd in modern systems.
<span><span>/etc/systemd/system/*.service</span></span>	User-defined systemd service unit files, supporting boot startup and custom service management.
<span><span>/usr/lib/systemd/system/*.service</span></span>	Default service file path for system services, usually written here by installation packages. Manual modifications are not recommended.
<span><span>/etc/init.d/</span></span>	SysV-init service script directory, compatible with older system startup methods.
<span><span>/etc/default/grub</span></span>	Default configuration file for the GRUB boot menu, modifications require running <span>update-grub</span> to generate the actual configuration.
<span><span>/boot/grub2/grub.cfg</span></span>	Configuration file actually used by the GRUB bootloader. Automatically generated by <span>/etc/default/grub</span>.
<span><span>/etc/sysctl.conf</span></span>	Configures kernel parameters, such as TCP cache, connection count, forwarding, etc. Can be loaded using <span>sysctl -p</span>.
<span><span>/etc/modprobe.d/*.conf</span></span>	Defines rules for loading or blacklisting kernel modules. For example, disabling USB storage modules.

4 Logs and System Auditing Related

File Path	Function and Purpose
<span><span>/var/log/messages</span></span>	System log summary (CentOS), contains operational information for services, hardware, network, etc.
<span>/var/log/syslog</span>	General system log (Ubuntu/Debian), similar functionality to messages.
<span>/var/log/auth.log</span>	Authentication log, records login, sudo usage, password verification, and other operations.
<span>/var/log/secure</span>	Authentication log for CentOS, functions similarly to auth.log.
<span><span>/var/log/dmesg</span></span>	Records kernel startup information and driver loading, suitable for checking hardware recognition status.
<span><span>/var/log/boot.log</span></span>	Service startup output during each system boot process.
<span><span>/var/log/wtmp</span></span>	Login and logout history (binary), viewable using <span>last</span>.
<span><span>/var/log/btmp</span></span>	Login failure records (binary), viewable using <span>lastb</span> to check for brute force risks.
<span><span>/var/log/cron</span></span>	Log of scheduled task executions, an important basis for checking if crontab is running normally.
<span><span>/var/log/kern.log</span></span>	Kernel-level log, records device drivers, kernel panic, exceptions, etc.

5 User Environment and Shell Configuration Related

File Path	Function and Purpose
<span><span>/etc/profile</span></span>	Shell initialization script read by all users upon login, suitable for setting global variables, umask, PATH, etc.
<span><span>~/.bash_profile</span></span>	User-level login shell script, executed only once during login shell.
<span>~/.bashrc</span>	User-level interactive shell configuration, executed every time a new terminal is opened.
<span>/etc/bashrc</span>	Bash shell configuration file for all users, also called by non-interactive shells.
<span>/etc/environment</span>	System-level environment variable configuration file, does not parse shell commands, only sets variable values.

6 Scheduled Tasks and Scheduling Related

File Path	Function and Purpose
<span><span>/etc/crontab</span></span>	System-level scheduled task file, can set the user to which the task belongs, suitable for system service scheduling.
<span><span>/etc/cron.d/</span></span>	Directory for crontab configurations for application-level or system-level services, supports custom scripts.
<span><span>/var/spool/cron/</span></span>	Directory where scheduled tasks created by users using <span>crontab -e</span> are saved, named after the username.

7 System Information and Status Viewing Related

File Path	Function and Purpose
<span><span>/proc/cpuinfo</span></span>	Displays information about the current CPU, including model, core count, thread count, frequency, etc.
<span><span>/proc/meminfo</span></span>	Displays detailed memory information, including total memory, remaining memory, buffer, cache, etc.

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