Understanding the Linux /etc Directory: Key Configuration Files Explained

/etc/profile

This file sets environment information for every user; it runs at the first login and can configure environment variables and aliases.

/etc/profile.d

A subdirectory of /etc/profile that stores additional environment configuration files, making classification and usage of new environments easier.

/etc/bashrc

Executed for each user running a bash shell; read when the shell starts and can also define aliases.

~/.bash_profile

User‑specific shell configuration executed once at login; it sets necessary environment variables and sources the user's .bashrc.

~/.bashrc

Per‑user bash configuration read at login and each time a new shell is opened.

~/.bash_logout

Executed each time a user logs out of the system or exits a bash shell.

/etc/motd

Stores the message‑of‑the‑day (welcome information). Edit with vi; changes take effect after reconnecting.

/etc/issue and /etc/issue.net

Both hold welcome text displayed before login. /etc/issue is shown on local console login (before the login prompt), while /etc/issue.net is used for remote telnet logins. The content is the same by default but can be edited separately. The ssh banner is controlled by the Banner field in /etc/ssh/sshd_config.

Escape characters in /etc/issue

\d – local date

\l – terminal line

\m – hardware architecture

\n – host name

\o – domain name

\r – OS version

pwconv and pwunconv

pwconv enables shadow passwords, moving password hashes from /etc/passwd and /etc/group to /etc/shadow and /etc/gshadow, restricting read access to root. pwunconv reverses this, moving hashes back to /etc/passwd and /etc/group, reducing security.

/etc/fstab

Controls automatic mounting at boot. Example commands:

# mount /dev/sr0 /mnt

# echo "/dev/sr0 /mnt iso9660 defaults 0 0" >> /etc/fstab

/etc/rc.local

Used for commands that should run automatically at system startup.