Shell Customization

Concepts

Shell Startup Files

When Bash starts, it reads configuration files — but which files depend on how the shell was started.

Type	When	Example
Login shell	First shell after authentication	SSH session, TTY login, `su - user`
Non-login shell	Shells opened inside an existing session	Opening a terminal in the desktop, running `bash`

Interactive vs Non-Interactive Shell

Type	When	Example
Interactive	You type commands at a prompt	Terminal window, SSH session
Non-interactive	Runs a script, no prompt	`bash script.sh`, cron jobs

Which Files Are Read?

Login shell (interactive):
  /etc/profile → ~/.bash_profile OR ~/.bash_login OR ~/.profile (first found)

Non-login shell (interactive):
  /etc/bash.bashrc → ~/.bashrc

Non-interactive shell (scripts):
  $BASH_ENV (if set)

In practice, on Ubuntu and Debian, ~/.profile sources ~/.bashrc:

[Read more]

Signals

Concepts

What Is a Signal?

A signal is a software notification sent to a process. Signals tell processes to do something: stop, terminate, reload configuration, or respond to a user action. They are the kernel’s way of communicating with processes.

When you press Ctrl + C, you are sending a signal. When you run kill 1234, you are sending a signal.

Common Signals

Signal	Number	Default Action	Sent By
`SIGHUP`	1	Terminate	Terminal closes, or `kill -1`. Many daemons reload config.
`SIGINT`	2	Terminate	`Ctrl + C`
`SIGQUIT`	3	Terminate + core dump	`Ctrl + \`
`SIGKILL`	9	Kill immediately	`kill -9`. Cannot be caught or ignored.
`SIGTERM`	15	Terminate gracefully	`kill` (default signal). Process can clean up.
`SIGSTOP`	19	Suspend process	Cannot be caught or ignored.
`SIGTSTP`	20	Suspend process	`Ctrl + Z`. Can be caught by the process.
`SIGCONT`	18	Resume a stopped process	`kill -CONT` or `fg`/`bg`
`SIGUSR1`	10	User-defined	Application-specific
`SIGUSR2`	12	User-defined	Application-specific

SIGTERM vs SIGKILL

This is the most important distinction:

[Read more]

Snap, Flatpak, and AppImage

Concepts

Beyond APT — Universal Package Formats

APT and .deb packages are the traditional way to install software on Debian/Ubuntu. But they have a limitation: packages are built for a specific distribution version. A .deb built for Ubuntu 24.04 may not work on Debian 12 or Ubuntu 22.04.

Universal package formats solve this by bundling the application with its dependencies, running in a sandbox, and working across distributions:

[Read more]

Sorting, Cutting, and Counting

Concepts

The Unix Philosophy in Action

These small, specialized tools each do one thing well. Their power comes from combining them with pipes. This lesson covers the remaining essential text-processing commands.

`sort` — Sort Lines

sort file.txt                    # alphabetical sort
sort -r file.txt                 # reverse order
sort -n file.txt                 # numeric sort (1, 2, 10 not 1, 10, 2)
sort -h file.txt                 # human-readable numbers (1K, 2M, 3G)
sort -k2 file.txt                # sort by 2nd field
sort -k2,2n file.txt             # sort by 2nd field numerically
sort -t: -k3 -n /etc/passwd      # sort by UID (field 3, delimiter :)
sort -u file.txt                 # sort and remove duplicates
sort -f file.txt                 # case-insensitive sort

The -k flag specifies which field to sort by. Fields are separated by whitespace by default (change with -t).

[Read more]

Special Permissions

Concepts

Beyond rwx

The standard read/write/execute permissions cover most scenarios. But Linux has three special permission bits that handle specific situations:

Bit	Name	On Files	On Directories
SUID	Set User ID	Runs as the file’s owner, not the user who executes it	No effect
SGID	Set Group ID	Runs with the file’s group, not the user’s group	New files inherit the directory’s group
Sticky	Sticky Bit	No modern effect	Only file owner (or root) can delete files

SUID — Set User ID

When SUID is set on an executable, the program runs with the permissions of the file’s owner instead of the user who runs it.

[Read more]