Linux Processes, Bash
& Serial Communication

What is a Process?

A process is a program that is currently running. Every application you open becomes a process, and Linux manages all running processes using the kernel.

Every process gets a unique Process ID (PID):

Many Linux commands use the PID to identify and control a specific process.

Ctrl+C vs Ctrl+Z

Common Linux Signals

Linux File Permissions

Linux protects files using permissions that determine who can read, modify, or execute a file. Every file has three permission groups:

ls -l
# -rwxr-xr-- 1 satwik satwik 1234 Jun 1 script.sh

stdin / stdout / stderr

Every Linux program communicates using three standard streams. Understanding these is the foundation of redirection and pipes.

Output Redirection

Input Redirection

sort < numbers.txt   # sort reads from file instead of keyboard

Pipes ( | )

A pipe sends the stdout of one command directly into the stdin of another — without creating any temporary files.

ps aux | grep python        # find running Python processes
find . -name "*.c" | wc -l  # count all C source files
dmesg | tail                 # see the last kernel messages

Error Redirection

cat missing.txt 2> errors.txt   # redirect only errors to file
make > build.log 2>&1           # redirect both stdout AND stderr to same file

The 2>&1 syntax means: "redirect file descriptor 2 (stderr) to wherever file descriptor 1 (stdout) is going." This is used constantly to capture complete build logs.

Bash Shell

Bash (Bourne Again SHell) is the default command-line shell on most Linux systems. It reads commands from the user, executes them, and displays the results. It also runs shell scripts — files containing lists of commands to automate tasks.

Shell Scripts

A shell script is a text file containing Linux commands. Instead of typing the same commands repeatedly, save them once and execute whenever needed.

#!/bin/bash

echo "Building..."
make
echo "Done"

Shebang — the first line

The #!/bin/bash line tells Linux which interpreter should run the script. It must be the very first line.

Make it executable and run it

chmod +x build.sh   # make executable (only needed once)
./build.sh          # run the script

Comments

Lines beginning with # are comments. They are ignored during execution but improve readability.

# Build firmware for ESP32
make

Variables & echo

PROJECT=ESP32          # assign variable (no spaces around =)
echo $PROJECT          # use with $ prefix → outputs: ESP32

PORT=/dev/ttyUSB0
echo "Connecting to $PORT"

read NAME              # read user input
echo "Hello $NAME"

Exit Status

Every command returns an exit status (also called a return code) when it finishes. This is the most important way scripts detect success or failure.

make
echo $?       # 0 = success, non-zero = error

set -euo pipefail

Professional shell scripts begin with this line to make them safe and reliable:

set -euo pipefail

Complete Embedded Script Example

#!/bin/bash

set -euo pipefail

west build
west flash
picocom -b 115200 /dev/ttyUSB0

This script builds firmware, flashes it to the board, and opens the serial monitor. If any step fails, the script stops immediately rather than continuing with a broken state.

UART & Serial Device Files

UART (Universal Asynchronous Receiver/Transmitter) is the hardware interface used for serial communication — transferring data one bit at a time between two devices.

Serial Device Files

Baud Rate

The baud rate is the communication speed. Both sides must use the same value. Common values: 9600, 115200, 230400, 460800. Most embedded projects use 115200.

dmesg & lsusb

dmesg | tail     # show last kernel messages (use after plugging in board)

lsusb

ss & tree

ss -tln    # show listening TCP ports
ss -tlnp   # show ports with process names

tree         # full tree from current directory
tree -L 2    # limit depth to 2 levels

Typical Embedded Workflow

This is the standard sequence every embedded developer follows when connecting a board for the first time:

Complete Command Reference