What is a Process?
A process is a running program. Every command you type, every service in the background, every browser tab — each is a process. The kernel tracks them all in the process table, and every process gets a unique number: the PID.
PIDs and the Process Table
What is a PID?
Process ID — a unique number the kernel assigns to every process. PID 1 is always systemd (or init). PIDs count up and wrap around when they hit the max (typically 32768 or 4194304 on modern kernels).
ps aux | head -5
# USER PID %CPU %MEM VSZ RSS TTY STAT COMMAND
# root 1 0.0 0.1 168260 12340 ? Ss /sbin/init
# root 2 0.0 0.0 0 0 ? S [kthreadd]
# Show PID and Parent PID
ps -o pid,ppid,comm
# PID PPID COMMAND
# 1 0 systemd
# 899 1 sshd (parent is systemd)
fork() and exec() — How Processes Are Created
How does a new process come into existence?
Through two system calls working together.
fork() creates a clone of the calling process. exec() replaces that clone's memory with a new program. Together they're called the fork-exec pattern.
When you type "ls" in bash:
1. bash calls fork()
- Creates exact copy of bash process
- Returns child's PID to bash (parent)
- Returns 0 to the child
2. Child process calls exec("/bin/ls")
- Kernel replaces child's memory with ls binary
- ls starts running from its main()
3. bash calls wait() — waits for child to finish
4. ls finishes, bash shows next prompt
What is copy-on-write in fork?
fork() doesn't immediately copy all parent memory — that would be slow. Instead, parent and child share the same physical pages, marked read-only. Only when either process writes to a page does the kernel make a private copy. This makes fork() very fast.
The Process Tree
All processes form a tree — every process has one parent. You can visualize this:
pstree -p
# systemd(1)
# ├─sshd(899)
# │ └─sshd(1234)
# │ └─bash(1235)
# │ └─pstree(5678)
# ├─nginx(1100)
# │ ├─nginx(1101)
# │ └─nginx(1102)
# └─cron(950)
Viewing Processes
# All processes, detailed
ps aux
# Tree format
ps axjf
# Specific process
ps -p 1234 -o pid,ppid,user,comm,args
# Interactive view
top
htop # if installed (better UI)
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