Learning Modern Linux - ePub

1. Introduction to Linux

2. The Linux Kernel

2.1 Linux Architecture

lspcu
cat /proc/cpuinfo

Example of different architectures:

• x86
• ARM
• RISC-V

2.2 Kernel Components

2.2.1 Process Management

• Sessions (SID)
• Process groups (PGID)
• Processes (PID)
• Threads (TID(TGID)
• Tasks

ps -j
PID   PGID  SID   TTY    TIME CMD
6756  6756  6756  pts/0  00:00:00 bash
6790  6790  6756  pts/0  00:00:00 ps
ls -al /proc/6756/task/6756/

2.2.2 Memory Management

grep MemTotal /proc/meminfo
grep VmallocTotal /proc/meminfo
grep Huge /proc/meminfo

2.2.3 Networking

• Sockets
• Transmission Control Protocol (TCP) and User Datagram Protocol (UDP)
• Internet Protocol (IP)

ip link
ip route

2.2.4 Filesystems

2.2.5 Device Drivers

ls -al /sys/devices/
mount

2.2.6 syscalls

strace ls
strace -c curl -s https://mhausenblas.info > /dev/null

Example syscalls by category:

• Process management: clone, fork, execve, wait, exit, getpid, setuid, setns, getrusage, capset, ptrace.
• Memory management: brk, mmap, munmap, mremap, mlock, mincore.
• Networking: socket, setsockopt, getsockopt, bind, listen, accept, connect, shutdown, recvfrom, recvmsg, sendto, sethostname, bpf.
• Filesystems: open, openat, close, mknod, rename, truncate, mkdir, rmdir, getcwd, chdir, chroot, getdents, link, symlink, unlink, umask, stat, chmod, utime, access, ioctl, flock, read, write, lseek, sync, select, poll, mount.
• Time: time, clock_settime, timer_create, alarm, nanosleep.
• Signals: kill, pause, signalfd, eventfd.
• Global: uname, sysinfo, syslog, acct, _sysctl, iopl, reboot.

2.3 Kernel Extensions

2.3.1 Modules

uname -srm
find /lib/modules/$(uname -r) -type f -name '*.ko*'
lsmod
cat /proc/modules

2.3.2 A Modern Way to Extend the Kernel: eBPF

3. Shells and Scripting

3.1 Basics

3.1.1 Terminals

3.1.2 Shells

Streams

• stdin (FD0)
• stdout (FD1)
• stderr (FD2)

FD: File Descriptor.

cat
start typing

• 2> redirect stderr
• 1> or > redirect stdout
• &> redirect both stderr and stdout

curl https://example.com &> /dev/null
curl https://example.com > /tmp/content.txt 2> /tmp/curl-status
head -3 /tmp/content.txt
cat /tmp/curl-status  #C-d to stop
cat > /tmp/interactive-input.txt
tr < /tmp/curl-status [A-Z] [a-z]
curl https://example.com 2> /dev/null | \ 
  wc -l 

• Ampersand (&) at the end of the command, execute in the background
• Backslash () continue a command next line
• Pipe (|) connect stdout and stdin

Variables

• Environmental variables
• Shell variable

env  # show environmental
set  # show shell
MY_VAR=42
set | grep MY_VAR
export MY_GLOBAL_VAR="Hello!!"
set | grep MY_
env | grep MY_
bash
echo $MY_GLOBAL_VAR
set | grep MY_
exit
unset MY_VAR
set | grep MY_

Environmental variables:

• EDITOR: default editor;
• HOME: home directory;
• HOSTNAME: hostname;
• IFS: characters separate fields;
• PATH: directories for executables;
• PS1: prompt string;
• PWD: working directory;
• OLDPWD: directory before cd;
• RANDOM: random integer 0-32767;
• SHELL: current shell;
• TERM: terminal emulator used;
• UID: current user UID;
• USER: current user name;
• _: last argument previous command;
• ?: exit status;
• $: ID of current process;
• 0: name of current process.

Built-in commands

which ls
type ls

Job control

watch -n 5 "ls" &
jobs
fg

In order to keep a background process running, even after closing the shell, nohup is to be prepended the command.

Further, for a process that is already running and wasn’t prepended with nohup, it could be used disown %JobN, example:

jobs
disown %1

3.1.3 Modern commands

exa
bat list.txt
touch apple.yaml
echo "sample" > apple.yaml
find . -type f -name "*.yaml" -exec grep "sample" '{}' \; -print 
rg -t "yaml" sample 
jq  # json data processing

3.1.4 Common Tasks

Navigating

Shell navigation and editing shortcuts

• Ctrl+a: Move cursor to start of line;
• Ctrl+e: Move cursor to end of line;
• Ctrl+f: Move cursor forward one character;
• Ctrl+b: Move cursor back one character;
• Alt+f: Move cursor forward one word;
• Alt+b: Move cursor back one word;
• Ctrl+d: Delete current character;
• Ctrl+h: Delete character left of cursor;
• Ctrl+w: Delete word left of cursor;
• Ctrl+k: Delete everything right of cursor;
• Ctrl+u: Delete everything left of cursor;
• Ctrl+l: Clear screen;
• Ctrl+c: Cancel command;
• Ctrl+_: Undo (bash only);
• Ctrl+r: Search history (some shells);
• Ctrl+g: Cancel search (some shells);

In addition, you might wantto know that these shortcuts are based on Emacs editing keystrokes. Should you prefer vi, you can use set -o vi in your .bashrc file.

File content management

cat << FINE > contenuto.txt
heredoc> Prova inserimento testo
heredoc> in nuovo file
heredoc> FINE
cat contenuto.txt

Altri comandi interessanti:

• diff
• head
• tail

3.2 Human-Friendly Shells

• Fish
• Z-shell
• Oil shell
• murex
• Nushell