Module 02: Linux and Shell¶

/           # root of the filesystem
├── bin/    # essential user binaries (ls, cp, mv — symlink to /usr/bin on modern systems)
├── etc/    # system configuration files (nginx.conf, passwd, hosts)
├── home/   # user home directories (/home/alice, /home/bob)
├── proc/   # virtual filesystem exposing kernel/process information
├── sys/    # virtual filesystem exposing kernel hardware state
├── tmp/    # temporary files (cleared on reboot)
├── usr/    # user programs and data (/usr/bin, /usr/lib, /usr/local)
└── var/    # variable data: logs, databases, mail (/var/log, /var/lib)

# View running processes
ps aux                           # all processes, BSD-style
ps aux | grep nginx             # filter for nginx
pgrep nginx                     # get PIDs of nginx processes
top                              # interactive process viewer
htop                             # better interactive viewer (install separately)

# Process signals
kill -SIGTERM <pid>             # graceful shutdown (process can clean up)
kill -SIGKILL <pid>             # immediate termination (cannot be caught)
kill -SIGHUP <pid>              # hangup — nginx uses this to reload config
killall nginx                   # kill all processes named nginx

# Job control
command &                        # run in background
jobs                             # list background jobs
fg %1                            # bring job 1 to foreground
nohup command &                  # run immune to hangup (survives terminal close)

#!/usr/bin/env bash
# safe-script.sh — demonstrates essential bash patterns

set -euo pipefail
# -e: exit on error
# -u: treat unset variables as errors
# -o pipefail: if any command in a pipe fails, the whole pipe fails

# Functions
deploy_service() {
    local service_name="$1"    # local scope, named parameter
    local version="$2"

    echo "Deploying $service_name version $version"

    if ! systemctl is-active --quiet "$service_name"; then
        echo "WARNING: $service_name is not running before deploy" >&2
    fi
    # ... deployment logic ...
}

# Arrays
SERVICES=("nginx" "postgresql" "redis")
for service in "${SERVICES[@]}"; do
    echo "Checking: $service"
done

# String manipulation
VERSION="v1.2.3"
MAJOR="${VERSION%%.*}"          # remove longest match from end: "v1"
WITHOUT_V="${VERSION#v}"        # remove shortest match from start: "1.2.3"

# Trap for cleanup on exit
cleanup() {
    echo "Cleaning up temporary files..."
    rm -f /tmp/deploy-lock-$$   # $$ is the current PID
}
trap cleanup EXIT

deploy_service "nginx" "1.25.0"

# Service lifecycle
systemctl start nginx
systemctl stop nginx
systemctl restart nginx
systemctl reload nginx          # reload config without restart (if supported)
systemctl status nginx          # detailed status including recent logs
systemctl enable nginx          # auto-start on boot
systemctl disable nginx

# Journal logs
journalctl -u nginx             # all logs for nginx
journalctl -u nginx -f          # follow (tail -f equivalent)
journalctl -u nginx --since "1 hour ago"
journalctl -p err               # only error-level messages

# crontab syntax: minute hour day-of-month month day-of-week command
# *  *  *  *  *  command
# |  |  |  |  |
# |  |  |  |  └── day of week (0-7, 0 and 7 are Sunday)
# |  |  |  └───── month (1-12)
# |  |  └──────── day of month (1-31)
# |  └─────────── hour (0-23)
# └────────────── minute (0-59)

# Examples:
0 2 * * * /opt/scripts/backup.sh           # 2 AM every day
*/15 * * * * /opt/scripts/health-check.sh  # every 15 minutes
0 9 * * 1 /opt/scripts/weekly-report.sh   # every Monday at 9 AM

# Octal permission notation
# 7 = rwx (read + write + execute)
# 6 = rw- (read + write)
# 5 = r-x (read + execute)
# 4 = r-- (read only)
# 0 = --- (no permissions)

chmod 755 script.sh              # owner: rwx, group: r-x, others: r-x
chmod 600 /etc/myapp/secret.key  # owner: rw-, group: ---, others: ---
chown myapp:myapp /opt/myapp     # set owner and group

# User and group management
useradd -r -s /bin/false myapp   # create system user (no login shell)
groupadd myapp
usermod -aG docker alice          # add alice to the docker group

# sudo configuration (/etc/sudoers.d/myapp)
# myapp ALL=(root) NOPASSWD: /bin/systemctl restart myapp.service