Invata Linux & Bash
de la zero

$ ssh user@192.168.1.100          # conectare cu user si IP
$ ssh -p 2222 user@server.ro      # port personalizat
$ ssh -i ~/.ssh/cheie.pem user@IP # cu cheie privata
$ exit                               # deconectare

-rwxr-xr--  1  andrei  dev  4096  Jan 10  fisier.sh
 ↑↑↑↑↑↑↑↑↑
 │└──┬──┘└──┬──┘└──┬──┘
 │   │      │      └── Others: r-- (citire)
 │   │      └───────── Group: r-x (citire+exec)
 │   └──────────────── Owner: rwx (toate)
 └──────────────────── Tip: - fisier, d director, l link

# ip addr add 192.168.1.50/24 dev eth0  # adauga IP
# ip link set eth0 up                    # activeaza interfata
# ip link set eth0 down                  # dezactiveaza
# ip route add default via 192.168.1.1    # gateway default

# Sintaxa cron: min  ora  zi  luna  zi_saptamana  comanda
#               *    *    *   *     *              (0-59)(0-23)(1-31)(1-12)(0-7)

0 2 * * *       /home/andrei/backup.sh      # Zilnic la 2:00
*/5 * * * *     /usr/bin/check.sh           # La fiecare 5 minute
0 0 * * 1       /scripts/weekly.sh          # Luni la miezul noptii
0 9-17 * * 1-5  /scripts/workhours.sh       # Luni-Vineri 9-17, la ora fixa
@reboot         /scripts/startup.sh         # La fiecare reboot
@daily          /scripts/daily.sh           # Zilnic (echivalent 0 0 * * *)

#!/bin/bash
# Descriere: Primul meu script bash
# Autor: Andrei | Data: 2025-01-10

# Variabile
NUME="ZimbruL"
DATA=$(date +%F)

# Output
echo "Salut, $NUME! Astazi este $DATA"

# Verificare exit code comanda anterioara
ls /inexistent 2>/dev/null
echo "Exit code: $?"    # 0=succes, altceva=eroare

$1 $2 $3          # argumente pozitionale scriptului
$@                 # toate argumentele
$#                 # numarul de argumente
$0                 # numele scriptului
$$                 # PID-ul scriptului curent
$?                 # exit code ultima comanda
${VAR:-default}    # valoare default daca VAR e goala
${#VAR}            # lungimea variabilei
${VAR^^}           # uppercase
${VAR,,}           # lowercase
$(comanda)         # substitutie comanda (rezultatul ei)
$((2 + 3))         # calcul aritmetic = 5

#!/bin/bash
FISIER="/etc/hosts"

if [ -f "$FISIER" ]; then
    echo "Fisierul exista"
elif [ -d "$FISIER" ]; then
    echo "Este un director"
else
    echo "Nu exista"
fi

# Comparatii numerice: -eq -ne -lt -le -gt -ge
if [ $NUM -gt 10 ]; then echo "mare"; fi

# String comparatii: = != -z (gol) -n (negol)
if [[ "$STR" == "test" ]]; then echo "match"; fi

# Conditii fisiere: -f fisier -d dir -r readable -w writable -x exec -e exista -s nenul

# for clasic
for i in 1 2 3 4 5; do
    echo "Nr: $i"
done

# for range
for i in {1..10}; do echo $i; done

# for pe fisiere
for f in /etc/*.conf; do
    echo "Config: $f"
done

# while loop
COUNT=0
while [ $COUNT -lt 5 ]; do
    echo "Count: $COUNT"
    COUNT=$(($COUNT + 1))
done

# citire fisier linie cu linie
while IFS= read -r linie; do
    echo ">> $linie"
done < fisier.txt

# until loop (opusul while)
until [ $COUNT -eq 10 ]; do
    COUNT=$(($COUNT+1))
done

case $1 in
  start)   echo "Pornire..."  ;;
  stop)    echo "Oprire..."   ;;
  restart) echo "Restart..."  ;;
  *)       echo "Uz: $0 {start|stop|restart}" ;;
esac

# ~/.ssh/config — configurare conexiuni SSH

Host server-prod
    HostName     192.168.1.100
    User         andrei
    Port         2222
    IdentityFile ~/.ssh/id_ed25519

# Acum poti conecta simplu cu: ssh server-prod

Host *
    ServerAliveInterval 60
    ServerAliveCountMax 3
    AddKeysToAgent      yes

Port 2222                    # schimba portul default
PermitRootLogin no           # NU permite root prin ssh
PasswordAuthentication no   # NUMAI chei SSH
PubkeyAuthentication yes    # activeaza chei
MaxAuthTries 3               # max 3 incercari
AllowUsers andrei            # doar utilizatorul andrei

# 1. Verificare load
uptime
top # sorteaza cu 'P' dupa CPU, 'M' dupa memorie

# 2. Memorie
free -h
cat /proc/meminfo | grep "MemAvailable"

# 3. I/O disk
iostat -x 1 5
iotop -ao

# 4. Retea
ss -tulnp
netstat -s | grep "errors"

# 5. Loguri recente
journalctl -p err --since "1 hour ago"
dmesg | tail -30

# Gaseste ce ocupa loc
df -h
du -sh /* 2>/dev/null | sort -rh | head -10

# Fisiere mari in tot sistemul
find / -type f -size +500M 2>/dev/null

# Log-uri mari
ls -lSh /var/log/ | head

# Curatenie sigura
journalctl --vacuum-time=7d    # pastreaza 7 zile journald
apt autoremove && apt clean     # curata pachete
find /tmp -mtime +7 -delete     # sterge tmp vechi de 7 zile