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Loops

Table of contents​

  1. for loop
  2. while loop
  3. until loop
  4. Flow control: break and continue
  5. Reading files
  6. Practical exercises

1 - for loop​

Basic syntax​

for variable in liste; do
    # commandes
done

Iterating over a list​

# Liste explicite
for fruit in pomme banane orange; do
    echo "Fruit: $fruit"
done

# Avec tableau
fruits=("pomme" "banane" "orange")
for fruit in "${fruits[@]}"; do
    echo "Fruit: $fruit"
done

Sequence expansion​

# Avec {start..end}
for i in {1..5}; do
    echo "Numéro: $i"
done

# Avec incrément
for i in {0..10..2}; do
    echo "Pair: $i"  # 0, 2, 4, 6, 8, 10
done

# Avec seq (plus ancien)
for i in $(seq 1 5); do
    echo "$i"
done

C-style for loop​

# Syntaxe (( ))
for ((i=0; i<5; i++)); do
    echo "Index: $i"
done

# Avec plusieurs variables
for ((i=0, j=10; i<5; i++, j--)); do
    echo "i=$i, j=$j"
done

Iterating over files​

# Tous les fichiers .txt
for file in *.txt; do
    echo "Fichier: $file"
done

# Fichiers dans un répertoire
for file in /var/log/*; do
    if [ -f "$file" ]; then
        echo "Fichier: $file"
    fi
done

# Avec find (plus robuste)
while IFS= read -r -d '' file; do
    echo "Fichier: $file"
done < <(find /var/log -name "*.log" -print0)

Iterating over the lines of a command​

# Résultat d'une commande
for user in $(cut -d: -f1 /etc/passwd); do
    echo "Utilisateur: $user"
done

# Attention aux espaces !
# Utiliser while read pour les lignes avec espaces

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2 - while loop​

Syntax​

while [ condition ]; do
    # commandes
done

Examples​

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

# Avec [[ ]]
i=0
while [[ $i -lt 10 ]]; do
    echo $i
    ((i++))
done

# Boucle infinie
while true; do
    echo "Appuyez Ctrl+C pour arrêter"
    sleep 1
done

Wait for a condition​

# Attendre qu'un fichier existe
while [ ! -f /tmp/ready ]; do
    echo "En attente..."
    sleep 1
done
echo "Fichier trouvé!"

# Attendre qu'un service réponde
while ! curl -s http://localhost:8080/health > /dev/null; do
    echo "Service non disponible, attente..."
    sleep 5
done
echo "Service UP!"

Interactive menu​

#!/bin/bash

while true; do
    echo ""
    echo "=== Menu ==="
    echo "1. Option 1"
    echo "2. Option 2"
    echo "q. Quitter"
    read -p "Choix: " choice
    
    case $choice in
        1) echo "Option 1 sélectionnée" ;;
        2) echo "Option 2 sélectionnée" ;;
        q|Q) echo "Au revoir!"; break ;;
        *) echo "Choix invalide" ;;
    esac
done

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3 - until loop​

Syntax​

until [ condition ]; do
    # commandes (tant que condition est FAUSSE)
done

until is the opposite of while: it runs as long as the condition is false.

Examples​

# Compteur
count=0
until [ $count -ge 5 ]; do
    echo "Count: $count"
    ((count++))
done

# Attendre qu'un service soit disponible
until curl -s http://localhost:8080 > /dev/null 2>&1; do
    echo "En attente du service..."
    sleep 2
done
echo "Service disponible!"

while vs until comparison​

# Ces deux boucles sont équivalentes:

# while
while [ $x -lt 10 ]; do
    echo $x
    ((x++))
done

# until
until [ $x -ge 10 ]; do
    echo $x
    ((x++))
done

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4 - Flow control: break and continue​

break - Exit the loop​

# Sortir au premier fichier vide
for file in *.txt; do
    if [ ! -s "$file" ]; then
        echo "Fichier vide trouvé: $file"
        break
    fi
done

# break N - sortir de N niveaux
for i in {1..3}; do
    for j in {1..3}; do
        if [ $i -eq 2 ] && [ $j -eq 2 ]; then
            break 2  # Sort des deux boucles
        fi
        echo "i=$i, j=$j"
    done
done

continue - Move to the next iteration​

# Ignorer les fichiers cachés
for file in *; do
    if [[ $file == .* ]]; then
        continue
    fi
    echo "Traitement: $file"
done

# Ignorer les nombres pairs
for i in {1..10}; do
    if (( i % 2 == 0 )); then
        continue
    fi
    echo "Impair: $i"
done

Practical example​

#!/bin/bash
# Traiter des fichiers avec gestion d'erreurs

for file in *.log; do
    # Ignorer si n'existe pas (glob ne match pas)
    [ -f "$file" ] || continue
    
    # Ignorer les fichiers vides
    [ -s "$file" ] || continue
    
    # Traiter
    echo "Traitement de $file..."
    
    # Arrêter en cas d'erreur critique
    if ! process_file "$file"; then
        echo "Erreur critique, arrêt"
        break
    fi
done

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5 - Reading files​

while read - The recommended method​

# Lire ligne par ligne
while IFS= read -r line; do
    echo "Ligne: $line"
done < fichier.txt

Explanation of the options​

OptionDescription
IFS=Preserves leading/trailing spaces
-rDisables backslash escaping

Examples​

# Lire un fichier CSV
while IFS=, read -r nom age ville; do
    echo "Nom: $nom, Age: $age, Ville: $ville"
done < users.csv

# Lire depuis une commande
while IFS= read -r line; do
    echo "Process: $line"
done < <(ps aux)

# Lire depuis un pipe
cat /etc/passwd | while IFS=: read -r user _ uid gid _ home shell; do
    echo "$user (UID:$uid) -> $shell"
done

Watch out for subshells​

# PROBLÈME: variable modifiée dans sous-shell
count=0
cat file.txt | while read line; do
    ((count++))  # Modifié dans sous-shell
done
echo $count  # Affiche 0 !

# SOLUTION: redirection
count=0
while read line; do
    ((count++))
done < file.txt
echo $count  # Affiche le bon compte

Read with line numbers​

linenum=0
while IFS= read -r line; do
    ((linenum++))
    echo "$linenum: $line"
done < fichier.txt

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6 - Practical exercises​

Exercise 1: Multiplication table​

# Afficher la table de multiplication d'un nombre
# donné en argument (de 1 à 10)
Solution
#!/bin/bash
n=${1:-5}
for i in {1..10}; do
    echo "$n x $i = $((n * i))"
done

Exercise 2: Count the lines of every file​

# Parcourir tous les fichiers .sh du répertoire courant
# et afficher le nombre de lignes de chacun
Solution
#!/bin/bash
total=0
for file in *.sh; do
    [ -f "$file" ] || continue
    lines=$(wc -l < "$file")
    echo "$file: $lines lignes"
    ((total += lines))
done
echo "Total: $total lignes"

Exercise 3: Read a CSV file​

# Lire users.csv (nom,age,email) et afficher
# chaque utilisateur de plus de 30 ans
Solution
#!/bin/bash
while IFS=, read -r nom age email; do
    if (( age > 30 )); then
        echo "$nom ($email) a $age ans"
    fi
done < users.csv

Quiz​

Q1. What is the syntax of a for loop over a range of numbers?

Answer

for i in {1..10}; do ... done or for ((i=1; i<=10; i++)); do ... done

Q2. What is the difference between break and continue?

Answer

break exits the loop completely. continue moves to the next iteration.

Q3. How do you read a file line by line while preserving spaces?

Answer

while IFS= read -r line; do ... done < fichier

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Key takeaways​

  • for iterates over lists or sequences
  • while runs as long as the condition is true
  • until runs as long as the condition is false
  • break exits the loop, continue moves to the next iteration
  • Always use IFS= read -r to read files
  • Watch out for subshells with pipes!
  • Use "${array[@]}" with quotes for arrays

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