Lab 02: Shell Scripting — Loops & Iteration

Time: 30 minutes | Level: Practitioner | Docker: docker run -it --rm ubuntu:22.04 bash

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Overview

Loops are the engine of automation. In this lab you'll master every major loop type in Bash: for loops over lists, ranges, globs, and arrays; while and until loops; flow control with break and continue; reading files line-by-line; and nesting loops for matrix operations.

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Step 1: for Loop — Iterating a List

The simplest loop: iterate over a space-separated list of values.

for fruit in apple banana cherry; do
  echo "Fruit: $fruit"
done

📸 Verified Output:

Fruit: apple
Fruit: banana
Fruit: cherry

💡 Tip: The list items can be any whitespace-separated values. If items contain spaces, quote them individually: for item in "two words" "another item".

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Step 2: for Loop — Numeric Range with Brace Expansion

Use {start..end} to generate a numeric sequence without external tools:

for i in {1..5}; do
  echo "Count: $i"
done

📸 Verified Output:

Count: 1
Count: 2
Count: 3
Count: 4
Count: 5

💡 Tip: You can also use a step: {1..10..2} produces 1 3 5 7 9. For dynamic ranges (variables), use seq: for i in $(seq 1 $max).

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Step 3: for Loop — Glob Pattern (Files)

Loops over matching filenames — essential for batch file processing:

mkdir -p /tmp/testdir
touch /tmp/testdir/file1.txt /tmp/testdir/file2.txt /tmp/testdir/notes.txt

for f in /tmp/testdir/*.txt; do
  echo "File: $(basename $f)"
done

📸 Verified Output:

File: file1.txt
File: file2.txt
File: notes.txt

💡 Tip: Always check that the glob matched something. If no files match, $f will literally be /tmp/testdir/*.txt. Guard with: shopt -s nullglob so the loop body is skipped when there are no matches.

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Step 4: for Loop — Array Iteration

Arrays let you store structured lists and iterate cleanly:

colors=("red" "green" "blue")
for color in "${colors[@]}"; do
  echo "Color: $color"
done

📸 Verified Output:

Color: red
Color: green
Color: blue

💡 Tip: Always use "${array[@]}" (double-quoted, @ not *) to iterate arrays safely. Using * joins all elements into one string; @ preserves element boundaries even when elements contain spaces.

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Step 5: while Loop

Runs while a condition is true — ideal when the iteration count isn't known in advance:

count=1
while [ $count -le 5 ]; do
  echo "While: $count"
  ((count++))
done

📸 Verified Output:

While: 1
While: 2
While: 3
While: 4
While: 5

💡 Tip: ((count++)) is shorthand for count=$((count + 1)). The (( )) form also returns exit status 1 when the result is 0 — be careful with set -e (covered in Lab 04).

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Step 6: until Loop

Runs until a condition becomes true — the logical inverse of while:

n=1
until [ $n -gt 3 ]; do
  echo "Until: $n"
  ((n++))
done

📸 Verified Output:

Until: 1
Until: 2
Until: 3

💡 Tip: until [ cond ] is equivalent to while [ ! cond ]. Use whichever reads more naturally. until is less common but can make polling logic read cleanly: until server_is_ready; do sleep 1; done.

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Step 7: break and continue

break exits the loop immediately; continue skips to the next iteration:

for i in {1..10}; do
  [ $i -eq 4 ] && { echo "Skipping 4"; continue; }
  [ $i -eq 7 ] && { echo "Breaking at 7"; break; }
  echo "i=$i"
done

📸 Verified Output:

i=1
i=2
i=3
Skipping 4
i=5
i=6
Breaking at 7

💡 Tip: In nested loops, break 2 exits two levels at once and continue 2 continues the outer loop. This is cleaner than setting a flag variable.

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Step 8: Capstone — Nested Loops and Reading Files

Combine reading a file line-by-line, nested loops, and break/continue in a realistic report generator:

# Setup: create sample data
printf "alice:admin\nbob:user\ncharlie:user\n" > /tmp/users.txt

declare -A PERMS
PERMS[admin]="read write execute"
PERMS[user]="read write"

echo "=== Permission Report ==="
while IFS=: read -r user role; do
  echo ""
  echo "User: $user  Role: $role"
  perms=${PERMS[$role]:-none}
  for perm in $perms; do
    echo "  - $perm"
  done
done < /tmp/users.txt
echo ""
echo "=== End Report ==="

📸 Verified Output:

=== Permission Report ===

User: alice  Role: admin
  - read
  - write
  - execute

User: bob  Role: user
  - read
  - write

User: charlie  Role: user
  - read
  - write

=== End Report ===

💡 Tip: IFS=: before read -r user role splits each line on : — a powerful pattern for parsing colon-separated files like /etc/passwd. Always use -r with read to prevent backslash interpretation.

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Summary

Loop Type	Syntax	Best Used For
for over list	for x in a b c; do ... done	Known set of values
for over range	for i in {1..10}; do ... done	Numeric sequences
for over glob	for f in *.txt; do ... done	Batch file processing
for over array	for x in "${arr[@]}"; do ... done	Structured data sets
while	while [ cond ]; do ... done	Unknown iteration count
until	until [ cond ]; do ... done	Poll until success
break	break / break N	Exit 1 or N loop levels
continue	continue / continue N	Skip to next iteration
Read file lines	while IFS= read -r line; do ... done < file	Line-by-line file parsing
Nested loops	for i in ...; do for j in ...; done; done	Matrix / cross-product ops