my-docs/Linux/Bash Script/09-For.md

Bash for Loop Cheat Sheet

A clear and concise reference for using for loops in Bash, with syntax variations, examples, and best practices.

---

1. Basic for Loop (List Iteration)

Iterates over a fixed list of items.

Syntax:

for VARIABLE in ITEM1 ITEM2 ITEM3 ...
do
  commands
DONE
done

Example:

for color in red green blue
do
  echo "Color: $color"
done

Output:

Color: red
Color: green
Color: blue

---

2. Numeric Iteration Using a List

When you explicitly list out numbers.

Example:

for i in 1 2 3 4 5
do
  echo "Number: $i"
done

Output:

Number: 1
Number: 2
Number: 3
Number: 4
Number: 5

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3. C-style for Loop (Arithmetic)

Similar to loops in C-style languages: initialize, condition, increment.

Syntax:

for (( init; condition; increment ))
do
  commands
ndone

Example:

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

Output:

Index: 0
Index: 1
Index: 2
Index: 3
Index: 4

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4. Looping Over Files (Globbing)

Use shell glob patterns to iterate over matching filenames.

Example:

for file in *.txt
do
  echo "Found text file: $file"
done

# If the directory contains a.txt and notes.txt, output might be:
Found text file: a.txt
Found text file: notes.txt

---

Summary

Style	Purpose	Notes
for var in list	Iterate over a static list or glob results	Most common in shell scripting
for (( ... ))	Numeric / arithmetic loops	C-style syntax, powerful for counters