Linux Basic — Reading & Writing Files

Reading & Writing Files

Almost every shell script reads input from somewhere and writes output somewhere else. Doing this safely — without corrupting data, losing newlines, or being broken by spaces in filenames — is a foundational scripting skill that many beginners get wrong.

This chapter shows the correct, modern, safe patterns for I/O in Bash.

---

What You Will Learn

By the end of this chapter you will be able to:

• Read files line-by-line correctly using while read
• Slurp a file into an array with mapfile/readarray
• Use printf instead of echo for portable, predictable output
• Perform atomic writes that never leave partial files
• Append to files safely from multiple writers using flock
• Use tee to write to a file and stdout at the same time
• Create race-free temporary files with mktemp and clean them up with trap

---

Topics in This Chapter

#	Topic	Description
1	Reading Files	The right way to read line-by-line
2	Writing Files	>, >>, printf, atomic writes
3	Appending to Files	Safe appends and flock
4	Writing with tee	Fan-out and sudo writes
5	Safe Temporary Files	mktemp + trap cleanup
6	Practice Exercises	Hands-on tasks

---

Why This Matters

A real-world example you will write hundreds of times:

# ❌ The classic broken pattern — beginners write this every day
for line in $(cat users.txt); do
    echo "Processing $line"
done
# Breaks on: spaces, tabs, wildcards (* expands!), empty lines

# ✅ The correct pattern
while IFS= read -r line; do
    echo "Processing $line"
done < users.txt
# Survives spaces, special chars, and never glob-expands

The difference is the line between a script that works on your laptop and one that silently corrupts production data.

---

Next → Reading Files