File Writing

Intermediate ~20 min read

Writing files is essential for creating logs, configuration files, reports, and data exports. Bash provides multiple methods for file writing, from simple echo to formatted printf output. You'll also learn safe writing techniques to prevent data loss!

Basic File Writing Methods

The most common ways to write files are using echo, printf, and here documents with output redirection.

#!/bin/bash
# File Writing in Bash

echo "=== Writing Files in Bash ==="
echo ""

# Method 1: echo with redirection
output_file="/tmp/output.txt"
echo "--- Method 1: echo with > (overwrite) ---"
echo "First line" > "$output_file"
echo "Second line" >> "$output_file"  # Append
echo "File contents:"
cat "$output_file"
echo ""

# Method 2: printf for formatted output
echo "--- Method 2: printf (formatted) ---"
printf "Name: %s\nAge: %d\nScore: %.2f\n" "Alice" 25 98.5 > "$output_file"
echo "Formatted output:"
cat "$output_file"
echo ""

# Method 3: Here document
echo "--- Method 3: Here document ---"
cat > "$output_file" << 'EOF'
[Configuration]
server=localhost
port=8080
debug=true
EOF
echo "Config file:"
cat "$output_file"
echo ""

# Method 4: Multiple lines with echo -e
echo "--- Method 4: echo -e (escape sequences) ---"
echo -e "Line 1\nLine 2\nLine 3\tTabbed" > "$output_file"
cat "$output_file"
echo ""

# Method 5: Command output to file
echo "--- Method 5: Command output ---"
date > "$output_file"
echo "Current date saved:"
cat "$output_file"

# Cleanup
rm -f "$output_file"

Output

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Method	Description	Example
`>`	Overwrite file (create if not exists)	`echo "text" > file.txt`
`>>`	Append to file	`echo "text" >> file.txt`
`cat > file << EOF`	Here document (multi-line)	`cat > config.txt << EOF ... EOF`
`printf`	Formatted output	`printf "%s: %d\n" "Count" 5 > file`
`tee`	Write to file AND stdout	`echo "text" \| tee file.txt`

Overwrite vs Append:

> - Creates new file or overwrites existing content (destructive!)
>> - Creates new file or appends to existing content (safe for logs)

Printf for Formatted Output

The printf command offers precise control over output formatting, making it perfect for tables, reports, and structured data.

#!/bin/bash
# Printf for Formatted File Writing

echo "=== Printf Formatting ==="
echo ""

output="/tmp/formatted.txt"

# Basic printf formats
echo "--- Format Specifiers ---"
printf "String: %s\n" "Hello"
printf "Integer: %d\n" 42
printf "Float: %f\n" 3.14159
printf "Float (2 decimals): %.2f\n" 3.14159
printf "Padded: %10s|\n" "right"
printf "Padded: %-10s|\n" "left"
printf "Zero-padded: %05d\n" 42
echo ""

# Creating formatted table
echo "--- Formatted Table ---"
{
    printf "%-15s %8s %10s\n" "Name" "Age" "Salary"
    printf "%-15s %8s %10s\n" "---------------" "--------" "----------"
    printf "%-15s %8d %10.2f\n" "Alice Smith" 28 75000.50
    printf "%-15s %8d %10.2f\n" "Bob Johnson" 35 82500.00
    printf "%-15s %8d %10.2f\n" "Carol White" 42 95000.75
} | tee "$output"
echo ""
echo "Table saved to: $output"
echo ""

# CSV generation
echo "--- CSV Generation ---"
csv_file="/tmp/data.csv"
{
    printf "%s,%s,%s\n" "name" "email" "status"
    printf "%s,%s,%s\n" "alice" "alice@example.com" "active"
    printf "%s,%s,%s\n" "bob" "bob@example.com" "inactive"
    printf "%s,%s,%s\n" "carol" "carol@example.com" "active"
} > "$csv_file"
echo "CSV file:"
cat "$csv_file"
echo ""

# JSON-like output
echo "--- JSON-like Output ---"
name="Test User"
age=30
active=true
printf '{"name": "%s", "age": %d, "active": %s}\n' "$name" "$age" "$active"

# Cleanup
rm -f "$output" "$csv_file"

Output

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Format	Description	Example
`%s`	String	`printf "%s" "hello"`
`%d`	Integer	`printf "%d" 42`
`%f` / `%.2f`	Float / with precision	`printf "%.2f" 3.14159`
`%10s`	Right-padded (10 chars)	`printf "%10s" "hi"`
`%-10s`	Left-padded (10 chars)	`printf "%-10s" "hi"`
`%05d`	Zero-padded integer	`printf "%05d" 42` → `00042`

Pro Tip: Use printf instead of echo when you need consistent behavior across systems. echo behaves differently on various platforms, while printf is standardized.

Safe File Writing Techniques

Production scripts need robust file writing that handles errors and prevents data loss. Learn professional patterns for safe file operations.

#!/bin/bash
# Safe File Writing Techniques

echo "=== Safe File Writing ==="
echo ""

output_file="/tmp/important.txt"
backup_file="/tmp/important.txt.bak"

# Initial content
echo "Original content" > "$output_file"
echo "Initial file:"
cat "$output_file"
echo ""

# Safe write with backup
echo "--- Safe Write with Backup ---"
if [[ -f "$output_file" ]]; then
    cp "$output_file" "$backup_file"
    echo "Backup created: $backup_file"
fi
echo "New content" > "$output_file"
echo "Updated file:"
cat "$output_file"
echo ""

# Atomic write using temp file
echo "--- Atomic Write (temp file) ---"
temp_file=$(mktemp)
echo "Writing to temp file: $temp_file"
{
    echo "Line 1 of atomic write"
    echo "Line 2 of atomic write"
    echo "Line 3 of atomic write"
} > "$temp_file"

# Only move if write succeeded
if [[ $? -eq 0 ]]; then
    mv "$temp_file" "$output_file"
    echo "Atomically moved to: $output_file"
fi
cat "$output_file"
echo ""

# Write with noclobber (prevent overwrite)
echo "--- Noclobber Prevention ---"
set -o noclobber
echo "test" >| "$output_file"  # >| forces overwrite even with noclobber
echo "Used >| to force write with noclobber"
set +o noclobber
echo ""

# Append with timestamp
echo "--- Append with Timestamp ---"
log_file="/tmp/app.log"
echo "[$(date '+%Y-%m-%d %H:%M:%S')] Application started" >> "$log_file"
echo "[$(date '+%Y-%m-%d %H:%M:%S')] Processing data..." >> "$log_file"
echo "[$(date '+%Y-%m-%d %H:%M:%S')] Complete" >> "$log_file"
echo "Log file contents:"
cat "$log_file"

# Cleanup
rm -f "$output_file" "$backup_file" "$log_file"

Output

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Safe Writing Patterns

# 1. Atomic write with temp file
temp=$(mktemp)
echo "content" > "$temp" && mv "$temp" "target.txt"

# 2. Write with backup
cp "file.txt" "file.txt.bak"
echo "new content" > "file.txt"

# 3. Check write success
if echo "data" > "file.txt"; then
    echo "Write succeeded"
else
    echo "Write failed!" >&2
    exit 1
fi

# 4. noclobber - prevent accidental overwrite
set -o noclobber
echo "test" > existing.txt  # Fails if exists
echo "test" >| existing.txt # Force overwrite

# 5. Write to stderr for errors
echo "Error: Something wrong" >&2

Caution: Always be careful with > redirection - it destroys existing file content immediately! Consider using set -o noclobber in interactive shells, or always create backups before overwriting important files.

Multi-line Writing with Here Documents

Here documents are perfect for writing multi-line content like configuration files, scripts, or templates.

# Basic here document
cat > config.txt << 'EOF'
[settings]
debug = true
port = 8080
host = localhost
EOF

# With variable expansion (unquoted EOF)
name="MyApp"
version="1.0"
cat > config.txt << EOF
app_name=$name
app_version=$version
generated=$(date)
EOF

# Append with here document
cat >> log.txt << EOF
[$(date)] New entry added
EOF

# Here string (single line)
cat <<< "Single line content" > file.txt

Quoted vs Unquoted Delimiter:

<< 'EOF' - No variable expansion (literal text)
<< EOF - Variables and commands are expanded

Common Mistakes

1. Accidentally overwriting important files

# Dangerous - overwrites without warning
echo "test" > /etc/important.conf

# Safer - check first
if [[ -f "important.conf" ]]; then
    cp "important.conf" "important.conf.bak"
fi
echo "test" > "important.conf"

2. Forgetting quotes in filenames

# Wrong - fails with spaces in filename
echo "data" > $filename

# Correct - always quote
echo "data" > "$filename"

3. Not checking write permissions

# Wrong - might fail silently
echo "log entry" >> /var/log/app.log

# Correct - check first
if [[ -w "/var/log/app.log" ]]; then
    echo "log entry" >> "/var/log/app.log"
else
    echo "Cannot write to log file" >&2
fi

4. Using echo -e inconsistently

# Inconsistent - echo -e behavior varies by system
echo -e "line1\nline2"

# Consistent - use printf
printf "line1\nline2\n"

# Or use $'...' syntax
echo $'line1\nline2'

Exercise: Log File Generator

Task: Create a script that generates a formatted log file!

Requirements:

Create a log file with a header (app name, generated date)
Add several timestamped log entries
Create a summary section with line count
Use printf for aligned columns

Show Solution

#!/bin/bash
# Log File Generator

log_file="/tmp/app.log"

# Write header
cat > "$log_file" << EOF
========================================
Application Log
Generated: $(date '+%Y-%m-%d %H:%M:%S')
========================================

EOF

# Add timestamped entries using printf
{
    printf "%-20s %-8s %s\n" "TIMESTAMP" "LEVEL" "MESSAGE"
    printf "%-20s %-8s %s\n" "--------------------" "--------" "------------------------"
    printf "%-20s %-8s %s\n" "$(date '+%H:%M:%S.%3N')" "INFO" "Application started"
    printf "%-20s %-8s %s\n" "$(date '+%H:%M:%S.%3N')" "DEBUG" "Loading configuration"
    printf "%-20s %-8s %s\n" "$(date '+%H:%M:%S.%3N')" "INFO" "Connected to database"
    printf "%-20s %-8s %s\n" "$(date '+%H:%M:%S.%3N')" "WARN" "Cache miss detected"
    printf "%-20s %-8s %s\n" "$(date '+%H:%M:%S.%3N')" "INFO" "Request processed"
} >> "$log_file"

# Add summary
{
    echo ""
    echo "========================================"
    echo "Summary"
    echo "Total entries: $(grep -c "^[0-9]" "$log_file")"
    echo "========================================"
} >> "$log_file"

echo "Log file created:"
cat "$log_file"

rm -f "$log_file"

Summary

Overwrite: > creates or overwrites file
Append: >> adds to existing file
printf: Formatted output with precise control (%s, %d, %.2f)
Here Document: Multi-line content with << EOF ... EOF
tee: Write to file AND display (echo | tee file)
Atomic Write: Use temp file then mv for safety
noclobber: set -o noclobber prevents accidental overwrites

What's Next?

Now that you can read and write files, let's learn about File Manipulation. You'll master copying, moving, deleting files, and working with file permissions using cp, mv, rm, chmod, and more!

Previous File Reading

Next File Manipulation

Basic File Writing Methods

Printf for Formatted Output

Safe File Writing Techniques

Safe Writing Patterns

Multi-line Writing with Here Documents

Common Mistakes

1. Accidentally overwriting important files

2. Forgetting quotes in filenames

3. Not checking write permissions

4. Using echo -e inconsistently

Exercise: Log File Generator

Summary

What's Next?

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