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File Writing: Create and Modify Files

Scripts need to write files. Create logs. Generate configs. Save data.

Here's the thing: Writing files is common. Learn the methods. Use them.

Basic Writing​

Overwrite​

echo "content" > file.txt

The >: Overwrites file. Creates if doesn't exist.

My take: Use > to create new files or overwrite existing.

Append​

echo "more content" >> file.txt

The >>: Appends to file. Creates if doesn't exist.

My take: Use >> to add to existing files. Logs. Output.

Multiple Lines​

Heredoc​

cat > file.txt << EOF
Line 1
Line 2
Line 3
EOF

My take: Heredoc is useful for multi-line content. Configs. Templates.

printf​

printf "Name: %s\nAge: %d\n" "John" 25 > file.txt

My take: printf formats output. Useful for structured data.

tee: Write and Display​

echo "content" | tee file.txt    # Write and display
echo "content" | tee -a file.txt  # Append and display

My take: tee writes and shows output. Useful for logging with visibility.

Common Patterns​

Logging​

echo "[$(date)] Message" >> log.txt

My take: Append to logs. Don't overwrite. Use >>.

Generate Config​

cat > config.txt << EOF
server=$SERVER
port=$PORT
EOF

My take: Generate configs from variables. Useful for automation.

Write with Error Handling​

if ! echo "content" > file.txt; then
    echo "Error: Cannot write to file.txt"
    exit 1
fi

My take: Check if write succeeds. Handle errors.

Common Mistakes (I've Made These)​

  1. Using > instead of >>: Overwrites logs. Use >> for logs.

  2. Not checking if write succeeds: Writes can fail. Check them.

  3. Not quoting content: Spaces break things. Quote content.

  4. Writing to wrong location: Check path. Make sure directory exists.

  5. Not handling permissions: Files might not be writable. Check permissions.

Real-World Examples​

Log Messages​

log() {
    echo "[$(date '+%Y-%m-%d %H:%M:%S')] $*" >> /var/log/app.log
}

log "Application started"
log "Processing file: $file"

Generate Config​

cat > /etc/app/config.conf << EOF
# Generated on $(date)
SERVER=$SERVER
PORT=$PORT
DEBUG=$DEBUG
EOF

Write Backup Script​

backup_file() {
    local file="$1"
    if [ -f "$file" ]; then
        cp "$file" "${file}.backup"
        echo "Backed up: $file" >> backup.log
    fi
}

What's Next?​

Now that you can write files, scripts can create output. Or learn about Error Handling to handle failures.

Personal note: When I started, I'd overwrite logs constantly. Then I learned >>. Now I append. Logs are preserved. Small detail. Big difference.