Writing Files

Learn to write data to files using redirection, appending, and here documents. Master safe file writing practices.

Basic Output Redirection

#!/bin/bash

# Overwrite file (creates if doesn't exist)
echo "Hello, World!" > output.txt

# Append to file
echo "Another line" >> output.txt

# Multiple lines
echo "Line 1" > file.txt
echo "Line 2" >> file.txt
echo "Line 3" >> file.txt

Command Output to File

#!/bin/bash

# Save command output
ls -la > listing.txt
date > timestamp.txt

# Save with append
ps aux >> processes.log

# Discard output
command > /dev/null

# Discard errors too
command > /dev/null 2>&1

Exercise: Write to File

Create and append to a file:

Here Documents

Write multi-line content:

#!/bin/bash

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

# With variable expansion
NAME="production"
cat << EOF > settings.txt
Environment: $NAME
Created: $(date)
EOF

# Literal (no expansion) - quote EOF
cat << 'EOF' > script.sh
#!/bin/bash
echo "Hello, $USER"
EOF

Here Strings

For single-line input:

#!/bin/bash

# Here string
cat <<< "This is a here string" > file.txt

# Useful with commands expecting stdin
grep "pattern" <<< "$VARIABLE"

Printf for Formatted Output

#!/bin/bash

# Formatted output to file
printf "Name: %s\nAge: %d\n" "Alice" 30 > person.txt

# With field widths
printf "%-10s %5d\n" "Alice" 95 > scores.txt
printf "%-10s %5d\n" "Bob" 87 >> scores.txt

# Hex, octal, etc.
printf "Decimal: %d, Hex: %x, Octal: %o\n" 255 255 255 > numbers.txt

Exercise: Here Document

Use here document for multi-line:

Writing from Loops

#!/bin/bash

# Redirect entire loop output
for i in {1..5}; do
    echo "Line $i"
done > output.txt

# Append from loop
for file in *.txt; do
    echo "$file: $(wc -l < "$file") lines"
done >> report.txt

# Build content in variable
CONTENT=""
for i in 1 2 3; do
    CONTENT+="Item $i\n"
done
echo -e "$CONTENT" > items.txt

Atomic File Writing

Write safely to avoid corruption:

#!/bin/bash

# Write to temp file, then move
TMPFILE=$(mktemp)
echo "New content" > "$TMPFILE"
mv "$TMPFILE" final.txt

# Or with trap for cleanup
TMPFILE=$(mktemp)
trap 'rm -f "$TMPFILE"' EXIT

cat << EOF > "$TMPFILE"
Configuration data
That could be complex
EOF

# Only move if successful
mv "$TMPFILE" config.txt
trap - EXIT    # Clear trap

Tee: Write and Display

Write to file while also showing output:

#!/bin/bash

# Write to file AND stdout
echo "Hello" | tee output.txt

# Append mode
echo "More text" | tee -a output.txt

# Write to multiple files
echo "Shared" | tee file1.txt file2.txt file3.txt

# Capture command output while viewing
ls -la | tee listing.txt

File Descriptors

Advanced file control:

#!/bin/bash

# Open file for writing (fd 3)
exec 3> output.txt

echo "Line 1" >&3
echo "Line 2" >&3
echo "Line 3" >&3

# Close file descriptor
exec 3>&-

# Append mode
exec 4>> append.txt
echo "Appended" >&4
exec 4>&-

Error Handling

Handle write failures:

#!/bin/bash

OUTPUT_FILE="/path/to/output.txt"

# Check directory exists
PARENT_DIR=$(dirname "$OUTPUT_FILE")
if [[ ! -d "$PARENT_DIR" ]]; then
    mkdir -p "$PARENT_DIR" || {
        echo "Failed to create directory" >&2
        exit 1
    }
fi

# Check file is writable (if exists)
if [[ -e "$OUTPUT_FILE" && ! -w "$OUTPUT_FILE" ]]; then
    echo "File not writable: $OUTPUT_FILE" >&2
    exit 1
fi

# Write with error check
if ! echo "Content" > "$OUTPUT_FILE"; then
    echo "Failed to write file" >&2
    exit 1
fi

echo "Successfully wrote to $OUTPUT_FILE"

Key Takeaways

• > overwrites, >> appends
• Here documents (<< EOF) write multiple lines
• Use printf for formatted output
• Quote 'EOF' to prevent variable expansion
• Use temp file + mv for atomic writes
• tee writes to file and stdout
• Always check write success in critical scripts
• Use file descriptors for advanced control