File Reading

Reading files is one of the most common operations in shell scripting. Whether you're processing logs, parsing configuration files, or transforming data, Bash provides multiple powerful methods for reading file content. In this lesson, you'll master techniques from simple reads to complex field parsing!

Basic File Reading Methods

Bash offers several ways to read file content. The right choice depends on whether you need the entire file, line-by-line processing, or specific portions.

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while IFS= read -r line; do
    echo "$line"
done < file.txt

Reading Parts of Files

Often you don't need the entire file. Use head, tail, and sed to extract specific portions efficiently.

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Parsing Fields from Files

Many files are structured with delimiters (CSV, colon-separated, etc.). Learn to parse these efficiently using IFS, cut, and awk.

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IFS (Internal Field Separator)

# Parse comma-separated values
while IFS=, read -r field1 field2 field3; do
    echo "Fields: $field1, $field2, $field3"
done < data.csv

# Parse colon-separated (like /etc/passwd)
while IFS=: read -r user pass uid gid desc home shell; do
    echo "User $user has shell $shell"
done < /etc/passwd

Reading from Other Sources

Beyond regular files, you can read from command output, URLs, and special files.

# Read from command output (process substitution)
while read -r line; do
    echo "Process: $line"
done < <(ps aux | head -5)

# Read multiple files
for file in *.txt; do
    while read -r line; do
        echo "[$file] $line"
    done < "$file"
done

# Read from here document
while read -r line; do
    echo "Config: $line"
done </dev/null | xxd

Common Mistakes

# Wrong - backslashes are interpreted
while read line; do echo "$line"; done

# Correct - backslashes preserved
while IFS= read -r line; do echo "$line"; done

# Wrong - leading/trailing spaces lost
while read line; do echo "$line"; done

# Correct - IFS= preserves whitespace
while IFS= read -r line; do echo "$line"; done

# Wrong - count stays 0 (subshell)
count=0
cat file.txt | while read line; do ((count++)); done
echo "$count"  # Still 0!

# Correct - no subshell with redirection
count=0
while read -r line; do ((count++)); done < file.txt
echo "$count"  # Correct count

# Wrong - error if file doesn't exist
while read -r line; do echo "$line"; done < missing.txt

# Correct - check first
if [[ -f "file.txt" ]]; then
    while read -r line; do echo "$line"; done < "file.txt"
else
    echo "File not found!"
fi

Exercise: Log File Analyzer

#!/bin/bash
# Log File Analyzer

# Create sample log file
log_file="/tmp/access.log"
cat > "$log_file" << 'EOF'
192.168.1.1 - - [01/Jan/2024] "GET /index.html" 200
192.168.1.2 - - [01/Jan/2024] "GET /error.html" 404
192.168.1.1 - - [01/Jan/2024] "POST /api" 500 Error
10.0.0.1 - - [01/Jan/2024] "GET /about.html" 200
192.168.1.3 - - [01/Jan/2024] "GET /contact" 200
10.0.0.1 - - [01/Jan/2024] "GET /error" 500 Error
EOF

echo "=== Log File Analysis ==="
echo "File: $log_file"
echo ""

# Count total lines
total=$(wc -l < "$log_file")
echo "Total lines: $total"

# Count error lines (case-insensitive)
errors=$(grep -ic "error" "$log_file")
echo "Lines with 'error': $errors"
echo ""

# First and last 3 lines
echo "--- First 3 lines ---"
head -n 3 "$log_file"
echo ""
echo "--- Last 3 lines ---"
tail -n 3 "$log_file"
echo ""

# Unique IPs (first field)
echo "--- Unique IP Addresses ---"
cut -d' ' -f1 "$log_file" | sort -u

# Cleanup
rm -f "$log_file"

Summary

What's Next?

Now that you can read files, let's learn how to write files. You'll discover how to create files, append content, use printf for formatted output, and safely write to files without data loss!