Essential Bash Commands

# File operations
ls -la                    # List all files (long format)
cp source dest            # Copy file
mv old new                # Move/rename
rm file                   # Remove file
rm -rf directory          # Remove directory (DANGEROUS!)
mkdir -p path/to/dir     # Create directories
touch file                # Create empty file

# File viewing
cat file                  # Print entire file
head -20 file             # First 20 lines
tail -20 file             # Last 20 lines
tail -f /var/log/syslog   # Follow log file in real time
less file                 # Interactive viewer (q to quit)

# Searching
find / -name "*.log"      # Find files by name
grep -r "error" /var/log  # Search file contents
grep -i "warning" file    # Case-insensitive search
grep -n "TODO" *.ts       # Show line numbers

# Pipe: output of one command becomes input to next
cat access.log | grep "404" | wc -l     # Count 404 errors

# Redirect stdout
echo "hello" > file.txt        # Overwrite file
echo "more" >> file.txt        # Append to file

# Redirect stderr
command 2> errors.log           # Errors to file
command 2>&1                    # Merge stderr with stdout
command > all.log 2>&1          # Both stdout and stderr

# Redirect both
command &> all.log              # Same as above (bash shorthand)

# /dev/null — discard output
command > /dev/null 2>&1        # Silence everything

# cut — extract columns
cat data.csv | cut -d',' -f1,3         # Columns 1 and 3 (comma-delimited)

# sort
cat names.txt | sort                    # Alphabetical
cat names.txt | sort -r                 # Reverse
cat data.csv | sort -t',' -k2 -n       # Sort by column 2, numeric

# uniq — remove duplicates (input must be sorted)
cat file.txt | sort | uniq -c          # Count occurrences

# awk — field processing
cat data.csv | awk -F',' '{print $1, $3}'        # Print columns 1 and 3
cat access.log | awk '$9 == 404'                   # Lines where status is 404
cat access.log | awk '{sum += $10} END {print sum}'  # Sum of column 10

# sed — stream editor (find/replace)
sed 's/old/new/g' file.txt             # Replace in output
sed -i 's/old/new/g' file.txt          # Replace in-place (modify file)
sed -n '10,20p' file.txt               # Print lines 10-20
sed '/^#/d' config.txt                 # Remove comment lines

# jq — JSON processing
cat data.json | jq '.users[] | {name, email}'
cat data.json | jq '.items | length'
cat data.json | jq '.users[0].name'

# Count most frequent IPs in access log
cat access.log | awk '{print $1}' | sort | uniq -c | sort -rn | head -10

# Find large files
find / -type f -size +100M -exec ls -lh {} \; | sort -k5 -hr

# Monitor log errors in real time
tail -f app.log | grep --color=auto "ERROR\|WARN"

# Parse CSV and calculate average
awk -F',' '{sum += $3; count++} END {print "Average:", sum/count}' data.csv

# Remove duplicate lines preserving order
awk '!seen[$0]++' file.txt