================ Operating System ================ .. meta:: :description: Linux/Unix command-line reference covering date manipulation, file searching with find, system information, process management, and file operations. :keywords: Linux commands, Unix utilities, find command, date command, system administration, process management, bash utilities .. contents:: Table of Contents :backlinks: none From date manipulation to process control, mastering command-line utilities is essential for effective system administration. Here you'll find practical examples for everyday tasks: parsing dates, searching files with find, managing processes, and handling compression—all organized for quick reference. An interactive demo script is available at `src/bash/os.sh `_ to help you experiment with the concepts covered in this cheatsheet. .. code-block:: bash ./src/bash/os.sh # Run all demos ./src/bash/os.sh date # Run date/time demo ./src/bash/os.sh find # Run find command demo ./src/bash/os.sh --help # Show all available sections Date and Time ============= The ``date`` command displays and manipulates system date and time. Format strings control output, while GNU and BSD versions differ in syntax for date arithmetic. Current Date and Time --------------------- .. code-block:: bash date # Full date and time date +"%Y-%m-%d" # 2024-01-15 date +"%Y%m%d" # 20240115 date +"%H:%M:%S" # 14:30:45 date +"%Y-%m-%d %H:%M:%S" # 2024-01-15 14:30:45 date -Iseconds # ISO 8601 format with timezone Common Format Specifiers ------------------------ .. code-block:: bash %Y # Year (2024) %m # Month (01-12) %d # Day (01-31) %H # Hour 24h (00-23) %M # Minute (00-59) %S # Second (00-59) %s # Unix timestamp %A # Weekday name (Monday) %B # Month name (January) %Z # Timezone (PST) Date Arithmetic --------------- .. code-block:: bash # Linux (GNU date) date +%Y-%m-%d -d "1 day ago" # Yesterday date +%Y-%m-%d -d "7 days ago" # 7 days ago date +%Y-%m-%d -d "1 month ago" # 1 month ago date +%Y-%m-%d -d "next monday" # Next Monday date +%Y-%m-%d -d "2024-01-15 + 10 days" # BSD/macOS date -j -v-1d +"%Y-%m-%d" # Yesterday date -j -v-7d +"%Y-%m-%d" # 7 days ago date -j -v-1m +"%Y-%m-%d" # 1 month ago date -j -v+10d +"%Y-%m-%d" # 10 days from now Convert Timestamps ------------------ .. code-block:: bash # Unix timestamp to date (Linux) date -d @1705334400 # Unix timestamp to date (BSD/macOS) date -r 1705334400 # Current timestamp date +%s File Searching with find ======================== The ``find`` command recursively searches directories for files matching specified criteria. It supports filtering by name, type, size, time, ownership, and permissions, with actions to execute on matched files. Basic Patterns -------------- .. code-block:: bash find /path -name "*.py" # By extension find /path -name "*test*" # By substring find /path -iname "*.PY" # Case insensitive find /path -path "**/test/*" # By path pattern File Types ---------- .. code-block:: bash find /path -type f # Regular files find /path -type d # Directories find /path -type l # Symbolic links find /path -type f -name "*.py" # Combine filters # Type codes: f=file, d=directory, l=symlink, # b=block, c=character, p=pipe, s=socket By Size ------- .. code-block:: bash find /path -type f -size +100M # Larger than 100MB find /path -type f -size -1M # Smaller than 1MB find /path -type f -size 50M # Exactly 50MB # Size units: c=bytes, k=KB, M=MB, G=GB By Time ------- .. code-block:: bash # Access time find /path -type f -atime +7 # Accessed > 7 days ago find /path -type f -atime -7 # Accessed < 7 days ago find /path -type f -amin +60 # Accessed > 60 min ago # Modification time find /path -type f -mtime +30 # Modified > 30 days ago find /path -type f -mmin -10 # Modified < 10 min ago # Change time (metadata) find /path -type f -ctime +7 # Changed > 7 days ago # Newer than file find /path -type f -newer ref.txt By Ownership and Permissions ---------------------------- .. code-block:: bash find /path -type f -user root # Owned by root find /path -type f -group admin # Group admin find /path -type f -perm 644 # Exact permissions find /path -type f -perm -644 # At least these permissions find /path -type f -perm /u+x # User executable Combining Conditions -------------------- .. code-block:: bash # AND (implicit) find /path -type f -name "*.log" -size +1M # OR find /path \( -name "*.py" -o -name "*.sh" \) # NOT find /path -type f ! -name "*.txt" # Complex find /path -type f \( -name "*.py" -o -name "*.sh" \) ! -path "*/venv/*" Actions ------- .. code-block:: bash # Delete matched files find /path -type f -name "*.tmp" -delete # Execute command for each file find /path -type f -name "*.sh" -exec chmod +x {} \; # Execute command with all files find /path -type f -name "*.txt" -exec grep -l "pattern" {} + # Delete directories recursively find /path -type d -name "__pycache__" -exec rm -rf {} + # Print with null delimiter (for xargs) find /path -type f -print0 | xargs -0 ls -la Loop Through Results -------------------- .. code-block:: bash # Using while loop (handles spaces in filenames) find /path -type f -name "*.txt" -print0 | while IFS= read -r -d '' file; do echo "Processing: $file" done # Process substitution (preserves variables) count=0 while IFS= read -r -d '' file; do ((count++)) done < <(find /path -type f -print0) echo "Found: $count files" # Sort results find /path -name "*.txt" -print0 | sort -z | xargs -r0 -I{} echo "{}" System Information ================== Commands for gathering system software and configuration information. System Information ------------------ .. code-block:: bash uname -a # All system info uname -s # Kernel name uname -r # Kernel release uname -m # Machine architecture hostname # System hostname uptime # System uptime cat /etc/os-release # OS info (Linux) sw_vers # macOS version Process Management ================== Commands for monitoring and controlling system processes. Viewing Processes ----------------- .. code-block:: bash ps aux # All processes ps aux | grep nginx # Filter by name ps -ef # Full format listing ps -p PID -o pid,ppid,cmd,%cpu,%mem # Specific process details pgrep -l nginx # Find PID by name pstree # Process tree top # Interactive process viewer htop # Enhanced top (if installed) Process Control --------------- .. code-block:: bash kill PID # Terminate (SIGTERM) kill -9 PID # Force kill (SIGKILL) kill -HUP PID # Reload config (SIGHUP) killall nginx # Kill by name pkill -f "python script.py" # Kill by pattern Background Jobs --------------- .. code-block:: bash command & # Run in background jobs # List background jobs fg %1 # Bring job 1 to foreground bg %1 # Resume job 1 in background disown %1 # Detach job from shell nohup command & # Run immune to hangups File Operations =============== Commands for file manipulation, compression, and text processing. File Management --------------- .. code-block:: bash cp -r src dest # Copy recursively mv old new # Move/rename rm -rf dir # Remove recursively mkdir -p a/b/c # Create nested dirs ln -s target link # Symbolic link chmod 755 file # Change permissions chown user:group file # Change ownership Compression ----------- .. code-block:: bash # tar tar -cvf archive.tar dir/ # Create tar tar -xvf archive.tar # Extract tar tar -czvf archive.tar.gz dir/ # Create gzipped tar tar -xzvf archive.tar.gz # Extract gzipped tar tar -cjvf archive.tar.bz2 dir/ # Create bz2 tar tar -tzvf archive.tar.gz # List contents # zip zip -r archive.zip dir/ # Create zip unzip archive.zip # Extract zip unzip -l archive.zip # List contents # gzip gzip file # Compress (replaces file) gunzip file.gz # Decompress zcat file.gz # View without extracting Viewing Files ------------- .. code-block:: bash cat file # Print entire file head -n 20 file # First 20 lines tail -n 20 file # Last 20 lines tail -f file # Follow file updates less file # Paginated view wc -l file # Line count wc -w file # Word count File Comparison --------------- .. code-block:: bash diff file1 file2 # Line differences diff -u file1 file2 # Unified format diff -r dir1 dir2 # Compare directories cmp file1 file2 # Byte comparison comm file1 file2 # Common/unique lines (sorted) md5sum file # MD5 checksum sha256sum file # SHA256 checksum Environment and Shell ===================== Commands for managing shell environment and configuration. Environment Variables --------------------- .. code-block:: bash env # All environment variables printenv PATH # Specific variable export VAR=value # Set variable unset VAR # Remove variable echo $PATH # Print variable Command Information ------------------- .. code-block:: bash which command # Command location type command # Command type whereis command # Binary/man/source locations man command # Manual page command --help # Help text alias # List aliases alias ll='ls -la' # Create alias History ------- .. code-block:: bash history # Command history history | grep pattern # Search history !n # Execute command n !! # Repeat last command !$ # Last argument of previous command ctrl+r # Reverse search