Linux - Common File System Directories

Linux File System Cheat Sheet

cheatsheet linux

Common Directories

• / : Root directory, the top-level of the file system.
• /bin : Essential command binaries for all users (e.g., ls, cp).
• /boot : Boot loader files, including the kernel.
• /dev : Device files, representing hardware components.
• /etc : System configuration files.
• /home : User home directories.
• /lib : Essential shared libraries and kernel modules.
• /media : Mount points for removable media.
• /mnt : Temporary mount point for filesystems.
• /opt : Optional software packages.
• /proc : Virtual filesystem providing process and kernel information.
• /root : Home directory for the root user.
• /run : Runtime variable data.
• /sbin : System binaries for administration (e.g., ifconfig, reboot).
• /srv : Data for services provided by the system.
• /sys : Virtual filesystem for kernel and hardware information.
• /tmp : Temporary files, cleared on reboot.
• /usr : Secondary hierarchy for user utilities and applications.
• /var : Variable data files, like logs and databases.

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/etc - Configuration Details

Path	Purpose
/etc/passwd	User account information
/etc/shadow	Encrypted passwords (root only)
/etc/group	Group definitions
/etc/hosts	Static hostname to IP mappings
/etc/resolv.conf	DNS resolver configuration
/etc/hostname	System hostname
/etc/fstab	Filesystem mount table
/etc/crontab	System-wide cron jobs
/etc/sudoers	Sudo permissions (edit with visudo)
/etc/ssh/sshd_config	SSH server configuration
/etc/ssl/certs/	CA certificates
/etc/systemd/system/	Custom systemd units
/etc/profile	System-wide shell profile
/etc/bashrc or /etc/bash.bashrc	System-wide bash settings
/etc/environment	System-wide environment variables

Network Configuration (varies by distro)

Distro	Path
RHEL/CentOS	/etc/sysconfig/network-scripts/
Debian/Ubuntu	/etc/network/interfaces or /etc/netplan/
All (NetworkManager)	/etc/NetworkManager/

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/var - Variable Data Details

Path	Purpose
/var/log/	Log files
/var/log/syslog	Main system log (Debian/Ubuntu)
/var/log/messages	Main system log (RHEL/CentOS)
/var/log/auth.log	Authentication logs (Debian/Ubuntu)
/var/log/secure	Authentication logs (RHEL/CentOS)
/var/log/dmesg	Kernel ring buffer
/var/log/journal/	systemd journal (binary)
/var/cache/	Application cache files
/var/spool/	Queued data (mail, print, cron)
/var/lib/	Persistent application data
/var/www/	Web server content (common location)
/var/tmp/	Temporary files preserved between reboots

Reading Logs

# Traditional logs
tail -f /var/log/syslog
grep "error" /var/log/syslog
 
# systemd journal
journalctl -f                    # Follow all logs
journalctl -u nginx              # Specific service
journalctl --since "1 hour ago"  # Time-based
journalctl -p err                # By priority

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/usr - User Programs Details

Path	Purpose
/usr/bin/	User command binaries
/usr/sbin/	System admin binaries
/usr/lib/	Libraries for /usr/bin and /usr/sbin
/usr/local/	Locally installed software (manual installs)
/usr/local/bin/	Locally installed binaries
/usr/share/	Architecture-independent data
/usr/share/doc/	Documentation
/usr/share/man/	Manual pages
/usr/include/	C header files
/usr/src/	Source code (kernel sources)

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/proc - Process and Kernel Information

Virtual filesystem - doesn’t exist on disk.

Path	Purpose
/proc/cpuinfo	CPU information
/proc/meminfo	Memory information
/proc/version	Kernel version
/proc/[PID]/	Per-process information
/proc/[PID]/cmdline	Command that started process
/proc/[PID]/fd/	Open file descriptors
/proc/[PID]/environ	Environment variables
/proc/net/	Network statistics
/proc/sys/	Kernel parameters (tunables)

Useful /proc Examples

# CPU info
cat /proc/cpuinfo | grep "model name"
 
# Memory (in KB)
cat /proc/meminfo | head -3
 
# Kernel parameters
cat /proc/sys/net/ipv4/ip_forward
 
# See what a process is doing
ls -la /proc/$(pgrep nginx | head -1)/fd

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/dev - Device Files

Path	Purpose
/dev/null	Discard output
/dev/zero	Source of null bytes
/dev/random	Random bytes (blocking)
/dev/urandom	Random bytes (non-blocking)
/dev/sda	First SCSI/SATA disk
/dev/sda1	First partition on sda
/dev/nvme0n1	First NVMe disk
/dev/tty	Current terminal
/dev/pts/	Pseudo-terminals (SSH sessions)

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Practical Guidance

Finding Configuration Files

# Find all config files for a package
dpkg -L package | grep /etc    # Debian/Ubuntu
rpm -qc package                # RHEL/CentOS

Checking Disk Usage by Directory

du -h --max-depth=1 /var | sort -h

Finding Large Files

find / -type f -size +100M 2>/dev/null

Cleaning Up Disk Space

# Check what's using space in /var
du -sh /var/log/*
du -sh /var/cache/*
 
# Clear old journal logs
journalctl --vacuum-time=7d
 
# Clear package cache
apt clean         # Debian/Ubuntu
yum clean all     # RHEL/CentOS