Master Linux System Commands: Essential Tools for Sysadmins

System Information

arch                # Show machine architecture (1)
uname -m            # Show machine architecture (2)
uname -r            # Show running kernel version
 dmidecode -q       # Display hardware components (SMBIOS/DMI)
 hdparm -i /dev/hda # List disk architecture features
 hdparm -tT /dev/sda# Perform test read on disk
 cat /proc/cpuinfo  # Show CPU info
 cat /proc/interrupts# Show interrupts
 cat /proc/meminfo  # Verify memory usage
 cat /proc/swaps    # Show used swap
 cat /proc/version  # Show kernel version
 cat /proc/net/dev # Show network adapters and stats
 cat /proc/mounts   # Show mounted filesystems
 lspci -tv          # List PCI devices
 lsusb -tv          # Show USB devices

date – Display System Date

cal 2007                # Show calendar for 2007
 date 041217002007.00   # Set date/time: MMDDhhmmYY.ss
 clock -w                # Save time to BIOS

Shutdown, Reboot, Logout

shutdown -h now   # Power off (1)
 init 0           # Power off (2)
 telinit 0        # Power off (3)
 shutdown -h 02:00& # Schedule shutdown
 shutdown -c      # Cancel scheduled shutdown
 shutdown -r now  # Reboot (1)
 reboot           # Reboot (2)
 logout           # Logout

Files and Directories

cd /home          # Enter /home directory
 cd ..            # Go up one level
 cd ../..         # Go up two levels
 cd               # Go to home directory
 cd ~user1         # Go to user1's home
 cd -              # Return to previous directory
 pwd               # Show current path

 ls                # List files
 ls -F             # List files with type indicators
 ls -l             # Detailed list
 ls -a             # Show hidden files
 ls *[0-9]*        # Show files containing numbers
 tree              # Tree view of files (1)
 lstree            # Tree view of files (2)

 mkdir dir1        # Create directory dir1
 mkdir dir1 dir2   # Create two directories
 mkdir -p /tmp/dir1/dir2 # Create directory tree
 rm -f file1       # Delete file1
 rmdir dir1        # Remove empty directory
 rm -rf dir1       # Delete directory and its contents
 rm -rf dir1 dir2  # Delete two directories
 mv dir1 new_dir   # Rename/move directory

 cp file1 file2    # Copy file
 cp dir/* .        # Copy all files from dir to current directory
 cp -a /tmp/dir1 . # Copy directory preserving attributes
 cp -a dir1 dir2   # Copy directory

 ln -s file1 lnk1  # Create symbolic link
 ln file1 lnk1      # Create hard link

 touch -t 0712250000 file1 # Change timestamp (YYMMDDhhmm)
 file file1 outputs the mime type of the file as text
 iconv -l           # List known encodings
 iconv -f fromEncoding -t toEncoding inputFile > outputFile

File Search

find / -name file1               # Search from root
 find / -user user1               # Search files owned by user1
 find /home/user1 -name *.bin    # Search .bin files in a directory
 find /usr/bin -type f -atime +100 # Find executables not accessed in 100 days
 find /usr/bin -type f -mtime -10 # Find files created/modified in last 10 days
 find / -name *.rpm -exec chmod 755 '{}' \; # Find .rpm files and set permissions
 find / -xdev -name *.rpm        # Search .rpm files ignoring removable media
 locate *.ps                     # Find .ps files (run updatedb first)
 whereis halt                     # Locate binary, source, man page
 which halt                       # Show full path of executable

Mount a Filesystem

mount /dev/hda2 /mnt/hda2          # Mount partition hda2 (ensure mount point exists)
 umount /dev/hda2                  # Unmount partition hda2
 fuser -km /mnt/hda2               # Force unmount if busy
 umount -n /mnt/hda2               # Unmount without writing /etc/mtab
 mount /dev/fd0 /mnt/floppy        # Mount floppy
 mount /dev/cdrom /mnt/cdrom       # Mount CD/DVD
 mount /dev/hdc /mnt/cdrecorder    # Mount CDRW/DVD
 mount /dev/hdb /mnt/cdrecorder    # Mount another CDRW/DVD
 mount -o loop file.iso /mnt/cdrom # Mount ISO image
 mount -t vfat /dev/hda5 /mnt/hda5 # Mount Windows FAT32 filesystem
 mount /dev/sda1 /mnt/usbdisk      # Mount USB flash drive
 mount -t smbfs -o username=user,password=pass //WinClient/share /mnt/share # Mount Windows network share

Disk Space

df -h               # Show mounted partitions
 ls -lSr | more      # List files/directories sorted by size
 du -sh dir1          # Estimate disk usage of dir1
 du -sk * | sort -rn # List files/directories by size descending
 rpm -qa --qf '%10{SIZE}t%{NAME}n' | sort -k1,1n # Show RPM package sizes (Fedora/RedHat)
 dpkg-query -W -f '${Installed-Size;10}t${Package}n' | sort -k1,1n # Show DEB package sizes (Ubuntu/Debian)

Users and Groups

groupadd group_name   # Create new group
 groupdel group_name   # Delete group
 groupmod -n new_group_name old_group_name # Rename group
 useradd -c "Name Surname" -g admin -d /home/user1 -s /bin/bash user1 # Create user in admin group
 useradd user1        # Create new user
 userdel -r user1     # Delete user and home directory
 usermod -c "User FTP" -g system -d /ftp/user1 -s /bin/nologin user1 # Modify user attributes

 passwd               # Change password
 passwd user1         # Change password for user1 (root only)
 chage -E 2005-12-31 user1 # Set password expiration date
 pwck                  # Verify /etc/passwd syntax
 grpck                 # Verify /etc/group syntax
 newgrp group_name     # Switch to new group for file creation defaults

File Permissions

ls -lh                # Show permissions in human readable form
 chmod ugo+rwx directory1   # Grant read/write/execute to user, group, others
 chmod go-rwx directory1   # Remove rwx from group and others
 chown user1 file1          # Change file owner
 chown -R user1 directory1 # Recursively change owner
 chgrp group1 file1         # Change group ownership
 chown user1:group1 file1   # Change owner and group
 find / -perm -u+s         # List files with SUID bit set
 chmod u+s /bin/file1      # Set SUID on binary
 chmod u-s /bin/file1      # Remove SUID
 chmod g+s /home/public    # Set SGID on directory
 chmod g-s /home/public    # Remove SGID
 chmod o+t /home/public    # Set sticky bit on directory
 chmod o-t /home/public    # Remove sticky bit

Special File Attributes

chattr +a file1   # Append-only attribute
 chattr +c file1   # Enable automatic compression
 chattr +d file1   # Exclude from backup dumps
 chattr +i file1   # Immutable; cannot delete/modify/rename/link
 chattr +s file1   # Secure deletion
 chattr +S file1   # Write changes immediately to disk
 chattr +u file1   # Undelete capability
 lsattr             # List special attributes

Packaging and Compression

bunzip2 file1.bz2   # Decompress .bz2 file
 bzip2 file1        # Compress file with bzip2
 gunzip file1.gz    # Decompress .gz file
 gzip file1         # Compress file with gzip
 gzip -9 file1      # Maximum compression

 rar a file1.rar test_file   # Create RAR archive
 rar a file1.rar file1 file2 dir1 # Add multiple items to RAR
 rar x file1.rar            # Extract RAR archive
 unrar x file1.rar          # Extract RAR archive

 tar -cvf archive.tar file1            # Create uncompressed tarball
 tar -cvf archive.tar file1 file2 dir1 # Create tarball with multiple items
 tar -tf archive.tar                  # List contents of tarball
 tar -xvf archive.tar                  # Extract tarball
 tar -xvf archive.tar -C /tmp          # Extract to /tmp
 tar -cvfj archive.tar.bz2 dir1       # Create bzip2-compressed tarball
 tar -jxvf archive.tar.bz2            # Extract bzip2-compressed tarball
 tar -cvfz archive.tar.gz dir1        # Create gzip-compressed tarball
 tar -zxvf archive.tar.gz             # Extract gzip-compressed tarball

 zip file1.zip file1          # Create zip archive
 zip -r file1.zip file1 file2 dir1 # Recursively add items to zip
 unzip file1.zip              # Extract zip archive

RPM Packages (Fedora/RedHat)

rpm -ivh package.rpm               # Install RPM package
 rpm -ivh --nodeps package.rpm     # Install ignoring dependencies
 rpm -U package.rpm                 # Upgrade without changing config files
 rpm -F package.rpm                 # Refresh already installed package
 rpm -e package_name.rpm           # Remove RPM package
 rpm -qa                           # List all installed RPMs
 rpm -qa | grep httpd              # Find RPMs containing "httpd"
 rpm -qi package_name              # Query detailed info of installed package
 rpm -qg "System Environment/Daemons" # List RPMs in a group
 rpm -ql package_name              # List files provided by installed RPM
 rpm -qc package_name              # List config files of installed RPM
 rpm -q --whatrequires package_name # Show packages that require this RPM
 rpm -q --whatprovides package_name # Show what this RPM provides
 rpm -q --scripts package_name     # Show install/remove scripts
 rpm -q --changelog package_name   # Show changelog
 rpm -qf /etc/httpd/conf/httpd.conf # Find which RPM provides a file
 rpm -qp package.rpm -l            # List files in an uninstalled RPM
 rpm --import /media/cdrom/RPM-GPG-KEY # Import GPG key
 rpm --checksig package.rpm        # Verify RPM integrity
 rpm -qa gpg-pubkey               # List installed GPG keys
 rpm -V package_name               # Verify package files
 rpm -Va                           # Verify all installed packages (use with care)
 rpm -Vp package.rpm               # Verify an uninstalled package
 rpm2cpio package.rpm | cpio --extract --make-directories *bin* # Run executable from RPM
 rpm -ivh /usr/src/redhat/RPMS/`arch`/package.rpm # Install built RPM from source
 rpmbuild --rebuild package_name.src.rpm # Rebuild RPM from source

YUM Package Manager (Fedora/RedHat)

yum install package_name          # Download and install RPM
 yum localinstall package_name.rpm # Install RPM using local repository for dependencies
 yum update package_name.rpm      # Update all installed RPMs
 yum update package_name          # Update a specific RPM
 yum remove package_name          # Remove RPM package
 yum list                         # List all installed packages
 yum search package_name          # Search for packages in repositories
 yum clean packages               # Clean package cache
 yum clean headers                # Remove header files
 yum clean all                    # Remove all cached data

DEB Packages (Debian/Ubuntu)

dpkg -i package.deb               # Install/upgrade DEB package
 dpkg -r package_name             # Remove DEB package
 dpkg -l                         # List installed DEB packages
 dpkg -l | grep httpd           # Find DEB packages containing "httpd"
 dpkg -s package_name            # Show detailed info of installed package
 dpkg -L package_name            # List files provided by installed DEB
 dpkg --contents package.deb      # List files in an uninstalled DEB
 dpkg -S /bin/ping               # Find which DEB provides a file

APT Tools (Debian/Ubuntu)

apt-get install package_name      # Install/upgrade DEB package
 apt-cdrom install package_name   # Install/upgrade from CD-ROM
 apt-get update                   # Update package list
 apt-get upgrade                  # Upgrade all installed packages
 apt-get remove package_name       # Remove DEB package
 apt-get check                    # Verify repository dependencies
 apt-get clean                    # Clean downloaded package cache
 apt-cache search searched-package # Search package names containing a string

Viewing File Contents

cat file1                # View file from start
 tac file1                # View file in reverse order
 more file1               # Paginated view of long file
 less file1               # Paginated view with backward navigation
 head -2 file1            # Show first two lines
 tail -2 file1            # Show last two lines
 tail -f /var/log/messages # Follow file updates in real time

Text Processing

cat file1 file2 ... | command > result.txt   # General PIPE syntax for text manipulation
 cat file1 | grep pattern > result.txt          # Filter and write to file
 cat file1 | grep pattern >> result.txt         # Append filtered output
 grep Aug /var/log/messages                     # Find lines containing "Aug"
 grep ^Aug /var/log/messages                    # Find lines starting with "Aug"
 grep [0-9] /var/log/messages                  # Find lines containing digits
 grep Aug -R /var/log/*                        # Recursive search for "Aug"

 sed 's/stringa1/stringa2/g' example.txt       # Replace stringa1 with stringa2
 sed '/^$/d' example.txt                        # Delete empty lines
 sed '/ *#/d; /^$/d' example.txt               # Delete comments and empty lines
 echo 'esempio' | tr '[:lower:]' '[:upper:]'    # Convert to uppercase
 sed -e '1d' result.txt                         # Delete first line
 sed -n '/stringa1/p' example.txt               # Print lines containing stringa1
 sed -e 's/ *$//' example.txt                    # Trim trailing spaces
 sed -e 's/stringa1//g' example.txt            # Remove stringa1
 sed -n '1,5p;5q' example.txt                  # Print lines 1-5
 sed -n '5p;5q' example.txt                    # Print line 5
 sed -e 's/00*/0/g' example.txt                 # Collapse multiple zeros to single zero

 cat -n file1               # Number lines
 cat example.txt | awk 'NR%2==1'   # Print odd-numbered lines
 echo a b c | awk '{print $1}'    # Print first field
 echo a b c | awk '{print $1,$3}' # Print first and third fields
 paste file1 file2                # Merge files column-wise
 paste -d '+' file1 file2         # Merge with '+' delimiter

 sort file1 file2                 # Sort combined files
 sort file1 file2 | uniq          # Unique lines (union)
 sort file1 file2 | uniq -u      # Lines only in one file (difference)
 sort file1 file2 | uniq -d      # Lines common to both files (intersection)

 comm -1 file1 file2   # Show lines only in file2
 comm -2 file1 file2   # Show lines only in file1
 comm -3 file1 file2   # Show common lines

Character Set Conversion and File Format

dos2unix filedos.txt fileunix.txt   # Convert DOS to UNIX format
 unix2dos fileunix.txt filedos.txt   # Convert UNIX to DOS format
 recode ..HTML < page.txt > page.html # Convert text to HTML
 recode -l | more                  # List supported conversions

Filesystem Analysis

badblocks -v /dev/hda1   # Check for bad blocks
 fsck /dev/hda1           # Check/repair filesystem
 fsck.ext2 /dev/hda1     # Check/repair ext2 filesystem
 e2fsck /dev/hda1        # Check/repair ext2 filesystem
 e2fsck -j /dev/hda1     # Check/repair ext3 filesystem
 fsck.ext3 /dev/hda1     # Check/repair ext3 filesystem
 fsck.vfat /dev/hda1      # Check/repair FAT filesystem
 fsck.msdos /dev/hda1     # Check/repair DOS filesystem
 dosfsck /dev/hda1        # Check/repair DOS filesystem

Initialize a Filesystem

mkfs /dev/hda1                # Create filesystem on partition
 mke2fs /dev/hda1             # Create ext2 filesystem
 mke2fs -j /dev/hda1          # Create ext3 (journaled) filesystem
 mkfs -t vfat 32 -F /dev/hda1 # Create FAT32 filesystem
 fdformat -n /dev/fd0         # Format floppy disk
 mkswap /dev/hda3             # Create swap area

SWAP Filesystem

mkswap /dev/hda3            # Create swap area
 swapon /dev/hda3           # Enable swap
 swapon /dev/hda2 /dev/hdb3 # Enable multiple swap partitions

Backup

dump -0aj -f /tmp/home0.bak /home   # Full backup of /home
 dump -1aj -f /tmp/home0.bak /home   # Interactive backup of /home
 restore -if /tmp/home0.bak           # Restore interactive backup

 rsync -rogpav --delete /home /tmp    # Synchronize directories
 rsync -rogpav -e ssh --delete /home user@host:/tmp   # Sync over SSH
 rsync -az -e ssh --delete user@host:/home/public /home/local   # Pull remote directory
 rsync -az -e ssh --delete /home/local host:/home/public   # Push local directory

 dd bs=1M if=/dev/hda | gzip | ssh user@host 'dd of=hda.gz'   # Remote backup via SSH
 dd if=/dev/sda of=/tmp/file1                               # Backup disk to file
 tar -Puf backup.tar /home/user                               # Incremental backup
 (cd /tmp/local && tar c .) | ssh -C user@host 'cd /home/share && tar x -p' # Remote copy via SSH
 (tar c /home) | ssh -C user@host 'cd /home/backup-home && tar x -p' # Remote copy of directory
 tar cf - . | (cd /tmp/backup && tar xf -)                     # Local copy preserving permissions

 find /home/user1 -name '*.txt' | xargs cp -av --target-directory=/home/backup/ --parents # Copy .txt files preserving hierarchy
 find /var/log -name '*.log' | tar cv --files-from=- | bzip2 > log.tar.bz2 # Archive logs

 dd if=/dev/hda of=/dev/fd0 bs=512 count=1   # Copy MBR to floppy
 dd if=/dev/fd0 of=/dev/hda bs=512 count=1   # Restore MBR from floppy

Optical Discs

cdrecord -v gracetime=2 dev=/dev/cdrom -eject -blank fast -force   # Erase rewritable CD
 mkisofs /dev/cdrom > cd.iso                                 # Create ISO image from CD
 mkisofs /dev/cdrom | gzip > cd_iso.gz                       # Create compressed ISO image
 mkisofs -J -allow-leading-dots -R -V "Label CD" -iso-level 4 -o ./cd.iso data_cd # Create ISO from directory
 cdrecord -v dev=/dev/cdrom cd.iso                           # Burn ISO to CD
 gzip -dc cd_iso.gz | cdrecord dev=/dev/cdrom -            # Burn compressed ISO
 mount -o loop cd.iso /mnt/iso                               # Mount ISO image

 cd-paranoia -B                                            # Extract audio tracks to WAV
 cd-paranoia -- "-3"                                      # Extract with parameter -3
 cdrecord --scanbus                                        # Scan SCSI bus
 dd if=/dev/hdc | md5sum                                   # Compute MD5 of CD device

Network – Ethernet and Wi‑Fi

ifconfig eth0               # Show Ethernet configuration
 ifup eth0                  # Enable eth0
 ifdown eth0                # Disable eth0
 ifconfig eth0 192.168.1.1 netmask 255.255.255.0 # Set IP address
 ifconfig eth0 promisc      # Enable promiscuous mode for sniffing
 dhclient eth0              # Obtain IP via DHCP

 route -n                  # Show routing table
 route add -net 0/0 gw IP_Gateway   # Set default gateway
 route add -net 192.168.0.0 netmask 255.255.0.0 gw 192.168.1.1 # Add static route
 route del 0/0 gw IP_gateway          # Delete static route

 hostname                    # Show host name
 host www.example.com        # Resolve host name to IP
 nslookup www.example.com   # DNS query for troubleshooting
 ip link show                # Show network interfaces
 mii-tool                    # Show/manage media interface status
 ethtool                     # Query and set NIC parameters
 netstat -tupl               # Show TCP/UDP socket status
 tcpdump tcp port 80         # Capture HTTP traffic

JPS Tool

jps                         # List Java processes (PID and class name)
 jps -q                      # Show only PIDs
 jps -m                      # Show arguments passed to main method
 jps -l                      # Show full package name or JAR path
 jps -v                      # Show JVM arguments
 jps 192.168.0.77            # List Java processes on remote host via RMI (default port 1099)