[Archive] Linux Commands – What to Check

Not sure what to check when troubleshooting Linux systems? Well, don’t waste countless hours at 3AM troubleshooting when you can have the issue resolved in a timely manner.

Simply put – your time is valuable.

Here is a guide of valuable commands to know when troubleshooting.

What Is Running? #

• pstree -a
  • Returns all running processes in a tree structure
• ps -aux
  • a lists the processes of all users on the system
  • u provides the information in detail
  • x lists processes with no controlling terminal – like daemons

Listening Services #

• netstat -a
  • List all ports with active connections
• netstat -l
  • List all LISTENING ports
• netstat -t
  • List all TCP ports
• netstat -u
  • List all UDP ports
• Combine them:
  • netstat -lt
    • List all LISTENING TCP Ports

CPU and RAM #

• free -m
  • List available memory in MB
  • -g to list available memory in GB
• uptime
  • Display system up time
• top
  • Displays information such as; tasks, memory, cpu and swap
  • q to quit
  • Shift+O to sort
• htop
  • Similar to top, but it’s a beefed up version

Hardware #

• lspci
  • List all PCI devices
• dmidecode
  • Display Hardware and BIOS information
• ethtool
  • Display and change ethernet card settings

IO Performance #

• iostat -x 2
  • Display extended disk I/O stats every 2 seconds
• vmstat 2 10
  • Display virtual memory statistics every 2 seconds up to 10 times
• mpstat 2 10
  • Display CPU statistics every 2 seconds up to 10 times
• dstat --top-io --top-bio
  • Display disk stats

Mount Points and Filesystems #

• mount
  • Mount a file system
• umount
  • Unmount a filesystem
• cat /etc/fstab
  • Display File System Tables
• vgs or vgdisplay
  • Display info about volume groups
• pvs
  • Display physical volume information
• lvs or lvdisplay
  • Display logical volume information
• df -h
  • “disk free,” use it to display disk size
• lsof +D /
  • List opened files under directory

Kernel, Interrupts and Network Usage #

• sysctl -a (use with grep)
  • Display all available kernel configuration parameters
• cat /proc/interrupts
  • Display number of interrupts per IRQ
• cat /proc/net/ip_conntrack (may take some time on busy servers)
  • Linux Netfilter system (iptables firewall) – Allows for advanced filtering for the state of a connection
• ss -s
  • Display sockets summary

System Logs and Kernel Messages #

• dmesg
  • Display message buffer of the kernel
• less /var/log/messages
  • Display system log messages
• less /var/log/secure
  • Display messages related to authentication and authorization. Includes failed login attempts
• less /var/log/auth
  • Display system authorization information

Cronjobs #

• ls /etc/cron* + cat
  • Display all crons
• for user in $(cat /etc/passwd | cut -f1 -d:); do crontab -l -u $user; done
  • Display user specific crons