Linux
Ankur Kumar
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Linux is pervasive in software deployment architecture, and being a hands-on technologist with Linux helps in various ways. This article summarizes most of the commonly used Linux commands & utilities every architect should know. As each variant or distribution of Linux might have differences, there might be some variation as per Linux variants: Debian Linux, RHEL & CentOS, Oracle Linux, Ubuntu Linux, Alpine Linux, and Arch Linux.
Before we start, use the below command to check Linux Kernel details:

 cat /etc/os-release* 
 uname -a
(* - except CentOS, which needs /etc/centos-release)

For simplicity, Linux commands or utilities have been grouped together into 4 areas with top 5 commands along with popular params for convenience:

#1 – Process Monitoring

  • top – To view CPU & memory usage information. General guidance is Load Average should be similar to the number of cores for the system not under stress.You can also use newer version htop command for more interactivity.
    Usage Example:
top -u <user> #user specific processes
Press M: sort by memory, P: sort process list by CPU usage, V: Forest view, R: for reversing the order
htop -u <user> #user specific processes
  • ps – Classic command to view list of running processes – Linux variants have different parameter options. Below examples are the most useful:
ps auwwx #list all the processes with wider format & easy to remember
ps -ef | grep 'java' #to filter specific process
ps -ef | awk '{print $1}' | sed '/UID/d' | sort | uniq -c | sort -nr
#count running process under each user
  • kill – Classic command for killing a running process. killall is useful to kill all the processes by a name.
kill -l #to list all available signals
kill -9 <pid> #SIGKILL signal to kill the process
killall top #kill all top processes; you can put any process name
  • lsof– List of Open Files by the process – very useful when debugging resources consumed by a process particularly when everything in Linux is a file. Note that it might not be installed by default in some variants (e.g. CentOS) and you need to install “lsof” package.
lsof -u <username>  #list of files opened by a user
lsof -c <process> #list of files opened by a process named
lsof -p <pid> #list of files opened by PID
lsof -i #files opened by network
lsof -i :443 #to find the process/service listening on a port
  • fuser – shows the PIDs of processes using the specified files or file systems in Linux.
    You need to install “psmisc” package before using this command – see installation section below for instruction as per Linux variant.
fuser 80/tcp #find the process/service listening on a particular port by running the command 
fuser <filename> #to find out file being used by the

#2 – File Operations

  • tail – for viewing the last part of files/logs, most commonly used parameter is “tail -f”
  • find – for searching the files. Commonly usage examples – click here:
find . -name "*.log" #to find the log files in current directory & sub-directory 
find . -type f -name "*.java"  #to find all java file types
find . -type f -perm 0777 -print #to find files with 777 permissions
find / -type f -perm 0777 -print -exec chmod 555 {} \; #to find files with 777 permissions and replace with 555
find / -size +50M -size -100M #to find files between 50-100MB size
  • grep – for searching pattern within file. Click here for different examples.
grep -i "linux" *.log #case insensitive search within log files
grep "REGEX" *.log #use any REGEX
grep -w "word" *.log #check for full word
grep -c "word" *.log #count the number of words matched
grep -v "word" *.log #invert the match to display non-matching
grep -r "word" *.log #to search recursively in all folders
grep -A 5 -i "word" *.log #display 5 lines after the match
  • Others: uniq sort diff cut ncat sed awk


#3 – Network Monitoring

  • tcpdump – for analyzing network packet level details. Useful for packet-level inspection, detecting denial of service attacks by inspecting large packets or source, debug the source & destination generating traffic, etc.- tcpdump not port 22 and not port 25 (exclude SSH to avoid unnecessary info)
    Usage examples:
tcpdump udp #for capturing specific protocol dump; find list of protocols in /etc/protocols)
tcpdump -c10 -i eth1 -n -A port 80 #dumps in ASCII format with (-A) with specific port and exit after receiving 10 packets (-c))
tcpdump -l | tee dat #Make stdout line buffered. Useful if you want to see the data while capturing it
tcpdump -i eth0 host 10.122.19.222 #capture for specific IP address
tcpdump -s 0 -A -vv 'tcp[((tcp[12:1] & 0xf0) >> 2):4] = 0x47455420'  #capture only HTTP GET or POST only
  1. Note: If not installed, install “tcpdump” package (e.g. for Alpine Linux: apk add tcpdump). Click here to read the troubleshooting article by RedHat and see 20 different examples by clicking here.
  • netstat – Print network connections, routing tables, interface statistics, masquerade connections, and multicast memberships
  • traceroute – print the route packets trace to network host
  • ss – utility to investigate sockets (very useful to check socket stats)
  • iperf – utility to check network bandwidth between two systems. Click here to read the most useful link found for iperf.
  • Others: strace, mtr, dig, sar, ifconfig (prints ip address)

#4 – Disk & Memory Monitoring

  • du / df – du (disk usage of the set of FILEs, recursively for directories), df (report file system disk space usage)
  • free – to find out free and used memory
  • iostat – for CPU stats and I/O stats for devices & partitions
  • mount – to mount a file system
  • fdisk – to manipulate disk partition

Installing Package

Below commands are example of installing packages for different variations:

$ yum install <package> # RHEL/CentOS 
$ apt install <package>	# Debian/Ubuntu
$ apk add <package>     # Alpine Linux
$ dnf install <package> # Fedora

Other Useful Commands

  • alias (for bash) – useful for creating shortcuts to frequently used commands
  • tree – shows a visual representation of the files in a directory
  • watch – run any command at regular intervals and displays the output
  • truncate – shrink or extend the size of a file to the specified size. For example, the below command will free up capacity quickly:
truncate -s 0 filename
  • Security:
    • Check Certificate Expiry and other details using openssl command
openssl x509 -in my-cert.pem -noout -issuer -subject -dates
  • Top 10 commands from bash history:
cat ~/.bash_history | sort |uniq -c|sort -nr|head -n 10
  • systemctl– to control the running system and other services
  • Miscellaneous: mpstat, pmap, kill, iostat, vmstat, chkconfig, uptime (shows how long system is running), pidof, cal (shows calendar), zip, unzip, ssh, whatis (locate the binary), whereis (one-line man page), finger (short dump of info about a user), w (current user info), chown (change ownership of file), chmod (change permission of files & directories), locate (to find a file), ping (to check connectivity),nl (cat with line numbers)

To conclude, these commands/utilities help not only to troubleshoot but to understand the functions of Linux operating system. Even though you are using application performance monitoring (APM) tools, as a software architect you should know these expand the full-stack knowledge.

Linux Useful Websites

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