Linux Commands Every Architect/Developer Should Know

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Linux Commands Every Architect/Developer Should Know

#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)

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

• Linux.org – for discussion, tutorial, and references
• Linux.com – by Linux Foundation for latest news, articles & updates
• Linux Foundation – community-driven site focused on Opensource
• man7.org – Linux Online Manual
• Linux Commands Cheatsheets
• Explain Shell – Explains each part of the command (useful when trying to understand complex commands used together with parameters)
• nixCraft/cyberciti – for Linux sysadmins
• Linux Today – for latest news on Linux

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