vijay@codes-clouds:~$ ./blog --post linux-commands

Linux Commands Every Software Engineer Must Know

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Vijay Roy Kondaveeti

Vijay Codes & Clouds Linux · DevOps · Engineering April 2026

When I moved into software engineering, Linux stopped being something I occasionally used and became something I lived inside. Every server I've worked on runs Linux. Every deployment, every debug session, every cron job — all of it happens in a terminal.

Most tutorials give you a list of 50 commands and call it a day. That's not useful. What's useful is knowing why a command exists, when you actually reach for it, and what the output is telling you. That's what this post is.

Who this is for: Engineers who are comfortable writing code but want to get faster and more confident inside a Linux terminal — whether for deployments, debugging, or daily development work.

1. Navigating the file system

You spend more time moving around directories than you'd expect. These three are constant.

pwd          # where am I right now?
ls -lah      # list files: permissions, sizes, hidden files
cd /var/log  # change directory

The -lah flags on ls are worth memorising together. -l gives long format, -a shows hidden files, -h makes sizes human-readable.

$ ls -lah /var/log
total 128M
drwxr-xr-x  12 root   root   4.0K Apr  5 09:12 .
-rw-r--r--   1 root   root    84M Apr  8 07:01 syslog
-rw-r-----   1 root   adm    2.1M Apr  8 06:25 auth.log

2. Reading and searching files

cat filename.txt          # print entire file
less filename.txt         # scroll through large file (q to quit)
head -n 20 filename.txt   # first 20 lines
tail -n 50 filename.txt   # last 50 lines
tail -f /var/log/syslog   # follow a log in real time

tail -f is the one you'll use most during debugging. It streams new lines as they're written. Combine it with grep for focused monitoring:

grep "ERROR" /var/log/app.log              # find ERROR lines
grep -i "timeout" /var/log/app.log         # case-insensitive
grep -r "database" /etc/myapp/             # search recursively
tail -f /var/log/app.log | grep "ERROR"  # live error stream only

3. File permissions

This trips up almost every engineer at least once. Here's how to read the permission string:

$ ls -l script.sh
-rwxr-xr-- 1 vijay dev 1.2K Apr 8 10:00 script.sh
# └─ owner:rwx │ group:r-x │ others:r--

Number

Permissions

Use case

7rwx

read + write + execute

Owner of scripts

6rw-

read + write

App config files

5r-x

read + execute

Group scripts

4r--

read only

Public files

chmod 755 script.sh      # rwx for owner, r-x for group/others
chmod +x script.sh       # add execute for everyone
chown vijay:dev file.txt # change owner and group

4. Process management

ps aux                    # all running processes
ps aux | grep python      # filter by name
top                       # live resource usage view
kill 1234                 # send SIGTERM
kill -9 1234             # force kill (SIGKILL)
lsof -i :5432            # what's using PostgreSQL port?
ss -tulnp | grep 5432   # modern alternative to netstat

5. Disk and memory

df -h              # disk usage per filesystem
du -sh /var/log/*  # size of each item in a folder
free -h            # memory: total, used, free, cached

# find what's eating disk space, sorted by size
du -sh /var/log/* | sort -rh | head -10

$ free -h
              total   used   free   buff/cache  available
Mem:           7.7G   3.1G   512M        4.1G       4.2G

6. SSH and remote access

ssh vijay@192.168.1.100             # connect to remote server
ssh -i ~/.ssh/mykey.pem user@host   # use specific key
scp file.txt vijay@host:/tmp/       # copy file to remote
scp vijay@host:/tmp/file.txt .      # copy file from remote

# generate a key pair
ssh-keygen -t ed25519 -C "vijay@myserver"

# copy public key to remote (no more password prompts)
ssh-copy-id vijay@192.168.1.100

7. Finding files

find /etc -name "*.conf"                  # all .conf files
find /home -type f -mtime -7              # modified last 7 days
find /tmp -size +100M                     # files over 100MB
find . -name "*.log" | xargs grep "refused"

8. Cron jobs

crontab -e    # edit your crontab
crontab -l    # list current cron jobs

# Syntax: minute hour day month weekday command
# ┌──── minute (0-59)
# │ ┌── hour   (0-23)
# │ │ ┌─ day   (1-31)
# │ │ │ ┌ month (1-12)
# │ │ │ │ ┌ weekday (0=Sun)
# * * * * * command

30 2 * * *   /home/vijay/scripts/backup.sh          # 2:30am daily
0  0 * * 0   /home/vijay/scripts/cleanup.sh         # Sunday midnight
*/5 * * * *  /home/vijay/scripts/health.sh >> /var/log/health.log 2>&1

9. Piping — where Linux gets powerful

# count ERROR lines in a log
grep "ERROR" /var/log/app.log | wc -l

# top 10 processes by memory
ps aux --sort=-%mem | head -10

# top IPs hitting your nginx server
cat /var/log/nginx/access.log | awk '{print $1}' | sort | uniq -c | sort -rn

# redirect output and errors to a file
./script.sh > output.log 2>&1

10. Putting it all together

A real debugging session on a production server usually follows this sequence:

# 1. SSH in
ssh vijay@prod-server-01

# 2. Check disk and memory first — always
df -h && free -h

# 3. What's using all the CPU?
ps aux --sort=-%cpu | head -15

# 4. Check recent app logs
tail -n 100 /var/log/app/app.log | grep -i "error\|warn\|fail"

# 5. Is the app listening on the right port?
ss -tulnp | grep 8080

# 6. Any system-level issues?
sudo tail -n 50 /var/log/syslog

Disk → Memory → Processes → Logs → Network → System
That order covers 90% of production incidents. Run it before reaching for anything else.

The terminal feels slow and unfamiliar at first. After a few months of daily use it becomes faster than any GUI tool for the same tasks. Learn these until they're automatic — the speed gain is real.

Next post: How Linux file permissions actually work — chmod, chown, and umask explained in depth.

#Linux #SoftwareEngineering #Terminal #DevOps #BashScripting #AWS #VijayCodesAndClouds

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Vijay Roy Kondaveeti

Software engineer working in Python, Linux, AWS and PostgreSQL. Ex-Tech Mahindra.
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