Linux command line essentials

What you'll do: learn the 20 or so Linux commands that developers and DevOps engineers use every day, then use them to investigate a small web-server log and write a tiny script. Almost every cloud server, Docker container and CI machine runs Linux, so this is the base for the rest of the track.

Prerequisites: none. You need a Linux shell. On Windows, install WSL (open PowerShell as administrator and run wsl --install, then restart). On macOS, open Terminal (the commands are nearly identical). Or use any free online Linux terminal.

Linux has no drive letters. Everything lives in one tree that starts at / (called root). Your personal folder is your home directory, written ~. A path is the address of a file: /home/asha/notes.txt is an absolute path; notes.txt or ../notes.txt are relative to where you are now (. means here, .. means the parent folder).

pwd
ls
ls -la
cd ~
mkdir -p lab/logs
cd lab
pwd

pwd prints the current folder. ls -la lists everything including hidden files (names starting with a dot) with sizes, owners and permissions. mkdir -p creates nested folders and does not complain if they exist.

Servers write a line to a log for every request. Reading logs fast is a core DevOps skill. We will create a small log for the Brew & Bean ordering API and answer questions about it using search, pipes and redirects.

cat > access.log <<'EOF'
10.0.0.5 GET /menu 200
10.0.0.7 POST /orders 201
10.0.0.5 GET /orders/12 200
10.0.0.9 GET /orders/99 404
10.0.0.7 GET /menu 200
10.0.0.5 POST /orders 500
EOF
wc -l access.log

The <<'EOF' ... EOF block (a here-document) writes everything between the markers into access.log. wc -l counts lines.

Search with grep. grep prints lines that match a pattern. Useful flags: -i ignore case, -n show line numbers, -v invert (lines that do NOT match), -c count matching lines, -r search inside all files in a folder.

grep POST access.log
grep -n ' 500' access.log
grep -v ' 200' access.log
grep -c ' 5[0-9][0-9]$' access.log

The last pattern is a regular expression: a space, 5, any two digits, then end of line. It counts server errors (500 to 599).

Pipes and redirects. The pipe | sends the output of one command into the next, so small tools combine into powerful ones. > writes output to a file (replacing it), >> appends. Here is a classic: which client IP sent the most requests?

cut -d' ' -f1 access.log | sort | uniq -c | sort -rn

cut -d' ' -f1 takes the first space-separated column (the IP). sort groups equal lines together, uniq -c counts them, and sort -rn puts the biggest number first.

grep ' 5[0-9][0-9]$' access.log > errors.txt
cat errors.txt
find . -name '*.txt'

Saves only the error lines to errors.txt, then uses find to locate files by name (. means start from here).

Every file has an owner and three sets of permissions (owner, group, others), each made of read, write and execute. ls -l shows them, for example -rwxr-xr--. A script must have the execute bit before you can run it directly. chmod +x file adds it. sudo runs one command as the administrator (root); use it only when you must, for example to install software.

#!/usr/bin/env bash
set -euo pipefail

file="${1:-access.log}"
errors=$(grep -c ' 5[0-9][0-9]$' "$file" || true)
echo "Server errors in $file: $errors"
if [ "$errors" -gt 0 ]; then
  exit 1
fi

The first line says which program runs the script. set -euo pipefail makes the script stop at the first error or unset variable instead of carrying on. ${1:-access.log} means the first argument, or access.log if none was given. exit 1 signals failure; exit code 0 means success. CI tools use exit codes to decide whether a step passed.

Environment variables are how apps get settings like database URLs and secrets without putting them in code. You will use them constantly with Docker and CI. For help on any command, run man grep (press q to leave) or grep --help.

Explain this Linux command piece by piece, in plain English, and tell me if anything in it could delete files, expose secrets or change system settings:

[paste command]

Then suggest a safer way to test it first.

What to expect: Check that every flag is explained and that the answer warns about destructive parts such as rm -rf, chmod -R or > file overwriting. Verify with man before running.