Topics
Security
Data formats
Text
Linux and network
Encoding and conversion
Where to start
First steps
15 stages · From system basics and command input to navigation, files, text, and pipelines.
Track map 11 tracks · 123 stages
Difficulty
New labs
Sections 183 articles
Command index 432
Popular
Web stack
Read the output
$ curl https://secure.lab.test/ curl: (60) SSL certificate problem: certificate is not yet valid $ openssl x509 -in /etc/nginx/tls/secure.crt -noout -dates notBefore=Oct 19 09:00:00 2026 GMT notAfter=Jan 17 09:00:00 2027 GMT $ date Sun Jan 4 12:00:00 UTC 2026