lsns: add page (#8078)

pages/linux/lsns.md

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+# lsns
+
+> List information about all namespaces or about the specified namespace.
+> More information: <https://man7.org/linux/man-pages/man8/lsns.8.html>.
+
+- List all namespaces:
+
+`lsns`
+
+- List namespaces in JSON format:
+
+`lsns --json`
+
+- List namespaces associated with {{pid}}:
+
+`lsns --task {{pid}}`
+
+- List the specified type of namespaces only:
+
+`lsns --type <mnt|net|ipc|user|pid|uts|cgroup|time>`
+
+- List namespaces, only showing the namespace ID, type, PID, and command:
+
+`lsns --output NS,TYPE,PID,COMMAND`