For saving configuration in Cisco devices we have "wr" and "copy running config to startup config" i am curious what is difference between these two command.
They essentially achieve the same things by saving the running configuration to the memory so that after a reload it retains the same configuration. Write memory is the "ancient" way, and copy running-config startup-config is the "newer way". Some newer platforms do not accept write memory, the Nexus platforms for instance. The workaround is to create an alias using
cli alias name wr copy run start in global configuration mode.
The "copy run start" command is just a variation of the "copy" command. The copy command can be used to copy any files in or out of the flash etc. - as opposed to just saving the configuration. Just remember though, if you are in the wrong configuration register "wr" will lose your configuration after a reload/when you change the configuration register whereas "copy run start" will just copy the contents of the running configuration to the start-up configuration.
When doing CCNA exams, the command "write" is not allowed. It has to be the official "copy running-config startup-config".
The reason why the "wr" or "write" command is very popular are:
A minimum of two characters to save a config;
It is easy to confuse "copy start run" with "copy run start".