How much memory is in this UNIX system?
This is a relatively simple question, "but alas, poor Yorick!", the answer isn't as plain sailing. All flavours of UNIX including nowadays Linux systems can and often are fundamentally different, as are the commands they use to get the simplest of information.
Some variants of UNIX require kernel probes, others simply provide a method of extracting the output from dmesg to provide the information, whilst others don't.
Some UNIX and Linux systems have utilities or scripts to interrogate it's hardware which generally reports a variety of things, including memory, and again some don't.
I could ramble on and on and on, but lets not. Below is a list of commands that I've used over and over again across various UNIX and Linux platforms that I encounter on a daily basis. Some are available on certain releases of each OE whilst others aren't.
dmesg | grep mem prtdiag | grep Memory prtconf | grep Memory psrinfo -v
dmesg | grep Physical machinfo | grep Memory print_manifest | grep Memory
dmesg | grep Memory grep -i memtotal /proc/meminfo cat /proc/meminfo free -m vmstat -s
dmesg | grep memory grep memory /var/run/dmesg.boot sysctl -a | grep mem
bootinfo -r lsattr -E1 sys0 -a realmem getconf REAL_MEMORY
show mem /page