Is there a Mac OS X Terminal version of the "free" command in Linux systems?

As @khedron says, you can see this info in Activity Monitor.

If you want it on the command line, here is a Python script that I wrote (or perhaps modified from someone else's, I can't remember, it's quite old now) to show you the Wired, Active, Inactive and Free memory amounts:

#!/usr/bin/python

import subprocess
import re

# Get process info
ps = subprocess.Popen(['ps', '-caxm', '-orss,comm'], stdout=subprocess.PIPE).communicate()[0].decode()
vm = subprocess.Popen(['vm_stat'], stdout=subprocess.PIPE).communicate()[0].decode()

# Iterate processes
processLines = ps.split('\n')
sep = re.compile('[\s]+')
rssTotal = 0 # kB
for row in range(1,len(processLines)):
    rowText = processLines[row].strip()
    rowElements = sep.split(rowText)
    try:
        rss = float(rowElements[0]) * 1024
    except:
        rss = 0 # ignore...
    rssTotal += rss

# Process vm_stat
vmLines = vm.split('\n')
sep = re.compile(':[\s]+')
vmStats = {}
for row in range(1,len(vmLines)-2):
    rowText = vmLines[row].strip()
    rowElements = sep.split(rowText)
    vmStats[(rowElements[0])] = int(rowElements[1].strip('\.')) * 4096

print 'Wired Memory:\t\t%d MB' % ( vmStats["Pages wired down"]/1024/1024 )
print('Active Memory:\t\t%d MB' % ( vmStats["Pages active"]/1024/1024 ))
print('Inactive Memory:\t%d MB' % ( vmStats["Pages inactive"]/1024/1024 ))
print('Free Memory:\t\t%d MB' % ( vmStats["Pages free"]/1024/1024 ))
print('Real Mem Total (ps):\t%.3f MB' % ( rssTotal/1024/1024 ))

As you can see, you can just call vm_stat from the command line, though it counts in 4kB pages, hence the script to convert to MB.

The script also counts up the "real memory" usage of all running processes for comparison (this won't match any specific value(s) from overall memory stats, because memory is a complex beast).

Here's an example of the output of the script on my system:

[user@host:~] % memReport.py
Wired Memory:           1381 MB
Active Memory:          3053 MB
Inactive Memory:        727 MB
Free Memory:            1619 MB
Real Mem Total (ps):    3402.828 MB

(very slightly adjusted to match the tab sizing on StackExchange ;)

The command you need is vm_stat - similar to the traditional Unix tool vmstat but with a few MACH-specific differences. The man page is well written.

It seems the reason it's slow is because top -l 1 always delays by one second after completing, the standard delay between refreshes. Adding -s 0 to the command makes it complete instantly:

top -l 1 -s 0 | grep PhysMem

Also, for clarity, I like showing each mem-component on its line, so I added 9 spaces for alignment with 'PhysMem: ' in the sed replacement string:

top -l 1 -s 0 | grep PhysMem | sed 's/, /\n         /g'

Here's a simple one-liner to make the whole vm_stat output more human friendly:

$ vm_stat | perl -ne '/page size of (\d+)/ and $size=$1; /Pages\s+([^:]+)[^\d]+(\d+)/ and printf("%-16s % 16.2f Mi\n", "$1:", $2 * $size / 1048576);'
free:                     2330.23 Mi
active:                   2948.07 Mi
inactive:                 1462.97 Mi
speculative:               599.45 Mi
wired down:                840.46 Mi
copy-on-write:           43684.84 Mi
zero filled:            385865.48 Mi
reactivated:               608.14 Mi