Two comma-separated values configure command line recall features.
a — Maximum number of entries held in the command line/read line recall buffer, subject to the space limitation in the second parameter. The range is 0–256 entries. The default is 32 entries.
b — Total size (in bytes) of all input strings to be stored in the command line/read line buffer. The range is 0–8192 bytes. The default is 1024 bytes.
As described above.
If you want history=a,b you can use the portal to set these values as follows:
On the page System Administration > Configuration > Additional Settings > Advanced Memory: