安裝套件時遇到問題 - BBS

請問一下
小弟是FreeBSD新手
在安裝套件 linux_base-fc-4_15.tbz
出現了以下訊息
pkg_add: find_play_pen: can't find enough temporary sapce to extract the
files,
please set your PKG_TMPDIR environment
variable to a location with at least 121M bytes free

請問要怎麼更改PKG_TMPDIR呢

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