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NASM提供了一個環境變量NASMENV，這個環境變量的值會被插入到nasm和在命令行指定選項之間。比如NASMENV=-ic:\nasmlib\，當你輸入nasm -felf時之際的效果是nasm -ic:\nasmlib\ -felf。
在NASMENV中選項默認是以空格分開的，如-s -ic:\nasmlib\會被是爲兩個選項-s和-ic:\nasmlib\。但這樣會引入一些問題，比如-dNAME="my name"，NASM會將其視爲-dNAME="my和name"。
因此NASM提供了一個解決方法，NASMENV的第一個字符如果不是減號-。NASM會認爲該字符爲選項的分割符。如!-s!-ic:\nasmlib\，NASM會認爲這是兩個選項分別-s和-ic:\nasmlib\。
在0.98.31版本之前這個環境變量名爲NASM，之後改成了NASMENV。