setnx 命令的使用方式如下：

SETNX key value

Set key to hold string value if key does not exist. In that case, it is equal to SET. When key already holds a value, no operation is performed. SETNX is short for "SET if Not eXists".

中文：如果key不存在，则创建key，并设置相应的值

源码

void setnxCommand(client *c) {c->argv[2] = tryObjectEncoding(c->argv[2]);// 调用通用的方法，并传递OBJ_SET_NX标号setGenericCommand(c,OBJ_SET_NX,c->argv[1],c->argv[2],NULL,0,shared.cone,shared.czero);
}

setGenericCommand 是一个通用方法，比较多的命令也都是通过该方法来处理的

void setGenericCommand(client *c, int flags, robj *key, robj *val, robj *expire, int unit, robj *ok_reply, robj *abort_reply) {long long milliseconds = 0; /* initialized to avoid any harmness warning */int found = 0;int setkey_flags = 0;if (expire && getExpireMillisecondsOrReply(c, expire, flags, unit, &milliseconds) != C_OK) {return;}if (flags & OBJ_SET_GET) {if (getGenericCommand(c) == C_ERR) return;}// 按key进行查询found = (lookupKeyWrite(c->db,key) != NULL);// 如果是SetNx命令，并且找到，则直接返回if ((flags & OBJ_SET_NX && found) ||(flags & OBJ_SET_XX && !found)){if (!(flags & OBJ_SET_GET)) {addReply(c, abort_reply ? abort_reply : shared.null[c->resp]);}return;}setkey_flags |= (flags & OBJ_KEEPTTL) ? SETKEY_KEEPTTL : 0;setkey_flags |= found ? SETKEY_ALREADY_EXIST : SETKEY_DOESNT_EXIST;setKey(c,c->db,key,val,setkey_flags);server.dirty++;notifyKeyspaceEvent(NOTIFY_STRING,"set",key,c->db->id);.. 省略部分代码if (!(flags & OBJ_SET_GET)) {addReply(c, ok_reply ? ok_reply : shared.ok);}.. 省略部分代码
}

这里只看关于setnx相关的代码，其实也很简单，从数据库获取key对应的值，如果存在，则直接返回。

如果不存在，则通过setkey方法进行创建key。

在官方文档的setnx命令的开始，就有如下的说明

As of Redis version 2.6.12, this command is regarded as deprecated.

It can be replaced by SET with the NX argument when migrating or writing new code.

从2.6.12 开始，setnx 已经被标识为deprecated， 需要通过set 命令中，通过指定nx参数来替代。

SET key value [NX | XX] [GET] [EX seconds | PX milliseconds |EXAT unix-time-seconds | PXAT unix-time-milliseconds | KEEPTTL]

set 命令将在下一节完整分析下。