log文档是记录程序及其运行的文档，它将所有在结果窗口出现的程序和结果输出到log文档（默认是scml格式，Stata特定的文件格式；也可以输出为txt，在log using时修改后缀名）。从log using 到 log close中间所有的程序及其运行结果都会被记录到log文件中。

global output "xxx" //define the output direction to store log files
log using $output\\mylog,replace  //open a log file (default format is smcl) to record codes and results
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log close //close the log file and save the log file

global output "xxx" //define the output direction to store log file
log using $output\\mylog,append  //append the codes and results to the origin log file mylog (default format is smcl)
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log close //close the log file and save the log file