009-head和tail

head用法

command

查看文件的开头N行，支持多文件，多文件时会有标题输出

head -n N file

head的多种实现

$ head -N file #N是用户指定的行数
$ awk 'FNR <= N' file
$ sed Nq file

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tail用法

command

查看文件的结尾N行，支持多文件，多文件时会有标题输出

tail -n N file

跟踪日志输出

$ tail -f logfile  #此法持续监控某个文件的输出
$ less 后加F  #等价形式
# man less : F
Scroll  forward,  and  keep trying to read when the end of file is reached.  Normally this command would be used when already at the end of the file.  It is a way to monitor the tail of a file which is growing while it is being viewed.  (The behavior is similar to the "tail -f" command.)