head 命令可查看文件的前N行/字节的数据：

root@li384-34:/# head --help

Usage: head [OPTION]... [FILE]...

Print the first 10 lines of each FILE to standard output.

With more than one FILE, precede each with a header giving the file name.

With no FILE, or when FILE is -, read standard input.

Mandatory arguments to long options are mandatory for short options too.

  -c, --bytes=[-]N         print the first N bytes of each file;

                             with the leading `-', print all but the last

                             N bytes of each file

  -n, --lines=[-]N         print the first N lines instead of the first 10;

                             with the leading `-', print all but the last

                             N lines of each file

  -q, --quiet, --silent    never print headers giving file names

  -v, --verbose            always print headers giving file names

      --help     display this help and exit

      --version  output version information and exit

N may have a multiplier suffix:

b 512, kB 1000, K 1024, MB 1000*1000, M 1024*1024,

GB 1000*1000*1000, G 1024*1024*1024, and so on for T, P, E, Z, Y.

tail 命令可查看文件的最后N行/字节的数据：

 root@li384-34:/# tail --help

Usage: tail [OPTION]... [FILE]...

Print the last 10 lines of each FILE to standard output.

With more than one FILE, precede each with a header giving the file name.

With no FILE, or when FILE is -, read standard input.

Mandatory arguments to long options are mandatory for short options too.

  -c, --bytes=N            output the last N bytes; alternatively, use +N to

                           output bytes starting with the Nth of each file

  -f, --follow[={name|descriptor}]

                           output appended data as the file grows;

                           -f, --follow, and --follow=descriptor are

                           equivalent

  -F                       same as --follow=name --retry

  -n, --lines=N            output the last N lines, instead of the last 10;

                           or use +N to output lines starting with the Nth

      --max-unchanged-stats=N

                           with --follow=name, reopen a FILE which has not

                           changed size after N (default 5) iterations

                           to see if it has been unlinked or renamed

                           (this is the usual case of rotated log files)

      --pid=PID            with -f, terminate after process ID, PID dies

  -q, --quiet, --silent    never output headers giving file names

      --retry              keep trying to open a file even when it is or

                             becomes inaccessible; useful when following by

                             name, i.e., with --follow=name

  -s, --sleep-interval=S   with -f, sleep for approximately S seconds

                             (default 1.0) between iterations

  -v, --verbose            always output headers giving file names

      --help     display this help and exit

      --version  output version information and exit

If the first character of N (the number of bytes or lines) is a `+',

print beginning with the Nth item from the start of each file, otherwise,

print the last N items in the file.  N may have a multiplier suffix:

b 512, kB 1000, K 1024, MB 1000*1000, M 1024*1024,

GB 1000*1000*1000, G 1024*1024*1024, and so on for T, P, E, Z, Y.

With --follow (-f), tail defaults to following the file descriptor, which

means that even if a tail'ed file is renamed, tail will continue to track

its end.  This default behavior is not desirable when you really want to

track the actual name of the file, not the file descriptor (e.g., log

rotation).  Use --follow=name in that case.  That causes tail to track the

named file by reopening it periodically to see if it has been removed and

recreated by some other program.