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user306023
Asked: 2018-09-06 14:21:01 +0800 CST

为什么“head -1”与“head -n -1”不等价，而是与“head -n 1”相同？

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head -numhead -n num与而不是相同head -n -num（其中num是任何数字）

例子： 

$ echo -e 'a\nb\nc\nd'|head -1
a

$ echo -e 'a\nb\nc\nd'|head -n 1
a

$ echo -e 'a\nb\nc\nd'|head -n -1
a
b
c

这head -1似乎没有记录在任何地方。

$ 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=[-]NUM       print the first NUM bytes of each file;
                             with the leading '-', print all but the last
                             NUM bytes of each file
  -n, --lines=[-]NUM       print the first NUM lines instead of the first 10;
                             with the leading '-', print all but the last
                             NUM lines of each file
  -q, --quiet, --silent    never print headers giving file names
  -v, --verbose            always print headers giving file names
  -z, --zero-terminated    line delimiter is NUL, not newline
      --help     display this help and exit
      --version  output version information and exit

NUM 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.

GNU coreutils online help: <https://www.gnu.org/software/coreutils/>
Full documentation at: <https://www.gnu.org/software/coreutils/head>
or available locally via: info '(coreutils) head invocation'

head（在 Fedora 28 上） 的手册页：

HEAD(1)                          User Commands                         HEAD(1)

NAME
       head - output the first part of files

SYNOPSIS
       head [OPTION]... [FILE]...

DESCRIPTION
       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=[-]NUM
              print  the  first  NUM bytes of each file; with the leading '-',
              print all but the last NUM bytes of each file

       -n, --lines=[-]NUM
              print the first NUM lines instead of  the  first  10;  with  the
              leading '-', print all but the last NUM lines of each file

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

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

       -z, --zero-terminated
              line delimiter is NUL, not newline

       --help display this help and exit

       --version
              output version information and exit

       NUM 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.

AUTHOR
       Written by David MacKenzie and Jim Meyering.

REPORTING BUGS
       GNU coreutils online help: <https://www.gnu.org/software/coreutils/>
       Report head translation bugs to <https://translationproject.org/team/>

COPYRIGHT
       Copyright  ©  2017  Free Software Foundation, Inc.  License GPLv3+: GNU
       GPL version 3 or later <https://gnu.org/licenses/gpl.html>.
       This is free software: you are free  to  change  and  redistribute  it.
       There is NO WARRANTY, to the extent permitted by law.

SEE ALSO
       tail(1)

       Full documentation at: <https://www.gnu.org/software/coreutils/head>
       or available locally via: info '(coreutils) head invocation'

GNU coreutils 8.29               December 2017                         HEAD(1)
linux head

1 个回答

  1. Best Answer

    GNU的信息页面和在线手册head包含以下部分：

    为了兼容性，head还支持过时的选项语法 -[NUM][bkm][cqv]，只有在首先指定时才能识别。

    head -1与此相同的想法是head -n 1破折号不是减号，而是命令行选项的标记。这是通常的习惯：以破折号开头的东西是控制如何进行处理的选项，命令行中的其他东西是文件名或其他要处理的实际目标。在这种情况下，它不是单字符选项，而是 的简写-n，但它基本上仍然是一个选项，而不是文件名。但是，在head +1orhead 1中，+1or1将被视为文件名。

    双破折号--或--something也有不同的含义，它本身 ( --) 它停止选项处理，当后面跟着其他东西时，它标志着一个 GNU 风格的长选项。因此，拥有head --1forhead -n -1与习俗不符。

    如果我猜的话，我会假设存在的古朴快捷方式是积极的，而不是消极的，因为前一种情况更经常有用并且更容易实现。（此外，该标准仅针对线的正值定义。）-n iiihead

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