![为什么“head -1”不等于“head -n -1”,而是与“head -n 1”相同?](https://linux22.com/image/139721/%E4%B8%BA%E4%BB%80%E4%B9%88%E2%80%9Chead%20-1%E2%80%9D%E4%B8%8D%E7%AD%89%E4%BA%8E%E2%80%9Chead%20-n%20-1%E2%80%9D%EF%BC%8C%E8%80%8C%E6%98%AF%E4%B8%8E%E2%80%9Chead%20-n%201%E2%80%9D%E7%9B%B8%E5%90%8C%EF%BC%9F.png)
head -num
head -n num
与代替head -n -num
(其中num
是任意数字)相同
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
似乎没有任何地方记录。
$ 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'
手册页head
(在 Fedora 28 上):
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)
答案1
信息页面和在线手册对于 GNUhead
包含这部分:
为了兼容性,
head
还支持过时的选项语法-[NUM][bkm][cqv]
,只有首先指定它才能被识别。
head -1
相同的想法head -n 1
是破折号不是减号,而是命令行选项的标记。这是通常的习惯:以破折号开头的内容是控制如何进行处理的选项,命令行中的其他内容是文件名或其他要处理的实际目标。在本例中,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 +1
or中head 1
,+1
或1
将被视为文件名。
双破折号--
或--something
也具有独特的含义,它本身 ( --
) 会停止选项处理,当后面跟有其他内容时,它标志着 GNU 风格的长选项。所以有head --1
forhead -n -1
不符合习俗。
如果我猜的话,我会假设存在正数的古怪捷径,但负数则不存在,因为前一种情况更常用且更容易实现。 (此外,-n i
i
i
标准head
仅针对正值线定义。)