Negative arguments to head

Question

I was trying using head command, in macOS using zsh, code below,

a.txt:

1
2
3
4
5
6
7
8
9
10

tail -n +5 a.txt   // line 5 to line end

tail -n -5 a.txt   // last line 5 to line end

head -n +5 a.txt // line 1 to line 5

head -n -5 a.txt  // # What did this do?

The last command shows an error.

head: illegal line count -- -5

What did head -n -5 actually do?

Shows *what* error? And have you tried to read [the `head` manual page](http://man7.org/linux/man-pages/man1/head.1.html)? — Some programmer dude, Feb 21 '20 at 09:36
@RobbyCornelissen it doesn't, because `man head` on a Mac does not show any information at all, and although `info head` does, it still shows valid negative `-n` flag, which isn't supported by the macOs. So the question is valid and the answer is not in the manual :) — gented, May 28 '21 at 12:41

imbr · Accepted Answer · 2020-02-21T11:01:06.020

2

Some implementations of head like GNU head support negative arguments for -n

But that's not standard! Your case is clearly not supported.

When supported The negative argument should remove the last 5 lines before doing the head

edited Feb 21 '20 at 11:01

answered Feb 21 '20 at 10:41

imbr

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score 0 · Answer 2 · answered Feb 21 '20 at 09:45

It becomes more clear, if using 3 instead of 5. Note the signs!

# print 10 lines:
seq 10

1
2
3
4
5
6
7
8
9
10

#-------------------------    
# get the last 3 lines:
seq 10 | tail -n 3

8
9
10

#--------------------------------------
# start at line 3 (skip first 2 lines)
seq 10 | tail -n +3

3
4
5
6
7
8
9
10

#-------------------------    
# get the first 3 lines:
seq 10 | head -n 3

1
2
3

#-------------------------    
# skip the last 3 lines:
seq 10 | head -n -3

1
2
3
4
5
6
7

btw, man tail and man head explain this behavior.

Negative arguments to head

2 Answers2

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