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seq: sequence generator 
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It may surprise some but it is not uncommon to number lines. UNIX has
two tools that meet this need: seq & nl. Here we discuss seq.

"seq" generates a sequence of numbers (with syntax similar to a "for"
loop)

CAUTION: Supposedly seq is not a "standard" tool in Unix/Linux. Use
"brew" to install


$ seq -s OFS -t lastchar -f FORMAT [first [incr]] last  #default first=1, incr=1

	#default for first is 1
	#if last > first then default incr = +1
	#if last < first then default incr = -1
	#OFS is the output field separator, default "\n"
	#lastchar is the ouput record separator  (last character added to output)
	#FORMAT specifies the output format

$ seq 3   	#same as "seq 1 3"
1 
2
3

$ seq 2 4
2
3
4

$ seq 4 2         #descending order
4
3
2

$ seq 4 -1 2 	#likely to be robust across platforms

$ seq 0 0.5 1	#step size other than unity are possible
0
0.5
1

$ seq -s "," -t "\n" 3
1,2,3,

$ seq -s " " -t "\n" 3
1 2 3

You can format the numbers: seq -w 10000 will format all integers to 5 digits
(zero fill). You can also use "C" formating e.g.

$ seq -f "%02g/03/2016" 31    #generates dates
01/03/2016
02/03/2016
...