Data Science Wizardry Blog by Attila Vajda

Echos of number sequences.

2 2 10 ' ' -s <<< $(echo seq)

seq prints a sequence of numbers, so 2, 2, 10 could be printed with seq.

man bash can be searched with /.

Since <<<word expands word and supplies it as input to the command, and ' ' cannot be expanded, <<< might expand something in the variable made by $.

$ seq -s 2 2 10
2232425262728292102 $

I don't understand what is going on there, but I will try to figure it out by looking at a simpler form of the problem.

$ seq -s 2 2
1222 $
$ seq 1
1
$ seq 1 2
1
2
$ seq 1 2 -s
seq: invalid floating point argument: -s
$ seq -s 1 2
1121 $
$ seq -s 1 1
11 $
$ seq -s 1 1 1
11 $
$ seq -s 1 1 1 1
11 $

seq is similar to range() in Python:

$ seq 1 10
1
2
3
4
5
6
7
8
9
10
$ seq -s 1 10
112131415161718191101 $

Since -s is a flag to use string to separate numbers, I assume that the string in this example is 1 and it connects the range of numbers from 1 to 10.

Testing this hypothesis validated my assumption:

$ seq -s "a" 10
1a2a3a4a5a6a7a8a9a10a

The absence of 1 suggests that a single integer input creates a number sequence with 1 as default initial value.

$ seq 3
1
2
3

What happens if we use negative values?

The default initial value is still 1:

$ seq -3
1
0
-1
-2
-3

The original puzzle could be every second number from 2 to 10, separated by the whitespace ' ', and supplied as an input to echo.

This work is fun, satisfying and although seemingly trivial, I can sense that it is deeply meaningful.

$ seq -s ' ' 2 10
2 3 4 5 6 7 8 9 10 $
$ seq -s ' ' 2 10 2
2 $

After struggling to solve this seq puzzle I now read man seq with much more awareness.

According to the manual, seq is structured to accommodate first, last and increment values.

Number sequences in the spotlight:

$ seq 2 2 10
2
4
6
8
10

The aha moment: the first value is the starting point, the second is the step, the third is the last. The mystery unfolds, showcasing the structure and meaning of seq and 2 2 10

<<<,

Why is (` ') used for blank characters in man?

Can echo, the writing of arguments to the standard output be connected with <<<word, the expansion of word and its contribution to a command on its standard input?

$ echo <<<"a"

$ echo <<<" " 

$ echo <<<"\n" 

$ echo <<<seq 10
10
$ echo <<<seq 1 10
1 10
$ echo <<<seq

$ echo <<<$(1)
-bash: 1: command not found
$ echo <<<$(2)
-bash: 2: command not found
$ echo <<<$(seq)
usage: seq [-w] [-f format] [-s string] [-t string] [first [incr]] last
$ echo <<<$(seq 10)

$ echo <<<$(seq 1 10)

$ echo <<<$(seq ' ' 10)
seq: invalid floating point argument:  

$ echo <<<$(seq ' ' 2 2 10)
usage: seq [-w] [-f format] [-s string] [-t string] [first [incr]] last
$ echo <<< $(seq ' ' 2 2 10)
usage: seq [-w] [-f format] [-s string] [-t string] [first [incr]] last

$(command) is command substitution, the command is replaced by its output value. In our puzzle, the output is the sequence of numbers returned by seq. Since echo requires an argument as its input, and $(command) is a value, <<< supplies $(command) to echo.

$ echo 'hello world'
hello world
$ echo <<<'hello world'

$ echo echo <<<'hello world'
echo