How can I calculate pi using Bash command

This calculates the value of π using Gregory–Leibniz series:

seq -f '4/%g' 1 2 99999 generates the fractions:

4/1
4/3
4/5
4/7
4/9
4/11
4/13
4/15
4/17
4/19

The paste pipeline paste -sd-+ combines those with alternate delimiters - and +.

Finally, bc -l performs the arithmetic to give the result.

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EDIT: As noted in the comment, this sequence converges very slowly. Machin's formula has a significantly higher rate of convergence:

Using the same expansion for tan^-1(x):

to compute π, we can see that it produces the correct value to 50 digits¹ using just the first 50 terms of the series:

$ { echo -n "scale=50;"; seq 1 2 100 | xargs -n1 -I{} echo '(16*(1/5)^{}/{}-4*(1/239)^{}/{})';} | paste -sd-+ | bc -l
3.14159265358979323846264338327950288419716939937510

With just 100 terms, the value of π is computed accurately to more than 100 digits:

$ { echo -n "scale=100;"; seq 1 2 200 | xargs -n1 -I{} echo '(16*(1/5)^{}/{}-4*(1/239)^{}/{})';} | paste -sd-+ | bc -l
3.1415926535897932384626433832795028841971693993751058209749445923078164062862089986280348253421170679

¹Pi

seq -f 4 %g 1 2 99999 

Gives the data:

4/1
4/3 
4/5 
...
4/9999

The paste command takes this list and inserts a - between the first two, a + between the second two, etc (and puts it on one line, so):

4/1-4/3+4/5-4/7......4/9999

Which is an approximation of pi. The 'bc' program calculates this and prints the value.

Not a direct answer to your question about using seq, but pi can be easily computed using bc:

 echo "scale=1000; 4*a(1)" | bc -l

a is arctan, and this give pi to 1000 digits.