One of the easiest ways to generate a list of numbers in Linux is to use the seq (sequence) command. In its simplest form, seq will take a single number and then list all the numbers from 1 to that number. For example:
$ seq 5 1 2 3 4 5
Unless directed otherwise, seq always starts with 1. You can start a sequence with a different number by inserting it before the final number.
$ seq 3 5 3 4 5
Specifying an increment
You can also specify an increment. Say you want to list multiples of 3. Specify your starting point (first 3 in this example), increment (second 3) and end point (18).
$ seq 3 3 18 3 6 9 12 15 18
You can elect to go from larger to smaller numbers by using a negative increment (i.e., a decrement).
$ seq 18 -3 3 18 15 12 9 6 3
The seq command is also very fast. You can probably generate a list of a million numbers in under 10 seconds.
$ time seq 1000000 1 2 3 … … 999998 999999 1000000 real 0m9.290s <== 9+ seconds user 0m0.020s sys 0m0.899s
Using a separator
Another very useful option is to use a separator. Instead of listing a single number on each line, you can insert commas, colons or some other characters. The -s option followed by the character you wish to use.
$ seq -s: 3 3 18 3:6:9:12:15:18
In fact, if you simply want your numbers to be listed on a single line, you can use a blank as your separator in place of the default linefeed.
$ seq -s' ' 3 3 18 3 6 9 12 15 18
Getting to the math
It may seem like a big leap to go from generating a sequence of numbers to doing math, but given the right separators, seq can easily prepare calculations that you can pass to bc. For example:
$ seq -s* 5 | bc 120
What is going on in this command? Let’s take a look. First, seq is generating a list of numbers and using * as the separator.
$ seq -s* 5 1*2*3*4*5
It’s then passing the string to the calculator (bc) which promptly multiplies the numbers. And you can do a fairly extensive calculation in a fraction of a second.
$ time seq -s* 117 | bc 39699371608087208954019596294986306477904063601683223011297484643104\ 22041758630649341780708631240196854767624444057168110272995649603642\ 560353748940315749184568295424000000000000000000000000000 real 0m0.003s user 0m0.004s sys 0m0.000s
Limitations
You only get to choose one separator, so your calculations will be very limited. Use bc by itself for more complicated math. In addition, seq only works with numbers. To generate a sequence of single letters, use a command like this instead:
$ echo {a..g}
a b c d e f g