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.

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$ 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