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sandra_henrystocker
Unix Dweeb

Generating numeric sequences with the Linux seq command

How-To
Jan 07, 20202 mins
Linux

The Linux seq command can generate lists of numbers and at lightning speed. It's easy to use and flexible, too.

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  

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
sandra_henrystocker
Unix Dweeb

Sandra Henry-Stocker has been administering Unix systems for more than 30 years. She describes herself as "USL" (Unix as a second language) but remembers enough English to write books and buy groceries. She lives in the mountains in Virginia where, when not working with or writing about Unix, she's chasing the bears away from her bird feeders.

The opinions expressed in this blog are those of Sandra Henry-Stocker and do not necessarily represent those of IDG Communications, Inc., its parent, subsidiary or affiliated companies.

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