Bash Sequence Expression

In this article, we will cover the basics of the sequence expression in Bash.

The Bash sequence expression generates a range of integers or characters by defining a start and the end point of the range. It is generally used in combination with for loops.

The sequence expression takes the following form:

• {START..END[..INCREMENT]}
• The expression begins with an opening brace and ends with a closing brace.
• START and END can be either positive integers or single characters.
• The START and the END values are mandatory and separated with two dots .., with no space between them.
• The INCREMENT value is optional. If present, it must be separated from the END value with two dots .., with no space between them. When characters are given, the expression is expanded in lexicographic order.
• The expression expands to each number or characters between START and END, including the provided values.
• An incorrectly formed expression is left unchanged.

Here’s the expression in action:

• echo {0..3}

When no INCREMENT is provided the default increment is 1:

• 0 1 2 3

You can also use other characters. The example below prints the alphabet:

• echo {a..z}
• a b c d e f g h i j k l m n o p q r s t u v w x y z

If the START value is greater than END then the expression will create a range that decrements:

• for i in {3..0}
• do
• echo “Number: $i”
• done
• Number: 3
• Number: 2
• Number: 1
• Number: 0

When an INCREMENT is given, it is used as the step between each generated item:

• for i in {0..20..5}
• do
• echo “Number: $i”
• done

Each generated number is greater than the preceding number by 5:

• Number: 0
• Number: 5
• Number: 10
• Number: 15
• Number: 20

When using integers to generate a range, you can add a leading 0 to force each number to have the same length. To pad generated integers with leading zeros prefix either START and END with a zero:

• for i in {00..3}
• do
• echo “Number: $i”
• done
• Number: 00
• Number: 01
• Number: 02
• Number: 03

The expression can be prefixed or suffixed with other characters:

• echo A{00..3}B
• A00B A01B A02B A03B

If the expression is not constructed correctly, it is left unchanged:

• echo {0..}
• 0..

Conclusion

The Bash sequence expression allows you to generate a range of integers or characters.