> ## Documentation Index > Fetch the complete documentation index at: https://docs.syntblaze.com/llms.txt > Use this file to discover all available pages before exploring further. # Bash Sequence Expression A sequence expression is a mechanism within Bash's brace expansion phase that generates a contiguous series of integers or characters. It evaluates strictly before other expansions (such as parameter expansion, command substitution, or arithmetic expansion) and produces a space-separated list of discrete string tokens. ```bash theme={"dark"} {..[..]} ``` ## Parameters * **`START`**: The initial boundary value. Must be an integer or a single alphabetic character. * **`END`**: The terminal boundary value (inclusive). Must match the data type of `START`. * **`STEP`** *(Optional)*: An integer defining the interval between generated values. Bash evaluates this as an absolute value, ignoring any negative sign. If omitted, Bash defaults to an interval of `1` or `-1`, automatically inferred from the relative values of `START` and `END`. ## Core Mechanics **Integer Sequences** Generates a sequence of base-10 integers. It natively supports negative numbers and automatically determines whether to increment or decrement based on the boundaries. ```bash theme={"dark"} # Ascending echo {1..5} # Output: 1 2 3 4 5 # Descending with negative integers echo {2..-2} # Output: 2 1 0 -1 -2 ``` **Zero-Padding** If either `START` or `END` is prefixed with a `0`, Bash forces all generated tokens to be zero-padded to match the maximum character width of the provided boundary values. ```bash theme={"dark"} echo {001..5} # Output: 001 002 003 004 005 echo {08..11} # Output: 08 09 10 11 ``` **Character Sequences** Generates a sequence of single characters based on the C locale's lexicographical order (ASCII code points). While the `START` and `END` boundaries must be standard alphabetic characters to trigger the expansion, the generated sequence itself is not strictly alphabetic. Mixing cases (e.g., `{a..Z}`) traverses the ASCII table between the two code points, outputting the non-alphabetic characters (`[`, `\`, `]`, `^`, `_`, `` ` ``) that reside between the uppercase and lowercase ranges. ```bash theme={"dark"} echo {a..e} # Output: a b c d e # Mixed case traversal including non-alphabetic ASCII characters echo {a..Z} # Output: a ` _ ^ ] \ [ Z ``` **Step Intervals** The `STEP` parameter forces the sequence to skip values. Bash evaluates the `STEP` as an absolute difference. Syntactically, Bash accepts negative integers for the step but ignores the negative sign, applying the absolute value regardless of whether the sequence is ascending or descending. ```bash theme={"dark"} # Integer step echo {10..0..2} # Output: 10 8 6 4 2 0 # Negative step value (sign is ignored, evaluated as absolute 2) echo {1..5..-2} # Output: 1 3 5 # Character step echo {a..z..5} # Output: a f k p u z ``` ## Parser Evaluation Constraints Because brace expansion is the absolute first step in the Bash shell expansion process, sequence expressions are evaluated as strict literals. They cannot dynamically interpolate variables directly. ```bash theme={"dark"} # This will NOT evaluate as a sequence expression END=5 echo {1..$END} # Output: {1..5} ``` To force a sequence expression to evaluate variables, the shell must be instructed to perform a secondary evaluation pass using `eval`, though this requires careful string escaping to prevent premature expansion.

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