11. What Lash Does Differently From Bash

Lash compiles to Bash, but it is not Bash syntax with a different file extension. This chapter lists the differences that matter most when reading or porting scripts.

Blocks close with end

if ready
    echo "ready"
end

for file in ./*.lash
    echo $"{file}"
end

Generated Bash uses fi, done, and esac; Lash source uses end.

Declarations are checked

Bash lets you assign almost anywhere. Lash distinguishes declaration from assignment.

let name = "lash"
name = "other" // error

Variables must be declared before use. let cannot be reassigned. readonly also lowers to Bash readonly behavior.

Conditions are expressions

if count > 0
    echo "non-zero"
end

Lash generates Bash tests for you. Numeric comparisons use arithmetic tests. String equality uses [[ ... ]]. Regex =~ is supported in conditions only.

Booleans are numeric at runtime

true lowers to 1; false lowers to 0.

Functions return values by output

fn name()
    return "lash"
end

This emits echo "lash" followed by return 0. It is designed for command-substitution style value flow, not Bash numeric return status.

argv is zero-based

let arg_count = #argv
echo $"{argv[0]}"
echo $"{arg_count}"

Bash positional parameters are one-based after $0. Lash argv[0] means the first script argument.

Maps are Bash associative arrays

let meta = {"name": "lash"}
echo $"{meta["name"]}"

The compiler lowers string-keyed maps to Bash associative arrays and enforces string keys for map literals.

Ranges are Lash syntax

for i in 1..5 step 2
    echo $"{i}"
end

Ranges lower through seq and are only supported as for iterables.

wait jobs is Lash-tracked

wait jobs waits for background subshell and coproc jobs the compiler tracks. It does not scan arbitrary Bash jobs started by raw command lines.

Bare Bash is still supported

These shell forms are intentionally accepted:

set -euo pipefail
export NAME=value
source ./env.sh
shopt -s nullglob
alias ll='ls -l'
unset NAME
declare -A table
local value=1

Command substitution, process substitution, globs, redirects, heredocs, traps, background jobs, and coproc are also supported. Lash adds structure around them; it does not remove Bash from the language.