Control Flow

Conditionals

Braces are required. No then/fi syntax:

if count > 10 {
    echo "many"
} else if count > 0 {
    echo "some"
} else {
    echo "none"
}

Conditions are evaluated in expression context, so bare names resolve as variables. To evaluate a command in a condition, wrap it in backticks:

if `grep -c "error" log.txt` > 0 {
    echo "errors found"
}

Boolean operators work as expected:

if verbose && count > 0 {
    echo "processing $count items"
}

if !found || retries > 3 {
    exit "giving up"
}

For Loops

Iterate over lists, ranges, or chain results:

# over a list literal
for name in ["alice", "bob", "carol"] {
    echo "hello $name"
}

# over a range
for i in 1..10 {
    echo "iteration $i"
}

# over functional chain results
for f in `ls`.filter(x => x.endsWith(".d")) {
    echo "compiling $f"
}

Object iteration destructures key-value pairs:

let config = {host: "localhost", port: "8080", proto: "https"}
for key, value in config {
    echo "${key}=${value}"
}

While Loops

mut attempts = 0
while attempts < 5 {
    echo "attempt $((attempts + 1))"
    attempts = attempts + 1
}

Match

Pattern match with | for alternatives and _ as a wildcard:

let ext = "json"
match ext {
    "d" | "di"  => echo "D source"
    "json"      => echo "JSON data"
    "yaml"      => echo "YAML config"
    _           => echo "unknown: $ext"
}

If no branch matches and there is no _ wildcard, lash produces a runtime error.

Break and Continue

Both work in for and while loops:

for line in `cat results.csv` {
    if line.startsWith("#") {
        continue
    }
    if line.contains("STOP") {
        break
    }
    echo $line
}

Exit

Terminate a script immediately:

exit              # exit code 0
exit 42           # exit code 42
exit "fatal: missing config file"  # prints to stderr, exits with code 1

exit propagates through loops and conditionals.

Loop Protection

lash prevents infinite loops. If a loop exceeds 1000 iterations with no variable mutations, it reports an error. Loops where variables are actively mutated are allowed to exceed the limit.

Configure the threshold with the LASH_LOOP_LIMIT environment variable:

# allow up to 50000 iterations
mut LASH_LOOP_LIMIT = 50000

In interactive mode, lash may prompt you to continue instead of erroring.

Exit Codes

$? holds the exit code of the last executed command:

grep "needle" haystack.txt
if $? == 0 {
    echo "found it"
} else {
    echo "not found"
}

In functional chains, the exit code reflects the chain outcome.

Job Control

Background a command with &:

make -j8 &

Command	Description
jobs	List all running and stopped background jobs
fg %n	Bring job n to the foreground
bg %n	Resume a stopped job in the background
Ctrl+Z	Suspend the foreground process (SIGTSTP)
Ctrl+C	Interrupt the foreground process (SIGINT)

A backgrounded functional chain runs the entire chain in the background. When brought to foreground, buffered output is printed.

If a chain receives SIGINT, lazy evaluation stops immediately and partially consumed ranges are discarded.