r = shell['echo hello']
log[r.status]
log[r.stdout]
home = env['HOME']
log[home.ok]
unset = env['CROW_NO_SUCH_VAR']
log[unset.ok]
log[unset.kind]
log[env['CROW_NO_SUCH_VAR', 'fallback']]
log[float[now[]] gt 0.0]
log[len[args[]]]
log[hash['crow', 'sha256']]

expected output — from the book, verified by crow test --book

0
hello
true
false
env
fallback
true
0
851d49d3087657e971533b2fd8b28be2ff490c34411c2e3cd2b400fe66f3a3c3

exit[code] ends the program with a status code, sleep[ms] pauses, and halt[reason] halts the program with the reason on the line of the call, as a builtin given what it cannot take does; this block builds but is not run:

sleep[10]
log['stopping']
if len[args[]] eq 0
    halt['a file to read is the first argument']
exit[0]

Reference: spec section 11 (System, Shell, Hashing).

The system: shell, environment, time, arguments