CODEWITHSUNDEEP
parametersluaoperating-systemexit
Vinni Marcon
Vinni Marcon

Reputation: 616

Why and when im gonna use the parameter "code" of the os.exit() function in Lua

In the Lua documentation, they say that the code parameter in os.exit([code]) returns a value other than 0 when exiting the script, like, if I run the following line:

os.exit(7)

it will produce the following output:

>Exit code: 7

My question is why and when would it be useful to change the exit value of the script? like, when and where would I work with this exit code "7"?

Upvotes: 2

Views: 728

Answers (1)

ad absurdum
ad absurdum

Reputation: 21367

The value is returned to the process that ran the Lua interpreter; the C language has the same facility.

Typically, 0 is returned on the successful execution of a script, and a nonzero value is returned when there is some kind of error. If a Lua script were called from another script, an error code could guide the calling script in handling the error.

In Bash, you can inspect the returned value by checking the $? shell variable:

$ lua -e "os.exit(7)"
$ echo $?
7

If you call a Lua script from another Lua script using os.execute, the exit code is the third of three returned values:

handy_script:

#!/usr/bin/env lua

io.write(string.format("Doing something important...\n"))
os.exit(7)

main_script:

#!/usr/bin/env lua

b, s, n = os.execute("./handy_script")
io.write(string.format("handy_script returned %s, %s: %d\n", tostring(b), s, n))

$  ./main_script
Doing something important...
handy_script returned nil, exit: 7

The first value returned by os.execute is the boolean true if the command was successfully executed, fail otherwise (as of Lua 5.4, fail is still equivalent to nil). The second value returned is the string "exit" if the command terminated normally, or "signal" if it was terminated by a signal. The third value returned is the exit code from the call to os.exit(), here 7.

Upvotes: 5

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