Difference between return and exit in Bash functio

问题:

What is the difference between the return and exit statement in BASH functions with respect to exit codes?

回答1:

From man bash on return [n];

Causes a function to stop executing and return the value specified by n to its caller. If n is omitted, the return status is that of the last command executed in the function body.

... on exit [n]:

Cause the shell to exit with a status of n. If n is omitted, the exit status is that of the last command executed. A trap on EXIT is executed before the shell terminates.

EDIT:

As per your edit of the question, regarding exit codes, return has nothing to do with exit codes. Exit codes are intended for applications/scripts, not functions. So in this regard, the only keyword that sets the exit code of the script (the one that can be caught by the calling program using the $? shell variable) is exit.

EDIT 2:

My last statement referring exit is causing some comments. It was made to differentiate return and exit for the understanding of the OP, and in fact, at any given point of a program/shell script, exit is the only way of ending the script with an exit code to the calling process.

Every command executed in the shell produces a local "exit code": it sets the $? variable to that code, and can be used with if, && and other operators to conditionally execute other commands.

These exit codes (and the value of the $? variable) are reset by each command execution.

Incidentally, the exit code of the last command executed by the script is used as the exit code of the script itself as seen by the calling process.

Finally, functions, when called, act as shell commands with respect to exit codes. The exit code of the function (within the function) is set by using return. So when in a function return 0 is run, the function execution terminates, giving an exit code of 0.

回答2:

return will cause the current function to go out of scope, while exit will cause the script to end at the point where it is called. Here is a sample program to help explain this:

#!/bin/bash

retfunc()
{
    echo "this is retfunc()"
    return 1
}

exitfunc()
{
    echo "this is exitfunc()"
    exit 1
}

retfunc
echo "We are still here"
exitfunc
echo "We will never see this"

Output

$ ./test.sh
this is retfunc()
We are still here
this is exitfunc()

回答3:

I don't think anyone has really fully answered the question because they don't describe how the two are used. OK I think we know that exit kills the script, where ever it is called and you can assign a status to it as well such as exit or exit 0 or exit 7 and so forth. This can be used to determine how the script was forced to stop if called by another script etc. Enough on exit.

return when called will return the value specified to indicate the function's behavior, usually a 1 or a 0. For example:

    #!/bin/bash
    isdirectory() {
      if [ -d "$1" ]
      then
        return 0
      else
        return 1
      fi
    echo "you will not see anything after the return like this text"
    }

check like this:

    if isdirectory $1; then echo "is directory"; else echo "not a directory"; fi

or like this:

    isdirectory || echo "not a directory"

In this example, the test can be used to indicate if the directory was found. notice that anything after the return will not be executed in the function. 0 is true but false is 1 in the shell, different from other prog langs.

For more info on functions: http://www.linuxjournal.com/content/return-values-bash-functions

NOTE: The isdirectory function is for instructional purposes only. This should not be how you perform such an option in a real script.

回答4:

Remember, functions are internal to a script and normally return from whence they were called by using the return statement. Calling an external script is another matter entirely, and scripts usually terminate with an exit statement.

The difference "between the return and exit statement in BASH functions with respect to exit codes" is very little. Both return a status, not values per se. A status of zero indicates success, while any other status (1 to 255) indicates a failure. The return statement will return to the script from where it was called, while the exit statement will end the entire script from whereever it is encountered.

return 0  # returns to where the function was called.  $? contains 0 (success).

return 1  # returns to where the function was called.  $? contains 1 (failure).

exit 0  # exits the script completely.  $? contains 0 (success).

exit 1  # exits the script completely.  $? contains 1 (failure).

If your function simply ends with no return statement, the status of the last command executed is returned as the status code (and will be placed in $?).

Remember, return and exit give back a status code from 0 to 255, available in $?. You cannot stuff anything else into a status code (e.g. return "cat"); it will not work. But, a script can pass back 255 different reasons for failure by using status codes.

You can set variables contained in the calling script, or echo results in the function and use command substitution in the calling script; but the purpose of return and exit are to pass status codes, not values or computation results as one might expect in a programming language like C.

回答5:

Sometimes, you run a script using . or source.

. a.sh

If you include an exit in the a.sh, it will not just terminate the script, but end your shell session.

If you include a return in the a.sh, it simply stops processing the script.

回答6:

In simple words (mainly for newbie in coding), we can say,

`return` : exits the function,
`exit()` : exits the program(called as process while running)

Also If you observed, this is very basic but...,

`return` : is the keyword
`exit()` : is the function

回答7:

• exit terminate the current process; with or without exit code, consider this a system more than a program function. Note that when sourcing, exit will end the shell, however, when running will just exit the script.

• return from a function go back to the instruction after the call, with or without a return code. return is optional and it's implicit at the end of the function. return can only be used inside a function.

I want to add that while being sourced, it's not easy to exit the script from within a function without killing the shell. I think, an example is better on a 'test' script

#!/bin/bash
function die(){
   echo ${1:=Something terrible wrong happen}
   #... clean your trash
   exit 1
}

[ -f /whatever/ ] || die "whatever is not available"
# now we can proceed
echo "continue"

doing the following:

user$ ./test
Whatever is not available
user$

test -and- the shell will close.

user$ . ./test
Whatever is not available

only test will finish and the prompt will show.

The solution is to enclose the potentially procedure in ( and )

#!/bin/bash
function die(){
   echo $(1:=Something terrible wrong happen)
   #... clean your trash
   exit 1
}

( # added        
    [ -f /whatever/ ] || die "whatever is not available"
    # now we can proceed
    echo "continue"
) # added

now, in both case only test will exit.

回答8:

First of all, return is a keyword and exit my friend is a function.

That said, here's a simplest of explanations.

return It returns a value from a function.

exit It exits out of or abandons the current shell.

回答9:

The OP's question: What is the difference between the return and exit statement in BASH functions with respect to exit codes?

Firtly, some clarification is required:

• A (return|exit) statement is not required to terminate execution of a (function|shell). A (function|shell) will terminate when it reaches the end of its code list, even with no (return|exit) statement.
• A (return|exit) statement is not required to pass a value back from a terminated (function|shell). Every process has a built-in variable $? which always has a numeric value. It is a special variable that cannot be set like "?=1", but is set only in special ways (see below *). The value of $? after the last command to be executed in the (called function | sub shell) is the value that is passed back to the (function caller | parent shell). That is true whether the last command executed is ("return [n]"| "exit [n]") or plain ("return" or something else which happens to be the last command in the called functions code.

In the above bullet list, choose from "(x|y)" either always the first item or always the second item to get statements about functions & return or shells & exit respectively.

What is clear is that they both share common usage of the special variable $? to pass values upwards after they terminate.

* Now for the special ways that $? can be set:

• When a called function terminates and returns to it's caller then $? in the caller will be equal to the final value of $? in the terminated function.
• When a parent shell impliciltly or explicitly waits on a single sub shell and is released by termination of that sub shell, then $? in the parent shell will be equal to the final value of $? in the terminated sub shell.
• Some built-in functions can modify $? depending upon their result. But some don't.
• Built-in functions "return" and "exit", when followed by a numerical argument both $? with argument, and terminate execution.

It is worth noting that $? can be assigned a value by calling exit in a sub shell, like this:

# (exit 259)
# echo $?
3