The Key Differences Between the return and exit Commands in Bash

The return and exit commands in Bash serve related but distinct purposes. Understanding when to use each one and how they differ is important for writing effective Bash scripts. In this comprehensive guide, we will explore the key differences between return and exit to help you use them properly in your Bash scripting.

What is the exit Command?

The exit command in Bash causes the current Bash process to exit with a specified exit status. Some key things to know about exit:

• It terminates the current Bash process and returns an exit status to the parent process
• Can be used to terminate a script early before reaching the end
• Exit status is an integer from 0 to 255 that indicates whether the script succeeded (0) or failed (non-zero)
• Not limited to use in functions – can be used anywhere in a script

Here is a simple example of using exit in a Bash script:

#!/bin/bash

echo "Starting script..."

# Do some tasks 

if [ $? -ne 0 ]; then
  echo "An error occurred. Exiting..."  
  exit 1
fi 

# Do more tasks

echo "Script finished successfully!"
exit 0

In this example, if any of the tasks return a non-zero exit code indicating an error, the script will call exit 1 to terminate early and return a failed exit status. The final exit 0 at the end ensures the script returns success if it runs to completion.

Some key things that happen when exit is called:

• Terminates the script process immediately
• Skips any remaining code in the script
• Returns the exit status to the parent shell

So in summary, exit terminates the current script and returns a status to indicate whether the script completed successfully or not.

What is the return Command?

The return command is used to return from a Bash function with an optional return value. Some key points about return:

• Used to return from a function to the calling script
• Can optionally return an integer value that can be stored as a variable
• Returns control flow to the line after the function call
• Only valid to use inside functions – causes an error if used outside

Here is a simple example of using return in a Bash function:

#!/bin/bash

# Define a function
function double() {
  local x=$1
  local result=$(( $x * 2 ))

  return $result
}

# Call the function
value=10
double_result=$(double $value)

echo "$value doubled is $double_result"

In this case, we define a double() function that accepts a parameter, doubles it, and returns the result via return. In the main script, we call double() and store the result to print out.

Some key things that happen when return executes:

• Returns control flow to the calling script
• Can return an integer status code or value
• Allows data to be returned from functions
• Does not exit the script – just the function

So in essence, return simply returns execution to the calling script after the function is complete, optionally passing back data.

Key Differences Between return and exit

Based on the above overviews, here are the key differences between the return and exit commands:

Return	Exit
Used to return from a function	Used to terminate the script
Returns control flow to calling script	Terminates script completely
Can return data from functions	Returns only an exit status code
Stays within the script after use	Exits the script process entirely
Only valid within functions	Valid anywhere in script or shell

In summary:

• return: returns from a function to calling script
• exit: terminates the entire script process

Knowing exactly whether you want to terminate an entire script early or just return from a function call is key to understanding which command to use.

Examples Comparing return and exit

Given the differences highlighted above, let‘s look at some examples to reinforce when and how to use return vs exit.

Example 1: Exit a script completely

If you want to terminate an entire Bash script process early, use exit:

if [ $1 == "" ]; then
  echo "Error: no argument supplied"
  exit 1
fi  

# Rest of script

This checks if the first argument is empty and exits the whole script with an error if so.

• Use exit because we want to stop the entire script
• An explicit exit status of 1 indicates an error

Example 2: Return data from a function

If you simply want to return data from a function, use return:

function double() {
  local x=$1
  local result=$(( $x * 2 )) 

  return $result
}

value=5
double_result=$(double $value)
echo "$value doubled is $double_result" #10

This allows the double function to do its work and return data to the main script via return.

• Use return because we only want to return from the function, not the whole script
• Allows data to be returned

Example 3: Exit early from a function

If you want to terminate just a function early, you can use return without a value:

function print_file() {
  if [ ! -f "$1" ]; then
    echo "Error: File not found"  
    return 1
  fi

  cat "$1"
}

print_file "some_missing_file"

Here return 1 allows the function to exit early with an error, without terminating the entire script.

So in summary:

• return to exit a function but continue script execution
• exit to terminate the entire script

Properly Handling Exit Status Codes

When terminating scripts or functions in Bash, properly handling exit status codes is important. Here are some tips:

• Use exit code 0 to indicate success
• Use non-zero (1-255) to indicate different failure errors
• Store and check exit codes in variables like $?
• Use logic like if [ $? -ne 0 ]; to check if an error occurred
• Provide context on errors with custom status codes

Handling exit codes properly ensures errors can be programmatically checked and handled correctly in Bash scripts.

Best Practices for Using return and exit

Based on all of the above, here are some best practices around properly using return and exit in Bash:

return Best Practices

• Only use return within function definitions
• Have functions return data or status codes when relevant
• Use return without values just to exit a function early
• Consider storing function return values in variables for further logic

exit Best Practices

• Avoid using exit inside functions (use return instead)
• Use exit to terminate scripts early when major errors occur
• Consider having a final exit 0 at the end of scripts to indicate success
• Make use of exit status codes to communicate errors/success
• Use common status conventions: 0=success, 1-255=different errors

Adhering to these best practices around return and exit will help you write better-organized Bash scripts.

Conclusion

The return and exit commands play related but distinct roles in Bash scripting. The return command returns control flow from a Bash function, optionally passing back a value. The exit command terminates the entire Bash script process and sets an exit status code.

Key differences include:

• return is for functions, exit is for whole scripts
• return returns data, exit returns status codes
• return stays in script, exitterminates entirely

Knowing when to use each one depends on whether you just want to exit a function or terminate the entire script. Use the concepts and best practices covered here to help master both of these important Bash commands.

Properly utilizing return and exit will allow you to write well-structured scripts that programmatically handle status codes and data returns from functions. Both are vital tools for any intermediate or advanced Bash scripter.