Executing bash shell scripts

Question

I am reading an article on executing bash shell scripts.

Method 1: Create a directory for your shell scripts and add the directory to the contents of the PATH variable, so that you can execute the script as shown below. When this executes, the variables, functions and aliases created in this subshell are only known to the particular bash session of that subshell. When that shell exits and the parent regains control, everything is cleaned up and all changes to the state of the shell made by the script, are forgotten.

$ hello_world.sh

Method 2: A script can also explicitly be executed by a given shell. The specified shell will start as a subshell of your current shell and execute the script.

$ sh script_name.sh

Method 3: If you don't want to start a new shell but execute the script in the current shell, you source it:

source script_name.sh

I really don't understand the benefits and disadvantages of each of these methods. Can anyone clarify?

Answer 1

sh

This executes the program called hello_world.sh in the program search path (PATH). If that program happens to start with a shebang line, that line indicates that the program must be passed to an interpreter in order to be executed; otherwise the program must be loaded as native code. The shebang line is read by the kernel. For example, if the first line of hello_world.sh is #!/bin/sh, then the kernel launches /bin/sh and passes it hello_world.sh as an argument.

If hello_world.sh neither begins with a shebang nor is valid native code, then the kernel will return an error when you try to execute it. If you're doing that from a shell, the shell will notice the error and try to execute hello_world.sh as a shell script (like the second form below).

sh script_name.sh

This runs the program called sh found in the PATH, with script_name.sh passed as an argument. The program sh, assuming that it's the usual shell, will open the file called script_name.sh in the current directory and interpret it. If the script starts with a shebang line, it's ignored. The script doesn't need to be executable.

. script_name.sh
source script_name.sh

When you execute this in a shell, either of these instructions tells the running shell to interpret the commands in the specified file. The script isn't executed as a separate program, but as part of the existing shell process. Thus the script can affect the current directory, the environment, … of the running shell. The builtin commands . and source are synonymous, except that . only searches script_name.sh in the PATH whereas in some shells source also looks in the current directory.

Answer 2

`hello_world.sh`

It use interpreter from script... Maybe it sh, but may be some other. Also, need executable bit. And find script from $PATH if dont have in current directory

`sh hello_world.sh`

Start new examplair of sh programm with this shell

`source script_name.sh`

Don't used early... Maybe somethink like inclusion, if yes - you can use current shell state in script (unlike first 2)

Answer 3

There are situations where it is your aim to modify the current environment. This is e.g. the case when reading variables or function / alias definitions from a config file (like the shell startup files like ~/.bashrc, ~/.aliases).

Calling a shell with the script as argument in certain situations has the advantage that you need not make the file executable and that you can easily modify the execution parameters:

bash -vx ./my/script.sh

You also need this when checking how different shells execute a script:

bash -vx ./my/script.sh

vs.

sh -vx ./my/script.sh

vs.

zsh -vx ./my/script.sh