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There are two ways for Linux to execute Scripts. The main difference lies in whether to establish subshell or not.

1. Source filename or. Filename

Do not create subshell, read and execute commands in filename in the current shell environment, which is equivalent to sequential execution of commands in filename

2. Bash filename or. / filename

Create a subshell and create a new child shell to execute the command in the filename in the current bash environment. The child shell inherits the variables of the parent shell, but the child shell cannot use the variables of the parent shell unless you use export [Note: this is similar to the namespace, even to the functions in c]

The properties inherited by the child Shell from the parent Shell are as follows:

Current working directory environment variables standard input, standard output, and standard error output signals ignored by all open file identifiers

The attributes that the child Shell cannot inherit from the parent Shell are summarized as follows:

Signal processing 3. $(commond) that Shell variables are not ignored except for environment variables and variables defined in .bashrc files

Its function is to make the command execute in the child shell

4. Commond

It's similar to $(commond). The "`" symbol here is a prime (anti-single quotation mark), not a single quotation mark, but the key below the Esc key on the keyboard. ]

5. Exec commond

Replacing the current shell does not create a new process. The pid of the process remains the same: the built-in command exec of shell will not start the new shell, but will replace the current shell process with the command to be executed, and clean up the environment of the old process, and other commands after the exec command will no longer be executed. When exec ls is executed in a shell, the current directory is listed, and the shell exits itself. (subsequent commands are no longer executed) because the shell has been replaced with a process that only executes the ls command, the end of execution naturally exits. You can use sub shell to avoid this effect when needed. Generally, put the exec command into a shell script, call the script with the main script, and use bash a.sh at the call point (a.sh is the script in which the command is stored). This will create a sub shell for a.sh to execute. When executed to exec, the subscript process will be replaced with the corresponding exec command.

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