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shell levels and subshells

summary

����This subchapter looks at shell levels and subshells.

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shell levels and subshells

����This subchapter looks at shell levels and subshells.

����As you advance to more sophisticated operations, such as the material on substitutions and variables, you will find that the bash shell has levels.

����Every program, command, or script that you run can have and modify variables. How does Unix/Linux keep programs, commands, and scripts from interfering with each others� variables?

����The answer is shell levels. Each time the shell starts a program, command, or script, it starts it in a subshell. The subshell has its own variables and environment. It can�t interfere with any other program, command, or script, even if they use the same name for a variable.

����Most of the time this system is great It can lead to frustration when attempting to share variables (you will have to use some work-around). And it can trip up beginners who aren�t expecting this.

����Technically, each subshell is run as a child process. We will discuss this more in the subchapter on processes.

shell levels

����You can use the $SHLVL built-in variable to detemrine the current shell level.

����In the following example, we will start up bash in a subshell. Yes, you can start-up another copy of bash from the current copy of bash.

����This example includes some materials that will be covered in more detail in later subchapters.

����$ echo $SHLVL
����1
����$ bash
����$ echo $SHLVL
����2
����$ exit
����$ echo $SHLVL
����1
����$

����The first echo command shows the current shell level (1).

����The bash command starts up another copy of bash in a subshell.

����The second echo command shows the subshell level (2).

����The exit command closes down the copy of bash running in the child process and returns us to the main copy of bash.

����The third echo command shows the original shell level (1).

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