I believe that people who have learned how to program know what variables are, and at first it can be understood as an alias that points to the data stored in memory.

Although Bash shell and Bourne shell come with many system variables, we could also customize variables ourselves.

Check System Variables

We can view existing system variables in the following ways:

Show System Variables with set

We can pass the output of the set command to the less or more command to see all the variables from the beginning of the output result:

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set | less    # or set | more

We can type the q key to exit the the less command’s interactive mode.

Show Environment Variables with printenv

We can use the printenv command to output the current environment variables:

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printenv

Show Environment Variables with env

We can also use the env command to output the current environment variable:

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env

The output is quite similar to printenv command’s output result.

Show Shell Variables with export

We can use the export command to output shell variables:

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export    # equivalent to export -p

Show Value of a Variable with echo

We can use the echo command to output variable’s value to the screen:

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echo $PATH    # prints PATH variable's value

Variable Operations

Create a Variable for the Current Shell

We can create a variable directly by the name=value format:

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MY_NAME='I am Dong'
echo $MY_NAME

We can try to access this variable in the offspring shell:

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bash    # switch into a child shell
echo $MY_NAME

From the screenshot, we can see that we can’t use the MY_NAME variable created by the parent shell in the child shell.

Set a Variable as Shell Variable

The variable we just created can only be accessed in the current shell, what should we do if we want to access it in our offspring shell?

We can set the created variable to the shell variable through the export command in the shell where the variable was created, so that all descendants of the shell can access the variable:

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export MY_NAME
bash
echo $MY_NAME

Reference a Variable in a String

We can reference the created variable in a string:

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echo "$MY_NAME, how are you?"

Create an Array Variable

We can create an array variable that contains multiple elements:

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MY_NAME[1]="Dong"
MY_NAME[2]="Chen"
echo "My first name is ${MY_NAME[1]}, my last name is ${MY_NAME[2]}"

Remove a Variable

We can delete a variable or an array variable through the unset command:

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echo ${MY_NAME[1]} ${MY_NAME[2]}: $MY_NAME
unset MY_NAME
echo ${MY_NAME[1]} ${MY_NAME[2]}: $MY_NAME

References 4.3.1 The Set Builtin, 19.3 printenv: Print all or some environment variables, 23.2 env: Run a command in a modified environment

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