The difference between "echo" and "export" an environment variable?

Question

The environment is Ubuntu 18.

If I add one line JAVA_HOME="/usr/java11" in /etc/environment, and execute source /etc/environment , I can echo this environment variable:

echo $JAVA_HOME
/usr/java11

But if I try to get it from export, this variable is not in the list:

export | grep JAVA_HOME
--result is blank--

Then I use export $JAVA_HOME=/usr/java8 to export a variable (notice here is java8, not java11), I can have:

export |grep JAVA_HOME
declare -x JAVA_HOME="/usr/java8"

Now, I can still echo $JAVA_HOME as /usr/java11:

echo $JAVA_HOME
/usr/java11

Question: what is the difference of echo $variable and export | grep JAVA_HOME?

I try a simple python program, os.environ.get("JAVA_HOME") returns /usr/java8 from the export, not the echo.

Answer 1

The issue here is not really the difference between echo and export, but rather the difference between an environment variable and a simple shell variable (and also about how the /etc/environment file is normally used).

In particular, although /etc/environment happens to contain lines of the form name=value that are valid as POSIX shell variable assignments, its primary purpose (in a modern Linux system) is to be read by the pam_env module during initialization of a user's session - it is pam_env that exports them to the user's environment.

When you source /etc/environment into your shell, there's no special magic that tells the shell that the assignments are those of environment variables (which are exported to the environment, and hence inherited by sub-processes) rather than regular shell variables (which are just available in the current shell scope).

Next time you log in, pam_env will do its magic and JAVA_HOME will then appear in the output of export | grep JAVA_HOME.

See also

• Difference between shell variables which are exported and those which are not in bash

• What scopes can shell variables have?

Answer 2

echo and export are very different commands in the first place.

• echo displays text. In echo $JAVA_HOME, the shell will substitute $JAVA_HOME with the contents of the shell variable JAVA_HOME it it is defined. Otherwise, $JAVA_HOME will return an empty string.
• export not only creates the variable if it does not already exist, it also marks the variable for automatic export to the environment of subsequently executed commands. Thus, the variable will also be available in the environment of any sub shell or sub process rather than in your current shell only.

In /etc/environment, environment variables are registered with the syntax of a variable assignment. The content of /etc/environment in a default Ubuntu install indeed can be executed. So if you execute the line you included:

`JAVA_HOME=/usr/java11`

Then, all that will do is assign the shell variable PATH the current value.

However, because you included the variable in /etc/environment, it should be effectively be exported during the next startup of your system. Then, it should exist in export and show up in echo $JAVA_HOME in the first terminal you open. So what you currently observe is because you did not yet restart the machine after modifying /etc/environment (and did not export the variable in other ways).

Answer 3

Building on the other answers here, some commands that parallel export but for other categories of variables are set (which works for e.g. VARIABLE=value then set | grep VARIABLE) and env

Each of these three commands, when given no arguments, prints a list of variables; which variables they will print has to do with the kinds of variables the command manages.

See What's the difference between set, export and env and when should I use each?

echo $VARIABLE always works since all kinds of variables can be read as $VARIABLE; however, this means it gives you no information on what kind of variable it is or where it comes from.

Answer 4

echo is a command for printing text and variables to stdout (or redirect).

export lists the current exported variables in the shell.

This Stack Overflow thread explains why you would use export much better than I can: What does "export" do in shell programming? It does a great job explaining what export is for.