Setting environment variable for one program call in bash using env
bash, environment, environment-variables, unix
Solution
It's because in your first case, your current shell expands the `$HELLO` variable before running the commands. And there's no `HELLO` variable set in your current shell.
env HELLO='Hello World' echo $HELLO
will do this:
- expand any variables given, in this case `$HELLO`
- run env with the 3 arguments `'HELLO=Hello World'`, `'echo'` and `''` (an empty string, since there's no `HELLO` variable set in the current shell)
- The `env` command will run and set the `HELLO='Hello World'` in its environment
- `env` will run `echo` with the argument `''` (an empty string)
As you see, the current shell expanded the `$HELLO` variable, which isn't set.
HELLO='Hello World' bash -c 'echo $HELLO'
will do this:
- set the variable `HELLO='Hello World` for the following command
- run bash with the 2 arguments `'-c'` and `'echo $HELLO'`
- since the last argument is enclosed in single quotes, nothing inside it is expanded
- the new bash in turn will run the command `echo $HELLO`
- To run echo `$HELLO` in the new bash sub-shell, bash first expands anything it can, `$HELLO` in this case, and the parent shell set that to `Hello World` for us.
- The subshell runs `echo 'Hello World'`
If you tried to do e.g. this:
env HELLO='Hello World' echo '$HELLO'
- The current shell would expand anything it can, which is nothing since `$HELLO` is enclosed in single quotes
- run env with the 3 arguments `'HELLO=Hello World'`, `'echo'` and `'$HELLO'`
- The env command will run and set the `HELLO='Hello World'` in its environment
- env will run echo with the argument `'$HELLO'`
In this case, there's no shell that will expand the `$HELLO`, so `echo` receives the string `$HELLO` and prints out that. Variable expansion is done by shells only.
Problem
I am trying to invoke a shell command with a modified environment via the command `env`. According to the manual ``` env HELLO='Hello World' echo $HELLO ``` should echo `Hello World`, but it doesn't. If I do ``` HELLO='Hello World' bash -c 'echo $HELLO' ``` it prints `Hello World` as expected (thanks to this answer for this info). What am I missing here?