Runing a command without inheriting parent's environment

Question

Is there a way to run a command "as if" it is in a new login session?

I've already tried env -i. However, I don't want to deal with various ENV variables I have to set or unset.

I've also tried bash -c "some command" and bash -l -c "some commmand", but they all copy the current environment.

The closest I have come is a not-so-clean solution:

ssh me@localhost "some command"`

Use `/bin/bash --login` to get that behavior. I use it e.g. to get a proper `$PATH`. — Daniel Beck, Aug 04 '11 at 14:50
That is the equivalent of `/bin/bash --l`, which I already tried. It copies the original environment. Try it: `export SOME_VAL=something`. Then `/bin/bash --login`. Then `env | grep SOME_VAL`. The value will be there. — dgo.a, Aug 04 '11 at 15:03

Elliott Slaughter · Accepted Answer · 2018-06-22T20:43:58.270

Here's an answer that doesn't require sudo privileges or the user's password, but still provides an environment like what you'd get on a fresh login.

env -i HOME="$HOME" bash -l -c 'your_command'

Example:

$ export ABC=123
$ env -i HOME="$HOME" bash -l -c 'env' | grep ABC
$ env HOME="$HOME" bash -l -c 'env' | grep ABC
ABC=123

Breaking this down for explanation:

env -i HOME="$HOME": Clears the environment. The -i sets up an empty environment with no variables whatsoever. This is problematic because it means that if you try to naively run bash -l it won't load your .bash_profile etc. because HOME isn't set. To mitigate this, we explicitly pass HOME="$HOME", creating an environment where HOME (and only HOME) is set.
bash -l -c ...: Runs the desired command in a login shell. You'll want a login shell for this because we're starting from a clean environment and need to reload everything.

Notably:

This doesn't require sudo privileges (the sudo version does).
This doesn't require typing the user's password (the su version does).
This doesn't require running an SSH server and having a passwordless key that can be used to log back in to the machine (the ssh version does).

score 13 · Answer 2 · answered Aug 04 '11 at 15:17

13

su -l $USER

sudo -u $USER -i

For something even more aggressive try env -i bash, but that unsets everything including $HOME and $TERM.

answered Aug 04 '11 at 15:17

u1686_grawity

2

Thanks! The second command gives me exactly what I want. I searched for more than 2 hours and did not find anything close to your reply. Based on your reply, I went back to `man sudo` and found: "if the target user is the same as the invoking user, no password is required." (The `su` seems to always asks for a password.) I'm such a fool to have overlooked something so simple in the first paragraph of a `man` page :( I avoided sudo in the first place because I assumed it always asked for a password. Thanks again! – dgo.a Aug 04 '11 at 16:15
1

just a hint for others... if your logged in as `$USER` you will need to login as root and then `sudo -u ...` as your user. If you just do `sudo -u` as the user you will inherit. – Adam Gent Aug 04 '13 at 21:48
this does not work, obviously, if you do not have `sudo` access. Is there an alternative method that does not require sudo? – user5359531 Jun 12 '17 at 14:34
@user5359531: Yes – `su -l $USER`. – u1686_grawity Jun 12 '17 at 14:49
1

that asks for a password; I am logged in via ssh key authentication, so I dont even know what the password is. And a password prompt is going to kill script-ability. So far its looking like `ssh $USER@localhost ` is working better – user5359531 Jun 12 '17 at 15:30
@grawity Do you have a better answer to user5359531's question? – Melab Sep 04 '17 at 00:27
@Melab: See [this answer](https://superuser.com/a/1333539/143224). – Elliott Slaughter Jun 22 '18 at 20:34

2 Answers2