Run: sh assumeCLI.sh arn:aws:iam::<TrustingAccount-ID>:role/assumeRole-RoleName
It's exactly like Switching Roles but using the CLI. Our already configured account will try to assume a role under the specified profile we set in the PROFILE variable in the script, of the "Trusing" Account (Reference for how to set this up between accounts), for the time duration in seconds we specify in SESSION_DURATION variable (This is limited by the "Trusting" account's Role policy).
This will spawn a (Very unstable) shell with the Trusting account ID, for example: [AWS 123456789012> ]. It can receive bash commands or whatever we already had in our Environment since we're just running Bash with AWS CLI assuming a different role. It's very unstable and doesn't handle any piping so it's better to spawn a shell by /bin/bash manually, once we assumed the role
Note: After assuming the role, customize the shell prompt temporarilly by export PS1='New_Prompt> ' so we don't forget we're on a "special" shell.
How it looks:
[AWS 123456789012> ] aws s3 ls
[AWS 123456789012> ] aws sts get-caller-identity
[AWS 123456789012> ] /bin/bash
user@hostname:~$ export PS1='ASSUMED_ROLE >'
ASSUMED_ROLE > aws lambda list-functions | jq ".Functions | group_by(.Runtime)|[.[]|{ runtime:.[0].Runtime, functions:[.[]|.FunctionName] } ]"