Depend on version you using :
Bash is enabled by running the feature bash-shell command.
The run bash command loads Bash and begins at the home directory for the user.
The following examples show how to enable the Bash shell feature and how to run Bash.
switch# config t
switch(config)# feature bash-shell
switch# run?
run Execute/run program
run-script Run shell scripts
switch# run bash?
bash Linux-bash
switch# run bash
bash-4.2$ whoami
admin
bash-4.2$ pwd
/bootflash/home/admin
bash-4.2$