[bash] How do I use su to execute the rest of the bash script as that user?

I've written a script that takes, as an argument, a string that is a concatenation of a username and a project. The script is supposed to switch (su) to the username, cd to a specific directory based upon the project string.

I basically want to do:

su $USERNAME;  
cd /home/$USERNAME/$PROJECT;  
svn update;  

The problem is that once I do an su... it just waits there. Which makes sense since the flow of execution has passed to switching to the user. Once I exit, then the rest of the things execute but it doesn't work as desired.

I prepended su to the svn command but the command failed (i.e. it didn't update svn in the directory desired).

How do I write a script that allows the user to switch user and invoke svn (among other things)?

It's not possible to change user within a shell script. Workarounds using sudo described in other answers are probably your best bet.

If you're mad enough to run perl scripts as root, you can do this with the $< $( $> $) variables which hold real/effective uid/gid, e.g.:

#!/usr/bin/perl -w
$user = shift;
if (!$<) {
    $> = getpwnam $user;
    $) = getgrnam $user;
} else {
    die 'must be root to change uid';
}
system('whoami');

Much simpler: use sudo to run a shell and use a heredoc to feed it commands.

#!/usr/bin/env bash
whoami
sudo -i -u someuser bash << EOF
echo "In"
whoami
EOF
echo "Out"
whoami

(answer originally on SuperUser)

Here is yet another approach, which was more convenient in my case (I just wanted to drop root privileges and do the rest of my script from restricted user): you can make the script restart itself from correct user. Let's suppose it is run as root initially. Then it will look like this:

#!/bin/bash
if [ $UID -eq 0 ]; then
  user=$1
  dir=$2
  shift 2     # if you need some other parameters
  cd "$dir"
  exec su "$user" "$0" -- "$@"
  # nothing will be executed beyond that line,
  # because exec replaces running process with the new one
fi

echo "This will be run from user $UID"
...

You need to execute all the different-user commands as their own script. If it's just one, or a few commands, then inline should work. If it's lots of commands then it's probably best to move them to their own file.

su -c "cd /home/$USERNAME/$PROJECT ; svn update" -m "$USERNAME" 

This worked for me

I split out my "provisioning" from my "startup".

 # Configure everything else ready to run 
  config.vm.provision :shell, path: "provision.sh"
  config.vm.provision :shell, path: "start_env.sh", run: "always"

then in my start_env.sh

#!/usr/bin/env bash

echo "Starting Server Env"
#java -jar /usr/lib/node_modules/selenium-server-standalone-jar/jar/selenium-server-standalone-2.40.0.jar  &
#(cd /vagrant_projects/myproj && sudo -u vagrant -H sh -c "nohup npm install 0<&- &>/dev/null &;bower install 0<&- &>/dev/null &")
cd /vagrant_projects/myproj
nohup grunt connect:server:keepalive 0<&- &>/dev/null &
nohup apimocker -c /vagrant_projects/myproj/mock_api_data/config.json 0<&- &>/dev/null &

Use a script like the following to execute the rest or part of the script under another user:

#!/bin/sh

id

exec sudo -u transmission /bin/sh - << eof

id

eof

Inspired by the idea from @MarSoft but I changed the lines like the following:

USERNAME='desireduser'
COMMAND=$0
COMMANDARGS="$(printf " %q" "${@}")"
if [ $(whoami) != "$USERNAME" ]; then
  exec sudo -E su $USERNAME -c "/usr/bin/bash -l $COMMAND $COMMANDARGS"
  exit
fi

I have used sudo to allow a password less execution of the script. If you want to enter a password for the user, remove the sudo. If you do not need the environment variables, remove -E from sudo.

The /usr/bin/bash -l ensures, that the profile.d scripts are executed for an initialized environment.

Use sudo instead

EDIT: As Douglas pointed out, you can not use cd in sudo since it is not an external command. You have to run the commands in a subshell to make the cd work.

sudo -u $USERNAME -H sh -c "cd ~/$PROJECT; svn update"

sudo -u $USERNAME -H cd ~/$PROJECT
sudo -u $USERNAME svn update

You may be asked to input that user's password, but only once.