Change Default Group in Script

Question

Is it possible to change a user's default group inside a script for the duration of that script's execution?

I need to generate files in a script that have the proper user and group but my user's primary group is not who should own the resultant output.

$ groups
groupa groupb

$ ./myscript.sh

$ ls -l
-rw-r--r-- 1 me groupa     0 Sep 17 09:42 myscript_output.txt

But I want "groupb".

myscript.sh:

#!/bin/bash

touch "myscript_output.txt"

Answer 1

Try the newgrp command, which changes the primary group of a user into another group of which that user is a member:

#!/bin/bash

newgrp groupb << END
    touch "myscript_output.txt"
END

Answer 2

The group can be set from a script. It only requires the "if" statement below. The group is checked and if it is incorrect, then the script is restarted with the sg command Nate mentioned.
A check for looping is employed(just in case the unforeseeable happens.)

To use, just change the group from "wheel" to the desired. Replace the "DEMO" section with the regular code.

Read on, below(after the script.)

#! /bin/sh
#    
# If the group(set with NEEDGRP) is already correct, or this code has already
# run, then this section is skipped and the rest of the 
# script is run; otherwise sg is called to restart the script with the
# desired group.  Assumes the command "id -ng" returns the group.

if ! [ "${SBREADY:=false}" = true -o $(id -ng) = ${NEEDGRP:=wheel} ] ; then
    export SBREADY=true
    exec sg $NEEDGRP "$0" "$@"
fi

# ---------------------- DEMO: CUT HERE ---------------------------
# This is a demonstration of creating files.
echo HELLO my group is $(id -ng), GID=$(id -g)
# NOTE: files are created with the current group only if the directory
# is not sgid.
# Show current directory and permissions on it
echo
pwd -P
ls -ld .
echo
# Create and list some new files, the remove them.
touch my-$$.{a,b,c}
echo Created  my-$$.{a,b,c}...
ls -l  my-$$.{a,b,c}
echo
rm -v  my-$$.{a,b,c}

Following are printouts of some tests run in order to explain why just changing groups my not be sufficient to ensure files have the right group ownership. Directory permissions also come into play.

This first log is the output from ruining in a regular directory. The script is run as user frayser, and group frayser. Files are created with the desired group. Compare to the next listing:

frayser@gentoo ~/src/Answers $ (cd /tmp; $OLDPWD/set-group.sh)
HELLO my group is wheel, GID=10

/tmp
drwxrwxrwt 16 root root 976 Sep 24 04:45 .

Created my-19201.a... my-19201.b... my-19201.c...
-rw-r----- 1 frayser wheel 0 Sep 24 04:53 my-19201.a
-rw-r----- 1 frayser wheel 0 Sep 24 04:53 my-19201.b
-rw-r----- 1 frayser wheel 0 Sep 24 04:53 my-19201.c

removed `my-19201.a'
removed `my-19201.b'
removed `my-19201.c'

Now this next run happens in a director that is sgid "conman" because as a policy, Configuration Management is given group ownership of all src directories. NOTE: The files inherit the group of the directory.

frayser@gentoo ~/src/Answers $ ./set-group.sh 
HELLO my group is wheel, GID=10

/usr/lucho/src/frayser/practice
drwxr-s--- 6 frayser conman 768 Sep 24 04:51 .

Created my-19214.a... my-19214.b... my-19214.c...
-rw-r----- 1 frayser conman 0 Sep 24 04:54 my-19214.a
-rw-r----- 1 frayser conman 0 Sep 24 04:54 my-19214.b
-rw-r----- 1 frayser conman 0 Sep 24 04:54 my-19214.c

removed `my-19214.a'
removed `my-19214.b'
removed `my-19214.c'
frayser@gentoo ~/src/Answers $ 

Because of directory permissions, it may be necessary for a script to explicitly set permissions and ownership.