Change Default Group in Script - linux

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 sg command can do this pretty well.
#!/bin/bash
sg groupb "touch myscript-output.txt"

Answer 3

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.

Answer 4

Normally that can be accomplished by applying to a program the modifications:
chgrp groupb myprog
chmod g+s myprog
But that works with normal programs - not with the shell scripts (for security reasons). For a shell script there is no other way (at least I'm not aware (*)) other than from inside script itself to call the chgrp:
#!/bin/bash
FNAME="myscript_output.txt"
GRP=groupb
touch $FNAME
chgrp $GRP $FNAME || { echo 2>&1 "Can't change group of $FNAME to $GRP"; exit 1; }
(*) Some people for the purpose write a tiny wrapper C program. But that is kludgy. Search net for "setuid shell scripts" - there would be lots of such example C programs and replace most commonly found there setuid(0) with getgrnam() + setgid().