You'll need a shell script/function that iterates over the assigned range from UIDLOW to UIDHIGH and stops when it finds a free UID.
How about something like this:
#!/bin/sh
iterateUID () { UIDLOW="$1" UIDHIGH="$2"
ASSIGN_UID=none CURR_UID="$UIDLOW"
while [ "$CURR_UID" -lt "$UIDHIGH" ]; do grep -q "^[^:]*:[^:]*:$CURR_UID:" /etc/passwd if [ $? = 0 ]; then # UID is in use CURR_UID=$( expr "$CURR_UID" + 1 ) continue else # found an UID ASSIGN_UID="$CURR_UID" break fi done if [ "$ASSIGN_UID" = "none" ]; then # could not find an UID echo "ERROR: could not find a free UID. Range full?" >&2 echo "none" return 1 fi
echo "$ASSIGN_UID" return 0 }
If the function manages to find a suitable UID, it outputs it to stdout. If no free UID was found, it outputs an error message to stderr, sends the string "none" to stdout and returns with an error code.
After defining this function near the beginning of your script, you can use it like this:
# to find an UID between 100 and 200 USER_UID=$( iterateUID 100 200 ) if [ $? -gt 0 ]; then # things to do if no UID found fi
There are probably other, more resource-saving ways to do this.
For GIDs, change the variable names as appropriate and the "grep" line to:
