set-default-printer

#!/bin/bash

this_lpadmin="`command -v lpadmin`"
if [ ! -f "$this_lpadmin" ]; then
    this_lpadmin=/usr/sbin/lpadmin
fi
if [ ! -f "$this_lpadmin" ]; then
    echo "Error: lpadmin is missing. Install cups-client first and run this script ($SCRIPTPATH) with root privileges (otherwise using a user in the lpadmin group may work)."
    exit 1
fi

lpstat -p -d
# ^ such as:
# printer MFCL2740DW is idle.  enabled since Wed 21 Apr 2021 02:15:31 PM EDT
# printer TS6200USB is idle.  enabled since Thu 11 Mar 2021 11:08:46 AM EST
# no system default destination

# lpstat -a
# ^ such as:
# MFCL2740DW accepting requests since Wed 21 Apr 2021 02:15:31 PM EDT
# TS6200USB accepting requests since Thu 11 Mar 2021 11:08:46 AM EST
P_COUNT=`lpstat -a | wc -l`
echo "* usb_printerid /dev/usb/lp0:"
usb_printerid /dev/usb/lp0
for NUM in 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
do
    echo "* usb_printerid /dev/usb/lp$NUM:"
    usb_printerid /dev/usb/lp$NUM
    if [ $? -ne 0 ]; then
        echo "^ ignore that error, this is a test to find each printer"
        break
    fi
done

# cat /etc/printcap
# ^ such as:
# MFCL2740DW:\
#         :mx=0:\
#         :sd=/var/spool/lpd/MFCL2740DW:\
#         :sh:\
#         :lp=/dev/usb/lp0:\
#         :if=/opt/brother/Printers/MFCL2740DW/lpd/filter_MFCL2740DW:

# lpstat -a | cut -f1 -d ' '
# ^ such as:
# MFCL2740DW
# TS6200USB

# ^ See <https://superuser.com/a/1016825> on
#   <https://superuser.com/questions/177445/how-to-list-printer-names-acceptable-for-use-with-lpr>

# lpstat -d
# ^ get default such as:
# no system default destination
# ^ (return is still 0; output goes to standard output)
# ^ or:
# system default destination: MFCL2740DW
THIS_PRINTER="`lpstat -d`"
if [ ! -z "$1" ]; then
    SET_PRINTER="$1"
    # ^ such as MFCL2740DW
fi
if [ "@$THIS_PRINTER" = "@no system default destination" ]; then
    if [ -z "$SET_PRINTER" ]; then
        if [ $P_COUNT -gt 0 ]; then
            echo "Error: You must specify a printer but there are 0 printers listed above via 'lpstat -a'."
            exit 1
        elif
            echo "Error: You must specify a printer from the $P_COUNT listed above via 'lpstat -a'."
            exit 1
        fi
    else
        echo "* setting default printer to \"$SET_PRINTER\" as specified"
    fi
    $this_lpadmin -d $SET_PRINTER
    echo "* See also: https://`hostname -I | cut -d' ' -f1`:631/printers/$SET_PRINTER"
else
    echo "* The default is present, so this script will leave it as \"$THIS_PRINTER\"."
fi