My shell script exits if I try to mount from within the script an already mounted drive How can I make my shell script resume?

You have to run a check to see if is mounted already, then determine to mount based on the conditional.

In your case, it's /dev/sdc.

#!/bin/bash

# Function to check state of the USB drive
function is_usb_ready() {
    # Check if the USB drive exists
    if lsblk -d -o NAME | grep -q "^sdc$"; then
        # Check if the USB drive is not mounted
        if ! mount | grep -q "/dev/sdc"; then
            return 0  # Plugged in and not mounted
        fi
    fi
    return 1  # Not plugged in or already mounted
}

# Call function and perform the proper task based on its return value
if is_usb_ready; then
    echo "USB drive exists, but not mounted. Mounting drive."
    sudo mount /dev/sdc /your/mount_point/here
else
    echo "USB drive is not plugged in or already mounted."
fi

# Rest of your code

exit 0

The first grep will check if the drive's already plugged in, and the second grep will determine if it's mounted. If not, do so, otherwise proceed.

This is a way to do the check via destination instead of source. Since both don't cover each other's potential scenario's, I'll put both, and leave the decision up to the user.

#!/bin/bash

mountpoint="/your/mount_point/here"

function is_usb_ready() {
    # Check for filesystem at the mount point
    if mountpoint -q "$mountpoint"; then
        echo "The USB drive is already mounted at $mountpoint."
        return 1  # Already mounted
    else
        # If not mounted, check for device
        if lsblk -d -o NAME,TRAN | grep -q "sdc.*usb"; then
            return 0  # USB drive exists and is not mounted
        fi
    fi
    return 1  # USB drive is not plugged in
}

# Call function and perform the proper task based on its return value
if is_usb_ready; then
    echo "USB drive exists, but not mounted. Mounting drive."
    # Mount the USB drive
    sudo mount /dev/sdc "$mountpoint"
else
    echo "USB drive is not plugged in or already mounted."
fi

# Rest of your code

exit 0

It's waltinator for the win! A very simple solution.

#!/bin/bash

mountpoint="/your/mount_point/here"

if sudo grep -q "$mountpoint" /etc/mtab; then
    echo "The USB drive is already mounted at $mountpoint."
else
    echo "Mounting the USB drive."
    sudo mount /dev/sdc "$mountpoint"
fi

# Rest of your code

exit 0

For the benefit of future readers who may be running a different OS, here is a more general solution that supports nested mount points:

mountpoint="/your/mount_point/here"
mountdevice="/dev/sdc"

if mount | grep -Fq " $mountpoint "; then
    printf "%s is already in use.\n' "$mountpoint"
else
    printf 'Mounting "%s" on "%s".\n' "$mountdevice" "$mountpoint"
    sudo mount "$mountdevice" "$mountpoint"
fi

# Rest of your code

findmnt can make detecting the mounted/not-mounted state easy, and ordinary shell || or && operators can decide whether to invoke the mount command:

#!/usr/bin/env bash

mount_point='/your/mount/point'

# pre-mount commands go here

# is the filesystem already mounted?  if not, mount it
findmnt -m -M "${mount_point}" || mount "${mount_point}"

# post-mount commands go here

Add redirection around findmnt if you don't want to see its messages. The -n option can get rid of the output's header line. The -m option tells the command to look in the /etc/mtab file for mounted filesystems, and the -M option gives the mount point's path to look for. When the path is mounted, the exit status is 0 (success), and non-zero otherwise (usually 1).

You can use more verbose if foo; then bar; else baz; fi clauses instead of the shortcut operators if you prefer.