Check out example codes for ".sh script: check if file exist". It will help you in understanding the concepts better.

Code Example 1

#The most readable option when checking whether a file exist or not is to use the
#test command or the old '[' or the new '[[' in combination with the if statement.
#Any of the snippets below will check whether the /etc/resolv.conf file exists:

FILE=/etc/resolv.conf
if test -f "$FILE"; then
    echo "$FILE exist"
fi

# or

FILE=/etc/resolv.conf
if [ -f "$FILE" ]; then
    echo "$FILE exist"
fi

# or

FILE=/etc/resolv.conf
if [[ -f "$FILE" ]]; then
    echo "$FILE exist"
fi

Code Example 2

#!/bin/bash
if [ -e x.txt ]
then
    echo "ok"
else
    echo "nok"
fi

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