1. How to check if specific bash command is available:

if command -v CMD &> /dev/null; then echo "it exists"; fi

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2. How to check if specific file exists:

if [ -f /tmp/file ]; then echo 1; fi

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3. How to check if scecific directory exists:

if [ -d /tmp ]; then echo 1; fi

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4. How to check if variable is empty

if [ -z "$variable" ]; then echo "\$variable is empty"; fi

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5. How to check if variable is not empty

if [ ! -z "$variable" ]; then echo "\$variable is not empty"; fi

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