`test` - evaluate an expression

test EXPR  |  [ EXPR ]

Options you will use

-f FILE / -d FILE / -e FILE: Regular file / directory / exists.
-r -w -x FILE: Readable / writable / executable by you.
-s FILE: Exists and is not empty.
-z STR / -n STR: String is empty / not empty.
STR = STR / !=: String comparison.
N -eq -ne -lt -le -gt -ge N: Integer comparison.

Gotchas

In bash prefer [[ ]]: it does not word-split, it has =~ for regex and && || instead of -a -o. Use [ ] only when the script must run under /bin/sh.

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