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This is tricky because Bash variables are string-like by default, but conditional expressions can compare values as either text or integers. So we have separate operators for strings and integers.

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String Operators

String ordering is affected by the current locale. The clearest way to use < and > is inside [[ ]]:
They can also be used with [ ], but the operators must be escaped so the shell does not treat them as redirections:
Inside [ ], quoting variable expansions prevents word splitting and pathname expansion and safely handles empty values:
Inside [[ ]], quoting the left-hand variable is usually unnecessary because those expansions do not undergo word splitting or pathname expansion:

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Integer Operators

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File Operators

These check properties of files and directories. We use them a lot in scripts that interact with the filesystem:

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Logical Operators

We combine them to build more complex conditions. Example:
${#var} gives us the string length directly. Using echo "$var" | wc -m would also count the newline printed by echo.