Conditionals are the primary means of making decisions in programming, especially Bash scripting. Conditionals in Bash are with the [ and [[ built-ins, which are also commands! It makes reasoning about conditionals challenging at first, but there are some useful flags available:

-a file

True if file exists.

-b file

True if file is an existing block device.

-c file

True if file is an existing character device

-d file

True if file is an existing directory.

-e

See -a.

-f file

True if file is an existing normal file.

-h file

True if file is a symbolic link (symlink).

-r file

True if file exists and is readable.

-s file

True if file exists and has a size greater than 0.

-w file

True if file exists and is writable.

-x file

True if file exists and is executable.

-v arg

True if arg is a set variable. Bash 4.2+ only.

• man bash — See "CONDITIONAL EXPRESSIONS"