Chapter 9: Conditional Logic

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if - else Statement

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Below is the syntax:

1 if [[ condition ]]; then
2 	code
3 elif [[ condition ]]; then
4 	code
5 else
6 	code
7 fi

There must be a space between the opening and closing brackets and the condition you write. Otherwise, the shell will complain of error.
There must be space before and after the conditional operator (=, ==, <= etc). Otherwise, you’ll see an error like "unary operator expected".

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Below are useful unary operators we can use with if statements:

Unary Operator	Description
`-e file_name`	True if file exists
`-d dir_name`	True if specified directory exists
`-h file_name`	True if the file exists & is a symlink
`-s file_name`	True if file exists and size > 0
`-r file_name`	True if file exists and is readable
`-w file_name`	True if file exists and is writable
`-x file_name`	True if file exists and is executable
`-v var_name`	True if variable is set (even if it has an empty value
`-z string_var`	True if length of string is zero
`-n string_var`	True if length of string is non-zero

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Operator	Behavior
;	Allows you to chain commands together. It will run all commands regardless of whether they succeeded or failed.
&&	Similar to semicolon, but next command runs ONLY IF previous command succeeded
		Next command is run ONLY IF previous command did NOT succeed
&	All the above (;, &&,	) will wait for execution of first command, then run 2nd command and so on. However, & will start the execution of first command and then it will start the execution of 2nd command, regardless of whether execution of first command was completed or not.
-z “$var_name”	Checks if the variable is set to nothing. Equivalent code would be “$my_var” = “”
[ -z “$my_var” ]	This is an implicit if statement - you don’t need if, then, else, fi etc.

1 # Using if-else
2 if [ "$EDITOR" = "" ]; then
3 	EDITOR=nano
4 fi
5 
6 # Not using if-else
7 [ -z "$EDITOR" ] && EDITOR=nano

Up next

Chapter 10: Expansions and Substitutions