When to use "or" and when to use || (double pipe) in Perl

Use || for assigning to a variable:

$a = $b || $c

Use "or" to control program flow:

$a = $b or die 'error: $b is unexpectedly false'

Mnemonic: || is a symbol like variables and numbers are, while "or" and "die" are both english words.

(source, source)

Difference between or and || in Perl

• Use || / && when you want to assign or return the result of the expression
• Use 'or' / 'and' when you only need to evaluate the truth or falseness, e.g. for an if statement

See also Perl Operator Precedence

Bash variable comparisons

...bash logic...

Note that integer and string comparison use a different set of operators.

integer comparison

-eq

is equal to

if [ "$a" -eq "$b" ]

-ne

is not equal to

if [ "$a" -ne "$b" ]

-gt

is greater than

if [ "$a" -gt "$b" ]

-ge

is greater than or equal to

if [ "$a" -ge "$b" ]

-lt

is less than

if [ "$a" -lt "$b" ]

-le

is less than or equal to

if [ "$a" -le "$b" ]

<

is less than (within double parentheses)

(("$a" < "$b"))

<=

is less than or equal to (within double parentheses)

(("$a" <= "$b"))

>

is greater than (within double parentheses)

(("$a" > "$b"))

>=

is greater than or equal to (within double parentheses)

(("$a" >= "$b"))

string comparison

=

is equal to

if [ "$a" = "$b" ]

==

is equal to

if [ "$a" == "$b" ]

This is a synonym for =.

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