What are the advantages and disadvantages of using the test command over the [ ] operator?

2 The [ ] operator

The [ ] operator is a syntactic sugar for the test command, meaning that it is equivalent to the test command but with a different syntax. The [ ] operator encloses the arguments in square brackets and requires spaces around them. For example, you can use the [ ] operator to check if a variable is empty or not:

[ -z "$var" ]

The [ ] operator is more concise and readable than the test command, and it can be used in the same way as the test command. However, the [ ] operator has some limitations, such as being less portable and compatible with some shells, requiring more care with quoting and escaping, and not supporting arithmetic expressions.

3 The [[ ]] operator

The [[ ]] operator is an extension of the [ ] operator, meaning that it is similar to the [ ] operator but with some additional features. The [[ ]] operator is supported by some modern shells, such as bash, ksh, and zsh, and it can check more types of conditions, such as pattern matching, regular expressions, arithmetic expressions, and string operations. For example, you can use the [[ ]] operator to check if a string matches a pattern:

[[ "$str" == foo* ]]

The [[ ]] operator is more powerful and flexible than the [ ] operator, and it does not require quoting or escaping in most cases. However, the [[ ]] operator is not standardized and not compatible with some shells, and it may behave differently depending on the shell options and settings.

4 The (( )) operator

The (( )) operator is another extension of the [ ] operator, meaning that it is similar to the [ ] operator but with a different purpose. The (( )) operator is used to evaluate arithmetic expressions and return a boolean value based on the result. The (( )) operator is supported by some modern shells, such as bash, ksh, and zsh, and it can perform various arithmetic operations, such as addition, subtraction, multiplication, division, modulo, and bitwise operations. For example, you can use the (( )) operator to check if a number is even or odd:

(( $num % 2 == 0 ))

The (( )) operator is more convenient and intuitive than the [ ] operator for arithmetic expressions, and it does not require spaces around the arguments or the $ sign for variables. However, the (( )) operator is not standardized and not compatible with some shells, and it may have some pitfalls, such as returning a zero exit code for false expressions and a non-zero exit code for true expressions.

5 Choosing the right operator

When writing conditional operators in shell scripting, there are various approaches to consider. Factors such as the type of condition, the shell environment, readability, and performance should be taken into account when making a decision. Generally speaking, the test command should be used when writing portable and compatible scripts that work with most shells and systems. The [ ] operator is ideal for concise and readable scripts that work with most shells and conditions. For powerful and flexible scripts that work with modern shells and advanced conditions, the [[ ]] operator is recommendable. Finally, if you want to write convenient and intuitive scripts that work with modern shells and arithmetic expressions, the (( )) operator is the way to go.