Performance Benchmark – [ Vs [[ In Bash Conditionals

Comparing Performance of [ vs [[ in Bash

Bash provides two ways of executing conditional expressions – the traditional [ command, and an improved [[ keyword added in Bash version 2.0. Understanding the differences between these can help explain the performance benchmarks outlined later in this article.

[ is the original conditional expression evaluation utility in Bash, having been adopted from earlier Bourne shells. It is actually an external command, not a shell builtin, so using it involks a separate subprocess. [[ was added to provide an alternative way to evaluate conditionals, implementing it as a builtin to avoid the overhead of an external process.

In addition to the performance implications, [[ adds features not available in [, such as pattern matching and regular expression handling. It also allows for negation of conditions using !. The [[ construct is designed as the de facto approach for evaluating conditionals in Bash, with [ left available for backwards compatibility with scripts written for older Bourne shells.

When to Use [ vs [[

The [[ keyword should be used for evaluating conditional expressions in Bash scripts, offering better performance and more flexibility than the legacy [ command.

However there are cases where [ may still be needed for compatibility with older systems. Scripts that need to run on minimal embedded Linux environments, or legacy commercial Unix platforms, may rely on the POSIX defined test utility instead of Bash-specific features. In those cases, the portability of [ comes at the cost of performance.

Performance Differences

The key difference in performance between [ and [[ lies in how they are executed – [[ is a bash keyword parsed by the shell itself, while [ invokes a separate process.

By being a builtin, tests using [[ avoid the need to spawn a subprocess and create a new process environment. All state is contained within the shell itself rather than a separate command context. Management of the pipeline from the parent process to the child process and back again adds measurable overhead, even if the end result is the same.

To demonstrate this, a simple benchmark test is shown below comparing the time to iterate a loop 100,000 times for each method:

$ time for i in {1..100000}; do [[ $i == 100000 ]]; done

real    0m0.084s
user    0m0.076s
sys     0m0.008s

$ time for i in {1..100000}; do [ $i == 100000 ]; done
real    0m0.188s
user    0m0.164s
sys     0m0.020s

This shows [[ finishing over 2x faster than the equivalent test using [. The savings come from not having to spawn a new [ process each iteration. While minor in this case, any nested conditionals or loops can see significant compound savings.

Recommendation

Based on both features and performance, the [[ keyword is recommended for evaluating conditional expressions in Bash scripts.

The [[ construct should provide noticeable speed improvements over [ in loops, nested if statements, and other areas of complex flow control. The benefits increase in proportion to the number of iterations or depth of nesting.

However, [ does still have a place when writing scripts intended for portability to a wide range of POSIX platforms. Understanding this tradeoff allows tuning Bash scripts for either performance or compatibility as needed.

When Performance Matters

While the [[ optimization provides a readily measurable performance gain, that may not always translate into meaningful benefits for overall script runtimes. The real world bottleneck depends greatly on the task being performed.

Logical evaluation using [ or [[ contributes very little to total runtime compared to actual work done inside conditional blocks. Operations like reading files, processing data, calling external programs or requesting network resources are orders of magnitude more expensive than any shell built-in.

The relative costs can be demonstrated in another micro-benchmark:

# Testing string comparison
$ time for i in {1..100000}; do [[ $i == 100000 ]]; done
real    0m0.063s

# Testing arithmetic
$ time for i in {1..100000}; do (( i == 100000 )); done
real    0m0.044s  

# Testing file stat
$ time for i in {1..100000}; do [ -f /var/log/syslog ]; done
real    0m5.768s

While the string and math evaluations are 20x faster with [[ and (( ), the file system stat call dominates overall runtime. Saving a fraction of a second on shell logic is irrelevant next to a task requiring tangible IO, CPU or network overhead.

The recommendation therefore, is to first profile scripts to identify where time is actually being spent. Only optimize logical evaluation code once other bottlenecks like disk, database access or number crunching have been addressed.

Portability Considerations

The [[ conditional expression syntax is a Bash-ism – it is not available by default in shells compliant only with the POSIX specification, like Dash and others.

Scripts written taking advantage of [[ will not work unmodified in minimal environments like Busybox. That is a reasonable tradeoff for simplicity and performance in cases where Bash is known to be the target platform.

However, shell code intended for widespread portability should rely only on POSIX defined features. That requires using [ for logical evaluation, and foregoing nice new things like pattern matching. The reasons for avoiding even common Bash extensions go beyond strict POSIX environments.

Even inline shell code in other languages like Python can face issues. For example, Python on Windows does not include full Bash compatibility. So a portable Python script may work fine on Linux using [[ but fail to parse conditional expressions when run elsewhere.

Understanding interoperability constraints is important if writing scripts meant for reuse across multiple scripting languages, operating systems, or customer deployment environments.

Summary

The [[ keyword provides noticeably faster evaluation of conditional expressions compared to the legacy [ command by avoiding spawning an external subprocess.

For Bash-specific shell scripting, [[ enables optimizations especially beneficial in loops, nested conditionals and math tests. The difference is easily measurable through microbenchmarks.

However the performance gains may not change real world runtimes dominated by physical work or resource access. Profiling end to end scripts can determine if optimizing conditionals provides meaningful benefits.

Meanwhile, relying solely on POSIX sh features ensures compatibility with other shells and scripting environments. That requires sticking to [ for conditionals, at the expense of some performance.

Understanding these tradeoffs allows tuning Bash scripts to either maximize performance, or ensure portability, depending on the intended purpose.