Backticks In Shell Scripts: Deprecated But Not Going Away

Why Backticks Persist in Shell Scripts

Despite being deprecated in favor of $( ) syntax, backticks continue to have a stubborn persistence in shell scripts. There are several key reasons this antiquated command substitution syntax has yet to fade away:

There is a massive amount of legacy shell scripts that rely on backticks for command substitution. Rewriting all of this code would require tremendous effort and risks breaking working production scripts.
Backward compatibility considerations have kept backtick support intact across shell implementations. Removing backtick functionality could break backwards compatibility.
In some specialized situations, backticks offer unique utility that makes them still useful in modern scripts. Examples include nesting command substitutions and variable substitutions.

The sheer prevalence of legacy scripts using backticks is the primary force keeping this deprecated syntax in widespread use. An estimated 80% of active bash scripts leverage backticks due to being initially written prior to the acceptance of $( ) as superior syntax. Rewriting even a fraction of these scripts would necessitate extremely high development and testing costs.

Software development shops and enterprises are loathe to touch functional legacy scripts solely to modernize syntax. With many mission critical business functions bound to legacy shell scripts, the risks of introducing regressions by rewriting working backtick notation typically outweigh any semantic benefits.

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Compounding inertia is the fact that all major shells continue to support backtick functionality for backwards compatibility. Bash, zsh, ksh, csh, ash and other shells retain backtick command substitution to avoid suddenly breaking the vast number of older shell scripts written prior to the widespread adoption of $( ) notation in the mid 2000’s.

Removing backtick support risks downstream impacts to systems relying on archaic shell scripts. Few developers are willing to take accountability for destabilizing production environments in exchange for modest syntactic improvements. So backtick persistence remains the path of least resistance for most.

Finally, while $( ) command substitution handles most use cases better, backticks do maintain some functionality advantages in niche applications. The most common area where backticks still excel is nesting command substitutions or variable substitutions.

Constructing elaborate nested parameter expansion expressions can often be made more readable by substituting intermediate commands with backticks instead of $( ). This takes advantage of the order of operations differences in how backticks parse before dollar signs.

Similarly, capturing multi-line output in variables may occasionally call for backticks over dollar parentheses due to subtle parsing deviations. So while 99% of uses mandate transitioning from backticks to modern syntax, they remain a convenient escape hatch for addressing edge case needs.

Demystifying Backticks

To help developers gain confidence in safely modernizing legacy backtick usage, it is important to demystify what backticks are and how they operate under the hood. We will examine the origins of backtick syntax, formally define its behavior, and clarify its interactions with other shell scripting constructs.

Definition and Origins of Backticks

Backticks ( ` ) indicate legacy Unix style command substitution. They invoke a subshell, execute the enclosed command in that context, and substitute the standard output of the command into the surrounding script. Semantically they can be thought of as executing a command and performing an inline variable assignment to capture the whole output stream.

Backticks originated all the way back in Version 7 Unix released in 1979. The syntax was introduced as a convenience shortcut to avoid needing to call echo to print command output. Their name refers to the backquote glyph itself which appears under the tilde on American keyboards.

At the initial time of creation, backticks represented the easiest way to inject raw command output directly into a script. But with the ascension of $( ) syntax providing similar inline substitution, backticks came to be viewed as antiquated and potentially confusing. Though the details took years to crystallize into best practice guidance, $( ) became the preferred notation starting in the mid 2000’s.

Contrast with $() for Command Substitution

The $( ) syntax serves as the modern substitution for backticks for command substitution inline in scripts. Besides being considered cleaner and easier to read, $() offers definitive improvements in composability, nesting capabilities, and variable compatibility.

Backticks provide a simplistic string replacement for the output stream. $( ), in contrast, tokens each nested substituion as a discrete unit prior to evaluation. This enables simple and safe composition of arbitrarily complex nested command invocations and output streaming.

Additionally, the $( ) notation forgoes multiple layers of subshells required by backtick Expansion. This increases performance and accuracy when invoking commands in a substitution context. The elimination of subshells also provides access to variables and state from the caller rather than isolation.

These advantages of readability, versatility and performance hold across every major scripting language and shell. $( ) syntax is unambiguously the future while backticks remain solely for legacy support until they can eventually be fully deprecated.

When to Still Favor Backticks Over $()

Given the performance and structural benefits detailed above, $( ) syntax is nearly always preferential in modern scripts. The exceptions relate primarily to nested substitutions and order of operations considerations.

In complex nested workflows with interpolated variables, backticks can occasionally provide better readability due to their order of evaluation relative to variables.

Similarly, by evaluating inline script blocks before parameter expansion, backticks enable easier capture of multi-line command output into a variable for later use. These niche cases excuse maintaining backticks functionality in modern shells despite their general undesirability.

For the vast majority of command substitution use cases though, $( ) syntax represents the uncontested modern method. The following sections cover migrating legacy backtick usage, safe deprecation practices, and reinforce why $( ) achieved ascendence as best practice.

Migrating from Backticks to $()

Modernizing backtick command substitutions is seen as a best practice to improve script quality and safety. The migration task may seem daunting, but following structured procedures can smooth the transition to $( ) syntax.

We will examine guidelines and methods for removing backticks, highlight benefits, and show before and after code samples. The goal is to equip developers to upgrade their scripts and take advantage of the strengths of $() going forward.

Benefits of Switching to $()

Transitioning from backticks to $() syntax yields several advantages:

Eliminates deprecated functionality and tech debt accrued from legacy artifacts
Enhances readability through clearer, more expressive syntax
Allows nesting command substitutions without quoted escaping
Reduces excess subshell usage which limits performance in high throughput contexts
Integrates cleanly with complex scripts leveraging extensive variable expansion and interpolation

These benefits keep $() adoption moving steadily forward despite the lingering prevalence of backtick usage. Taking the time to proactively migrate to $() will improve script quality now while future proofing code against the planned eventual backtick deprecation.

Step-by-Step Process for Replacing Backticks

Handling the entire migration manually could risk breaking functionality despite testing. Instead, leverage git diffs coupled with find and replace:

Copy the backtick script to a test branch:

git checkout -b backtick-migration

Use find and replace all backticks with $():

find . -type f -exec sed -i 's/`/$(/g' {} \;

Run tests and fix any identified regressions:

./test-suite.sh

Diff the upgraded script against the backtick version:

git diff master

Carefully inspect any complex logic changes, nesting depth impacts etc.
Merge to master after final validation passes

This process transitions syntax while allowing for iterative improvements before integration. The diffs specifically highlight changes in script flow and nesting impacts to inspect.

Isolating the transition via a branch also enables easy rollback if issues emerge. But typically directly substituting $() will work seamlessly without logical changes to workflows.

Before and After Code Examples

As a concrete illustration, consider this simple script to clean apt caches leveraging backticks:

  
#!/bin/bash
  
echo "Cleaning apt caches"
`find /var/cache/apt/archives -type f -delete`  
`apt clean`
  
echo "Apt cache cleanup complete!"

Transitioned to $() syntax this would become:


#!/bin/bash
  
echo "Cleaning apt caches"  
$(find /var/cache/apt/archives -type f -delete)
$(apt clean)
  
echo "Apt cache cleanup complete!"

The logic remains identical but has modernized to leverage $() for improved readability, nesting capability, and performance.

More intricate multi-line nested backtick workflows can be upgraded similarly. The only catch is balancing nested quotation marks which is smoothed by regex find and replace.

Taking time to migrate legacy backticks to $() helps improve scripts and prevent future surprises when backticks finally reach full deprecation down the road.

Safe Usage of Backticks

Where justified retaining legacy backticks, care should be taken to implement appropriate security practices. We will examine risks around uncontrolled inputs combined with backticks, remediations, and prudent preventative tactics.

Security Implications of Backticks

Like many substitution mechanisms allowing dynamic execution logic, unsafe user inputs paired with backticks introduces vulnerabilities.

If a script accepts unchecked user arguments that get concatenated into a backtick command string, arbitrary command injection becomes possible. Constructing purposeful input can enable running attacker specified bash commands on the host system.

This Execute arbitrary code capability provides power for everything from information exposure to denial of service attacks to compromised credentials for further network intrusion.

Functions like escapeshellcmd() offer partial protection but still permit meta characters that could impact control flow logic. So additional precautions are imperative around scrubbing dangerous inputs.

Best Practices for Sanity Checking Inputs

Rigorously sanitizing any dynamic inputs prior to backtick inclusion prevents injection issues. Key recommended validations include:

Explicitly whitelisting permitted parameter options
Blocklisting known dangerous metacharacters and values
Calling escapeshellarg() around the full concatenated substitution string
Parsing the intended command syntax from input to validate structure

Combining these mitigations provides defense in depth assurance against passing unsafe inputs to backtick evaluation. Input constraints prevent uncontrolled syntax while escapeshellarg() handles edge cases.

For optimal security, also minimize inputs exposed directly to users. Instead utilize service accounts with limited permissions, API tokens, or multiple authentication factors to reduce attack surface.

Built-in Backtick Handling Functions

Legacy shells offer native utilities intended to help safely manage backtick command evaluation:

escapeshellarg() – Escape a string to safely pass as an argument
escapeshellcmd() – Escape meta characters for safe inclusion in syntax
bash -c – Run command string argument safely with bash

However these APIs still permit some potentially dangerous arguments. So they should only be used as secondary assurances rather than complete safety mechanisms.

Ultimately abstaining from backticks remains ideal. But where retaining legacy backticks, be vigilant about hardening input handling and sanitization to prevent command injection risks.