Csh/Tcsh: Backticks Still Relevant Despite $(...) Dominance

Legacy Shell Scripts Still Using Backticks

Despite the widespread adoption of $(…) command substitution in most major shells like Bash, Zsh, and modern versions of CSH/TCSH, backticks (`) remain widely used in legacy shell scripts. Systems that have not been upgraded often contain old shell scripts that still rely on backticks for command substitution.

Upgrading these legacy scripts can be time-consuming and risky, leading many administrators to leave old scripts using backticks in place to avoid issues. As such, backticks continue to be relevant despite most new scripts using $(…) instead.

Some examples of places where backtick usage still persists:

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Scientific computing clusters running old versions of Linux/UNIX
Legacy server management and automation scripts
User-written scripts for research computing
Building and packaging scripts for legacy software

While best practice dictates migrating to $(…) where possible, the reality is that backticks remain embedded in old code that would require significant rewrite efforts to modernize fully. As long as these legacy use cases remain, backticks will persist.

Backtick Pros Over $() Command Substitution

Backticks have a few advantages over $(…) command substitution in certain contexts. These help explain why they continue to see niche usage despite $(…) being the modern standard.

Visually distinguishable: The backtick character stands out more than $(…) which can blend in with surrounding code.
Better whitespace handling: With backticks, whitespace before and after the command substitution construct is ignored, allowing for neater code formatting.
Quoting rules more intuitive: Quoting variables inside $(…) can be unintuitive, whereas backticks have straightforward quoting semantics.

Additionally, some find backticks easier to type than $(…) which requires using the shift key. Others are simply used to typing backticks from prior experience with CSH/TCSH and continue to use them out of habit.

None of these pros outweigh moving to $(…) where possible, but they do explain why some users prefer backticks for command substitution under certain circumstances.

When to Favor Backticks Over $()

In most cases, $(…) should be preferred for new code. However, reasons to consider retaining backticks include:

Legacy CSH/TCSH scripts: Rewriting complex legacy scripts can introduce bugs. Leaving backtick syntax untouched avoids risk.
Reader familiarity: If readers are expected to know CSH/TCSH, backticks may be more intuitive.
Typing convenience: Some users find backticks slightly easier to type.
Visual distinction: If command substitutions are deeply nested, backticks stand out more.

The last two points are minor, but for legacy scripts orMaking changes to complex legacy CSH/TCSH scripts utilizing backticks risks introducing new issues. In many cases, the adage “if it ain’t broke, don’t fix it” applies. Requiring readers to know CSH/TCSH conventions can also justify retaining backticks for command substitution.

Aside from these niche cases, $(…) should be preferred in new code for wider compatibility across shells.

Basic csh Backtick Syntax and Examples

The basic syntax for backticks in CSH/TCSH is straightforward:

`command`

For example, to store the output of the ls command into a variable:

set files = `ls`

Some more examples:

set cwd = `pwd`
echo "User is `whoami`"
set date = `date +%F`

As seen above, the command substitution occurs, with the standard output result replacing the backticks. This allows capturing the result to store or reuse.

Quoting within backticks works similarly to regular Bash quoting:

set arg1 = "some value"
set vars = `echo "$arg1"`

That covers the basic usage. More complex cases build upon this foundation.

Common Backtick Use Cases

Some common use cases taking advantage of backtick substitution in CSH/TCSH include:

Storing Command Output in Variables

Storing command output is the most basic backtick usage:

set files = `ls`

This idiom is ubiquitous in shell scripts to both capture output and set variables to compute further logic off of.

Nested Command Substitutions

Backticks allow arbitrary nesting of command substitutions:

set user = `whoami`; echo "Logged in as `id -un $user`"

Here the inner backtick substitution provides the username, which the outer substitution inserts into the id command.

Such nesting provides powerful capabilities for CSH/TCSH scripts to utilize multiple commands in a single line pipeline.

Migrating from Backticks to $()

For compatible shells, migrating scripts from backticks to $(…) provides long-term compatibility and consistency:

Use sed, find/replace to change ‘`cmd`’ -> ‘$(cmd)’
Manually verify each substitution, ensure spacing/quoting is correct
Test script logic end-to-end to catch any bugs
Update any documentation referring to backtick syntax

The most risk comes from the mass find/replace changing syntax. Carefully verifying each substitution catches errors from any spacing or quoting issues.

Isolating script logic into functions can minimize risk. Change function internals separately, validating logic still works.

With careful verification and incremental testing, migrating from backticks can retire legacy syntax without introducing new bugs.

Long-term Viability of Backticks

While $(…) command substitution is the modern standard across shells, backticks fill ongoing niche use cases to subsist indefinitely:

Legacy CSH/TCSH scripts requiring high rewrite effort to migrate
Users with muscle memory favoring backtick syntax
Scripts requiring maximal visual distinction for nested command substitutions

However, long-term coding best practices should utilize $(…) where possible. CSH/TCSH compatibility scenarios, legacy codebases, and user habit form the enduring niches keeping backticks relevant despite $(…) dominance.

Csh/Tcsh: Backticks Still Relevant Despite $(…) Dominance