Comparative Analysis of Efficient Methods for Removing Leading and Trailing Quotes from Strings in Shell Scripts

Introduction

String manipulation is a common task in shell script development, particularly when handling configuration files, parsing JSON data, or processing user input. This paper systematically analyzes and compares multiple approaches for removing leading and trailing quotes from strings based on real-world development scenarios.

Problem Context and Original Approach

In the original problem, the developer used sed commands to remove leading and trailing quotes:

#!/bin/sh
opt="\"html\\test\\\""
temp=`echo $opt | sed 's/.\(.*\)/\1/' | sed 's/\(.*\)./\1/'`
echo $temp

While functional, this approach suffers from significant efficiency issues. It employs two separate sed commands, requiring two external process invocations. Moreover, the pattern s/.\(.*\)/\1/ and s/\(.*\)./\1/ removes the first and last characters regardless of whether they are quotes, potentially leading to unintended data modifications.

Efficient Solution: Shell Built-in Parameter Expansion

The most recommended solution leverages shell built-in parameter expansion, which executes without external process overhead:

temp="${opt%\"}"
temp="${temp#\"}"
echo "$temp"

The mechanism operates as follows:

• ${opt%\"}: Removes the double quote from the end of the variable value
• ${temp#\"}: Removes the double quote from the beginning of the variable value

This method excels not only in efficiency but also in precision—it only removes quotes if they are indeed at the start or end, avoiding accidental alteration of other characters.

Improved sed Command Approach

If sed must be used, a more efficient implementation is available:

sed -e 's/^"//' -e 's/"$//' <<<"$opt"

This enhanced version features:

• Single sed process handling both substitution operations
• Avoidance of echo command through <<< (here-string)
• Exact matching of quotes at the string start (^") and end ("$)

Compared to the original approach, this reduces process creation overhead and improves execution efficiency.

Limitations of the tr Command

An alternative suggestion uses the tr command:

echo "$opt" | tr -d '"'

While simple, this method has a critical flaw—it removes all double quotes in the string, not just the leading and trailing ones. When processing strings containing internal quotes, this can corrupt the data.

Special Application of jq Tool

For JSON data processing, the jq tool offers a specialized solution:

echo '{"foo": "bar"}' | jq '.foo'
echo '{"foo": "bar"}' | jq -r '.foo'

The -r (raw output) option outputs string content without quotes, which is particularly useful in JSON processing contexts.

Performance Comparison and Applicable Scenarios

Based on practical testing and analysis, the performance ranking of methods is as follows:

1. Shell built-in parameter expansion: Most efficient, no external process overhead
2. Improved sed approach: Moderate efficiency, suitable for complex text processing
3. Original sed approach: Lower efficiency, with design flaws
4. tr approach: Not suitable for this scenario, risks data integrity

In file processing scenarios like those mentioned in the reference article, combining parameter expansion with file redirection enables efficient dynamic file content updates.

Best Practice Recommendations

Based on the analysis, the following best practices are recommended:

• Prefer shell built-in parameter expansion
• Consider the improved sed approach for complex text processing
• Avoid the tr approach that removes all quotes
• Use jq's -r option for JSON processing
• Always verify data integrity after processing

Conclusion

String manipulation in shell scripts requires selecting appropriate methods based on specific contexts. For the common need of removing leading and trailing quotes, shell built-in parameter expansion demonstrates clear advantages in efficiency, precision, and maintainability. Developers should thoroughly understand the characteristics of various tools and choose optimal implementations while ensuring functional correctness.