How to Check if String Starts With

Quick Answer: Check if String Starts With a Prefix

To check if a string starts with a specific prefix in Bash, use pattern matching with [[ "$string" == prefix* ]]. For substring matching, use parameter expansion with ${string:0:${#prefix}}.

Quick Comparison: String Prefix Testing Methods

Method	Speed	Best For	Readability
[[ == pattern ]]	Very fast	Wildcard patterns	Very clear
Parameter expansion	Fastest	Substring prefix	Moderate
grep -q ^	Fast	Piped input	Simple
case statement	Very fast	Multiple patterns	Clear
expr match	Slower	Legacy scripts	Moderate

Bottom line: Use pattern matching with [[ ]] for simplicity and clarity.

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Checking if String Starts With

Checking string prefixes is essential for validating input, routing logic, and parsing data. Bash provides several clean ways to test if a string starts with specific characters.

Using Wildcard Matching

The simplest approach with pattern matching:

#!/bin/bash

filename="document.txt"

if [[ "$filename" == *.txt ]]; then
  echo "Is text file"
fi

Exact Prefix Check

Check for exact prefix string:

#!/bin/bash

text="Hello World"

if [[ "$text" == "Hello"* ]]; then
  echo "Starts with 'Hello'"
fi

Practical Examples

#!/bin/bash

# Check if URL starts with https
url="https://example.com"
if [[ "$url" == https://* ]]; then
  echo "Secure URL"
fi

# Check if user is root
user="root"
if [[ "$user" == root* ]]; then
  echo "Root user"
fi

# Check file type by extension
file="data.json"
if [[ "$file" == *.json ]]; then
  echo "JSON file"
fi

Function for Reusable Checks

#!/bin/bash

starts_with() {
  local string="$1"
  local prefix="$2"

  [[ "$string" == "${prefix}"* ]]
}

if starts_with "production" "prod"; then
  echo "This is production"
fi

Case Statements for Multiple Prefixes

#!/bin/bash

file="$1"

case "$file" in
  *.log)
    echo "Log file"
    ;;
  *.sh)
    echo "Shell script"
    ;;
  *)
    echo "Other file type"
    ;;
esac

Real-World Example: Route Processing

#!/bin/bash

process_request() {
  local path="$1"

  if [[ "$path" == "/api/"* ]]; then
    echo "API request"
  elif [[ "$path" == "/admin/"* ]]; then
    echo "Admin request"
  elif [[ "$path" == "/static/"* ]]; then
    echo "Static file"
  else
    echo "Regular page"
  fi
}

process_request "/api/users"
process_request "/admin/dashboard"

Using Parameter Expansion

Extract everything after prefix:

#!/bin/bash

text="version:1.2.3"

if [[ "$text" == version:* ]]; then
  # Get everything after prefix
  version="${text#version:}"
  echo "Version is: $version"
fi

Performance: Wildcard vs Regex

#!/bin/bash

text="production environment"

# Fast - pattern matching
[[ "$text" == prod* ]] && echo "Yes"

# Also works - regex (slightly slower)
[[ "$text" =~ ^prod ]] && echo "Yes"

Important Notes

• Use == with [[ ]] for pattern matching
• Add * after the prefix to match anything after
• Wildcard matching is faster than regex
• Always quote variables: [[ "$var" == pattern* ]]
• Use =~ with ^ for regex anchoring

Quick Reference

# Check if starts with
[[ "$string" == "prefix"* ]]

# Function approach
starts_with() { [[ "$1" == "${2}"* ]]; }

# Using regex
[[ "$string" =~ ^prefix ]]

# Multiple checks with case
case "$string" in
  prefix*) echo "Match" ;;
esac

Summary

Use [[ "$string" == "prefix"* ]] to check if a string starts with a prefix. This pattern is fast, readable, and doesn’t require external commands.