How to Concatenate String in Bash

String concatenation is a core concept in Bash scripting that lets you join multiple text fragments into a single string. It’s essential for tasks like building file paths, composing dynamic messages, and assembling command-line arguments or configuration lines within shell scripts.

This article explains various ways to concatenate strings in Bash, including simple variable joining, using curly brace notation for clarity, formatting numbers alongside text, and assembling structured strings with loops. These techniques help you write flexible, readable, and maintainable Bash scripts.

Concatenation of Multiple Variables

Bash allows you to concatenate strings by directly placing variables next to each other or by using curly braces for clarity and control.

Basic Variable Concatenation

1. Create a script file.

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   $ nano basic_concatenation.sh

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2. Add the following content to the file:

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   #!/bin/bash
   # Basic string concatenation examples
   
   first_name="John"             # Given name
   last_name="Doe"               # Surname
   domain="example.com"          # Domain used for building an email address
   
   full_name_spaced="$first_name $last_name"   # Concatenate with a literal space
   echo "Full name: $full_name_spaced"
   
   email="$first_name.$last_name@$domain"      # Join with '.' and '@' to form an email
   echo "Email: $email" 

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   Save and close the file.

3. Make the script executable.

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   $ chmod +x basic_concatenation.sh

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4. Run the script.

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   $ ./basic_concatenation.sh

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   Output:

   Full name: John Doe
   Email: John.Doe@example.com

Using Curly Braces for Safer Expansion

1. Create a second script.

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   $ nano advanced_concatenation.sh

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2. Add the following content to the file:

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   #!/bin/bash
   # Demonstrates brace expansion for safe concatenation
   
   prefix="server"   # Text prefix for the resource name
   number="01"       # Zero-padded sequence as a string
   suffix="prod"     # Environment tag
   
   server_name="${prefix}_${number}_${suffix}"  # ${var} makes variable boundaries unambiguous; '_' is a literal separator
   echo "Server name: $server_name"

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   Save and close the file.

3. Make the script executable.

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   $ chmod +x advanced_concatenation.sh

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4. Run the script.

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   $ ./advanced_concatenation.sh

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   Output:

   Server name: server_01_prod

Concatenation of Numbers and Strings

Bash lets you combine numeric values with strings to create URLs, IDs, version tags, or other formatted output. You can insert variables into strings directly or use arithmetic expressions to calculate and embed results.

Combine Numbers with Text

1. Create a script file.

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   $ nano number_concatenation.sh

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2. Add the following content to the file:

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   #!/bin/bash
   # Build a URL by concatenating protocol, hostname, and port
   
   port=8080                                  # Numeric port; will be treated as a string during concatenation
   protocol="https"                           # Scheme
   hostname="www.example.com"                 # Hostname under example.com
   
   url="${protocol}://${hostname}:${port}"    # Use ${} to avoid ambiguity next to ':' and '/'
   echo "Complete URL: $url"

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   Save and close the file.

3. Make the script executable.

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   $ chmod +x number_concatenation.sh

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4. Run the script.

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   $ ./number_concatenation.sh

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   Output:

   Complete URL: https://www.example.com:8080

Arithmetic in Concatenation

1. Create a new script file.

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   $ nano arithmetic_concatenation.sh

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2. Add the following content to the file:

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   #!/bin/bash
   # Demonstrates arithmetic expansion and string concatenation
   
   base_number=100                                                 # Integer operand
   increment=25                                                    # Integer operand
   result=$((base_number + increment))                             # $(( )) performs integer arithmetic; spaces are allowed inside
   
   message="The result of $base_number + $increment is $result"    # Insert values into a string
   echo "$message"
   
   id_prefix="USER"                                                # String prefix for IDs
   id_number=1001                                                  # Current numeric ID
   next_id=$((id_number + 1))                                      # Arithmetic expansion to get the next ID
   
   current_user_id="${id_prefix}_${id_number}"                     # Use ${} for clear boundaries; '_' is a literal
   next_user_id="${id_prefix}_${next_id}"
   
   echo "Current user: $current_user_id"               
   echo "Next user: $next_user_id"

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3. Make the script executable.

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   $ chmod +x arithmetic_concatenation.sh

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4. Run the script.

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   $ ./arithmetic_concatenation.sh

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   Output:

   The result of 100 + 25 is 125
   Current user: USER_1001
   Next user: USER_1002

Concatenation of Numeric Strings

You can format and join numeric values stored as strings in Bash to generate dates, timestamps, prices, and other structured outputs.

1. Create a script file.

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   $ nano numeric_strings.sh

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2. Add the following content to the file:

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   #!/bin/bash
   # Numeric values stored as strings and concatenated in different formats
   
   num1="42"                                 # First numeric string
   num2="17"                                 # Second numeric string
   
   concatenated="$num1$num2"                 # Simple concatenation: no operator needed in Bash
   echo "Concatenated: $concatenated"
   
   with_separator="$num1-$num2"              # Concatenate with a literal '-' separator
   echo "With separator: $with_separator"
   
   price="99"                                # Whole dollar part as a string
   cents="95"                                # Cents part as a string
   formatted_price="\$${price}.${cents}"     # Escape $ as \$ to print a literal dollar sign; ${} clarifies boundaries
   echo "Price: $formatted_price"
   
   hour="09"                                 # Zero-padded hour
   minute="05"                               # Zero-padded minute
   time_formatted="${hour}:${minute}"        # Use ':' as a literal separator
   echo "Time: $time_formatted"

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3. Make the script executable.

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   $ chmod +x numeric_strings.sh

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4. Run the script.

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   $ ./numeric_strings.sh

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   Output:

   Concatenated: 4217
   With separator: 42-17
   Price: $99.95
   Time: 09:05

Concatenation Using for Loops

You can use for loops in Bash to iteratively build strings from arrays, ideal for generating lists, CSV headers, or formatted content.

1. Create a script file.

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   $ nano loop_concatenation.sh

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2. Add the following content to the file:

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   #!/bin/bash
   # Build comma-separated strings from arrays
   
   servers=("web01" "web02" "web03" "db01" "cache01")    # Array of server names
   
   all_servers=""                                        # Accumulator starts empty
   for server in "${servers[@]}"; do                     # "${array[@]}" preserves elements with spaces
       if [ -z "$all_servers" ]; then                    # -z tests for an empty string
           all_servers="$server"
       else
           all_servers="$all_servers, $server"           # Append with a leading comma and space
       fi
   done
   
   echo "All servers: $all_servers"
   
   fields=("Name" "Age" "Email" "Department")            # Array of column names
   csv_header=""                                         # Header accumulator
   for field in "${fields[@]}"; do
       if [ -z "$csv_header" ]; then
           csv_header="$field"
       else
           csv_header="$csv_header,$field"               # Join with commas, no trailing comma
       fi
   done
   
   echo "CSV Header: $csv_header"

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   Save and close the file.

3. Make the script executable.

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   $ chmod +x loop_concatenation.sh

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4. Run the script.

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   $ ./loop_concatenation.sh

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   Output:

   All servers: web01, web02, web03, db01, cache01
   CSV Header: Name,Age,Email,Department

Conclusion

In this article, you explored multiple techniques for string concatenation in Bash. You learned how to combine text using direct placement and curly brace notation, append numbers to strings for building URLs or identifiers, format numeric strings cleanly, and construct complex output using for loops.

These techniques allow you to generate dynamic values and maintain readable scripts for automation, logging, and configuration tasks. For additional reference, consult the Bash manual.