How to concatenate string variables in Bash ?

To concatenate string variables in Bash, you can use the following methods:

1. Basic Concatenation

Place variables/literals adjacent to each other:

str1="Hello"
str2="World"
result="$str1 $str2!"  # Output: "Hello World!"

2. Using += Operator

Append to an existing variable:

greeting="Hello"
greeting+=" World"  # greeting becomes "Hello World"

3. With Literal Text

Mix variables and strings:

name="Alice"
message="Hello, $name! Today is $(date)"  # Includes command substitution

4. Using printf (For Formatting)

Control the output format:

first="John"
last="Doe"
printf -v full_name "%s, %s" "$last" "$first"  # full_name="Doe, John"

5. Concatenate in a Loop

Build a string incrementally:

parts=("I" "am" "learning" "Bash")
sentence=""
for part in "${parts[@]}"; do
    sentence+="$part "
done  # sentence="I am learning Bash "

Key Notes

• Always quote variables to handle spaces/special characters:

  # Bad (breaks with spaces):
  result=$str1$str2

  # Good:
  result="$str1$str2"

• Use ${var} to avoid ambiguity:

  animal="cat"
  echo "I have a ${animal}tail"  # Without braces: "I have a cattail"

Examples

Concatenate with a Separator

dir_path="/home/user"
file="document.txt"
full_path="$dir_path/$file"  # "/home/user/document.txt"

Combine Variables and Commands

user="$USER"
timestamp=$(date +%s)
log_entry="User $user logged at $timestamp"

Summary

Method	Use Case	Example
Adjacent	Simple concatenation	result="$str1$str2"
+=	Append to existing strings	str+=" new text"
printf	Format strings with precision	printf -v var "..."
Loop	Build strings incrementally	for ...; do str+="..."

By following these patterns, you can safely concatenate strings in Bash scripts.