Bash Concatenate Strings

Quick Reference

$ X="String"
 $ X+="Concatenation" 
 $ echo $X
 
 # Output
 StringConcatenation

What is Concatenation?

Concatenation is the process of joining two strings together into one result. Some common use cases include creating a string containing multiple variables, and formatting messages containing user-defined inputs.

Basic Concatenation

In Bash, when variables or strings are written one after another, they automatically concatenate. In the following examples, the code will echo the same output to the terminal.

Concatenating variables:

$ X="String"
 $ Y="Concatenation!"
 $ echo "${X}${Y}"
 
 # Output
 StringConcatenation!

Concatenating a variable and a string:

$ X="String"
 $ echo "${X}Concatenation!"
 
 # Output
 StringConcatenation!

The curly braces are used to “protect” the names of the X and Y variables from other characters in the string. They make it clear to Bash that you are using a variable with a specific name. Use double quotes for strings containing variable names - this tells Bash to interpolate, or substitute in, the values of the variables.

Concatenation with multiple variable types

Bash infers the type of variables depending on how they are used. So, this means we can use integer or boolean variables in string concatenation as well.

Concatenating an integer and a string:

$ X="The number four: "
 $ Y=4
 $ echo "$X$Y"
 
 # Output
 The number four: 4

Concatenating multiple numbers:

$ X=314
 $ Y=159
 $ echo "$X$Y"
 
 # Output
 314159

Concatenation with += operator

You can use the += operator to append to the end of a string variable.

$ X="Pi is the number "
 $ X+=3.14159
 $ X+= " and it is very handy."
 $ echo "$X"
 
 # Output
 Pi is the number 3.14159 and it is very handy.