Expanding

In shell scripting, variable expansion uses the dollar sign ($) to tell the shell to replace the variable name with its stored value.

#!/bin/bash
var="value of var"
echo ${var}

Running this script:

$ ./var-sample.sh
value of var

Braces vs No Braces

You can reference variables with or without braces. Braces become essential when you append characters immediately after the variable name.

With Braces

#!/bin/bash
var="value of var"
echo ${var}

Without Braces

#!/bin/bash
var="value of var"
echo $var

Both scripts output:

$ ./var-sample.sh
value of var

Delimiting Variable Names

Without braces, the shell cannot determine where the variable name ends:

#!/bin/bash
height=170

# Incorrect: $heightcm is undefined
echo "Your height is = $heightcm"

# Correct: ${height}cm expands properly
echo "Your height is = ${height}cm"

$ ./height.sh
Your height is = 
Your height is = 170cm

Quoting and Word Splitting

By default, unquoted expansions are split on whitespace defined by IFS (space, tab, newline). Use quotes to preserve the exact value.

Unquoted Expansion

#!/bin/bash
string="One Two Three"

# Splits into words
for element in ${string}; do
  echo "${element}"
done

Quoted Expansion

#!/bin/bash
string="One Two Three"

# Preserves the entire string as one element
for element in "${string}"; do
  echo "${element}"
done

$ ./string.sh
One
Two
Three

$ ./string2.sh
One Two Three

Intentional Splitting

Sometimes you want to iterate over each word in a list:

#!/bin/bash
readonly SERVERS="server1 server2 server3"

for server in ${SERVERS}; do
  echo "${server}.example.com"
done

$ ./expanding.sh
server1.example.com
server2.example.com
server3.example.com

Quoting the variable in this case treats the entire list as one element:

#!/bin/bash
readonly SERVERS="server1 server2 server3"

for server in "${SERVERS}"; do
  echo "${server}.example.com"
done

$ ./expanding2.sh
server1 server2 server3.example.com

Best Practices

Use this quick reference to decide when to quote or brace variables:

Further Reading

• Bash Parameter Expansion
• Advanced Bash-Scripting Guide
• POSIX Shell Command Language