Bash syntax: Variables

The syntax of the variables in Bash programming language is flexible. Unlike many other programming languages, variables are untyped. That means that if you do not explicitly declare its type, the content of a variable can be arbitrarily a numeric value or a character string depending on which way you use it in.

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1. Declaration

Declare a variable by simply typing its name immediatly followed by the operator =:

MY_VAR=

Fill its content by happening a value immediatly after the operator:

MY_VAR=42
MY_VAR="42"
MY_VAR="Forty two"
MY_VAR='Forty two'

These commands are similars. They assign a sequence of characters to the variable. For greater clarity, we commonly do not use quotes or double-quotes when we want the variable to contain a numeric value, as in the first case.

Beware about spaces when declaring variables! The following commands will result in error:

MY_VAR = Something                # No spaces around the operator
MY_VAR=Something else             # Enclose character strings by quotes or double quotes
MY_VAR="Quotes are "life""        # Protect special characters by adding a backslash like \"

If you want a variable to be typed, use the builtin command declare and its options:

declare -i MY_VAR                 # Integer
declare -r MY_VAR                 # Read-only
declare -i -r MY_VAR              # Read-only integer
declare -a MY_VAR                 # Array
declare -f MY_VAR                 # Function

Now we have declared a variable named MY_VAR and filled its content. This variable is only available in the environment of the current script. That means that when the script is terminated, its assigned value is destroyed. In order to export a variable outside of the environment of the script, we can use the following option declare -x MY_VAR.

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2. Substitution

To manipulate an assigned value of a variable, use the special character $ followed by the variable name:

MY_VAR="42FileChecker"
echo "The name of the script is $MY_VAR"

These commands display the value of the variable on stdout. We call it a substitution (or expansion) because Bash replaces the sequence $MY_VAR by the pointed value "42FileChecker" before executing the command line.

Depending on the context, it is sometimes necessary to use a less ambiguous syntax as you can see in the following example:

echo "script_name_$MY_VAR_rocks"
echo "script_name_${MY_VAR}_rocks"

Second line with braces { and } is safer because it differenciates the variable $MY_VAR and a possible other existing variable named $MY_VAR_rocks.

You can use a set of special symbols to make some special substitutions:

MY_VAR="42FileChecker"
MY_SECOND_VAR="MY_VAR"

${#MY_VAR}               # string length --> returns 13
${MY_VAR:2:4}            # offset and length, returns 4 chars starting at 2 --> returns "File"
${!MY_SECOND_VAR}        # substitute by referenced name --> returns "42FileChecker"

declare -a MY_ARRAY=(val1 val2 val3)

${MY_ARRAY[@]}           # values of the array --> returns "val1 val2 val3"
${!MY_ARRAY[@]}          # indexes of the array --> returns "0 1 2"

42FileChecker often use the substitution by referenced name, especially with array typed variables:

declare -a MY_ARRAY=(val1 val2 val3)

MY_VAR="MY_ARRAY[1]"
echo "${!MY_VAR}"        # displays "val2"

MY_ARRAY is an array filled with 3 elements. MY_VAR store a reference to the second element of the array MY_ARRAY. Finally, the symbol ! indicates that we want to display a substitution of the reference $MY_VAR that is equivalent to $MY_ARRAY[1].

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3. Positional and special parameters

A shell script or a shell function can be invoked with a list of arguments. We call it the positional parameters which are similar to the pointer to strings 'argv' in a C program. A positional parameter is like a variable whose name is a number other than 0. Get its value by substitution:

echo "$1"                # displays the first argument
echo "$2"                # displays the second argument
echo "${14}"             # add braces if the number contains more than one digit

Calling your script with the command ./my_script "arg1" "arg2" will declare two positional parameters 1 and 2 whose assigned value are "arg1" and "arg2".