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Bash Scripting

Quotation Marks

This can get a bit complicated.

Quoting can be used to disable special treatment for special characters,
to prevent reserved words from being recognized as such, and to prevent parameter expansion.

Ordinary strings (without 'special' characters) do NOT need to be quoted.

Quoting is used to remove the special meaning of certain characters or words to the shell.

Quoting in bash is accomplished 4 different ways:

escape character: \
preserves the literal value of the next character that follows, with the exception of newline
single quotes: '
preserves the literal value of each character within the quotes, but cannot occur between single quotes
double quotes: "
preserves the literal value of all characters within the quotes, with the exception of ‘$’, ‘`’, ‘\’
back-tick: `
used for command substitution - executing a command but replacing the expression with the output of the command:
echo "Today is `date`"
Since it is easily mistaken for a single quote, best practice is to use the $(...) syntax.

Single quotes treat contents literally
Double quotes evaluate contents, such as variables

Variables are a special case in bash use.

On the command line (not in a script), the words typed are separated by spaces, tabs, or linebreaks. These are known as whitespace.
If a word or command here includes a whitespace character, you have to use single or double quotes to treat it as a single unit. We saw this previously.

However, in a script, double quotes are used to evaluate the contents of a variable.

Single quotes in a script print contents literally.

name="Jeff"
echo 'Hello $name'
Hello $name

However, double quotes are what we should have used:

name="Jeff"
echo "Hello $name"
Hello Jeff

name="Jeff Brown"
echo "$name"
Jeff Brown

echo $name
Jeff Brown

echo '$name'
$name

Note that we were able to print a variable containing spaces without quotes.

Safer Quote

Printing variables WITHOUT double quotes is NOT Best Practice and is in fact DANGEROUS.

Bash splits the variable contents on spaces, tabs, or newlines. If the variable contains commands they are treated as such.

name=*
echo $name
Laid Back - ...Keep Smiling (1983) Laid Back - Bakerman (1990) Laid Back - Cosmic Vibes (2011) Laid Back - Forevergreen (2024) Laid Back - Happy Dreamer (2004) Laid Back - Hole In The Sky (1990 DSF) Laid Back - People (2005) Laid Back - Road To Fame (2023)

echo "$name"
*

The asterisk is a globbing character, meaning all files in the current directory. Not something you probably want strangers to see.

If you are asking for some user input in a script, ALWAYS double quote the resulting variable.

Imagine if someone entered '/*'