In many languages, quotes are primarily used to denote string literals. In the shell paradigm many constructs are interpreted as strings by default, so quotes play other important roles. Quotes demarcate the arguments of a command into units called "words", as well as modify the evaluation of their contents in numerous context-dependent ways. It is critical to understand how quoting affects the interpretation of code in a given context. It's something no one should avoid learning. Improper quoting is among the most common shell programming errors. Do not guess about quotes!

I'm Too Lazy to Read, Just Tell Me What to Do

cp $file $destination         # WRONG
cp -- "$file" "$destination"  # Right

When in doubt, double-quote every expansion in your shell commands.

Types of quoting

There are three standard types of quotes (or four if you count backslash escaping), and two nonstandard Bash extensions. This is the brief description of each of them. Read below for the verbose version.

You may concatenate the various types of quoting if you need to. For example, if you have one section of a string that has lots of special characters that you'd like to single-quote, and another section with a parameter expansion in it which must be double-quoted, you may mix them:

$ foo=bar
$ printf '%s\n' '!%$*&'"$foo"
!%$*&bar

Any number of quoted substrings, of any style, may be concatenated in this manner. The result (after appropriate expansions in the double-quoted sections) is a single word.

Effects of Quoting

Preserve unescaped metacharacters

A shell command is parsed by the shell into words, using whitespace other metacharacters. The first function of quoting is to permit words to contain these metacharacters.

echo '&'

Without quotes, the & would put the echo command into the background. With quotes, the & is simply made into a word, and passed as an argument to the echo command instead.

The quotes are not actually passed along to the command. They are removed by the shell (this process is cleverly called "quote removal"). In the example above, the echo command sees only the &, not the quotes.

Prevent field splitting and ignore glob pattern characters

The second purpose of quoting is to prevent word splitting and globbing. The result of a double-quoted substitution does not undergo any further processing (whereas the result of an unquoted substitution does).

cp -- "$filename" "$destination"

In this example, the double quotes protect the value of each parameter (variable) from undergoing word splitting or globbing should it happen to contain whitespace or wildcard characters (* or ? or [...]). Without the quotes, a filename like hot stuff.mp3 would be split into two words, and each word would be passed to the cp command as a separate argument. Or, a filename that contains * with whitespace around it would produce one word for every file in the current directory. That is not what we want.

With the quotes, every character in the value of the filename parameter is treated literally, and the whole value becomes the second argument to the cp command.

When in doubt, always double-quote your parameter expansions.

Expand argument lists

Double-quoting $@ or ${array[@]} has a special meaning. "$@" expands to a list of words, with each positional parameter's value being one word. Likewise, "${array[@]}" expands to a list of words, one per array element. When dealing with the positional parameters or with the contents of an array as a list of words, always use the double-quoted syntax.

Double-quoting $* or ${array[*]} results in one word which is the concatenation of all the positional parameters (or array elements) with the first character of IFS between them. This is similar to the join function in some other languages, although the fact that you can only have a single join character can sometimes be a crippling limitation.

When Should You Quote?

The basic rule of thumb is that you should double-quote every expansion. This prevents unwanted word splitting and globbing. When in doubt, quote it.

There are a few cases where double quotes may be safely omitted:

Use single quotes when protecting complex strings, especially ones that contain shell syntax which you don't want evaluated.

Examples

Here are some assorted examples, to show how things should be done. Some of these examples use bash/ksh syntax that won't work in strict POSIX shells.

Proper iteration over the positional parameters using a quoted "$@". Never use an unquoted $@ or $*.

for file in "$@"; do
...
done

As above, except with an array:

for element in "${array[@]}"; do
...

Proper iteration over array indexes:

# bash 3.0 and higher
for index in "${!array[@]}"; do
...

All of the usual expansions apply to text within the parentheses of a compound array assignment including word splitting and pathname expansion, and must be quoted and escaped in the same way as though they were to be passed as arguments to a command:

# bash or ksh93
find_opts=( \( -iname '*.jpg' -o -iname '*.gif' -o -iname '*.png' \) )
find . "${find_opts[@]}" -print

There are generally three alternatives for encoding a literal string containing quotes. Which works best depends on the context. First, a single quoted string that can contain anything other than single-quotes. In this case, an escaped, unquoted single-quote is concatenated with the argument between two single quoted strings. Second, a double-quoted string with all expansions and double-quotes within escaped. Third, a less portable equivalent using $'...':

printf '%s\n' 'Don'\''t walk!'
printf '%s\n' "Don't walk!"
printf '%s\n' $'Don\'t talk!'

$(...)-style command substitutions are unique in that the quoting of their contents is completely independent to their surroundings. This means you don't have to worry about nested quote escaping problems:

printf '%s\n' "The matching line is: $(grep foo "$filename")"

# Note that the quotes inside the $() command substitution are nested.
# This looks wrong to a C programmer, but it is correct in shells.

printf '%s\n' "The matching line is: $(grep foo "$filename")"
#                                      ^---------^    inner layer (quotes)
#                           ^^--------------------^   middle layer (command sub)
#    ^---------------------------------------------^  outer layer (quotes)

An example showing an array being "joined" with "${a[*]}":

# bash
ip=192.168.1.30
netmask=255.255.254.0
IFS=. read -ra ip_octets <<<"$ip"
IFS=. read -ra netmask_octets <<<"$netmask"
for i in 0 1 2 3; do
  ((ip_octets[i] &= netmask_octets[i]))
done
IFS=.; network="${ip_octets[*]}"; unset IFS

The ansi-c $'...' quoting style is used to interpret backslash escapes:

IFS=$' \t\n'
# sets the IFS variable to the three-byte string containing
# a space, a tab, and a newline

Examples showing backslash sequences:

# These are equivalent:

printf '%s\n' hi $'there\n'

# ksh / zsh:
print 'hi\nthere\n'

Patterns

The shell uses quotes to suppress interpretation of special syntax within patterns and regular expressions, so that any literal or expanded string may be easily included in a pattern matching context without individually escaping each character.

if [[ $path = foo* ]]; then
# unquoted foo* acts as a pattern

if [[ $path = "foo*" ]]; then
# quoted "foo*" is a literal string

if [[ $path =~ $some_re ]]; then
# the contents of $some_re are treated as a POSIX extended regular expression.

if [[ $path =~ "$some_re" ]]; then
# the contents of $some_re are treated as a literal string
# despite the =~ operator

# the "quoted" branch is taken.

g=a*[b]
case $g in
    $g) echo 'unquoted pattern' ;;
    "$g") echo 'quoted pattern'
esac

Here Documents

Quote-removal never applies to the contents of a here document

 $ arr=(words in array); cat <<EOF
> These are the "${arr[*]}"
> EOF
These are the "words in array"

Quoting or escaping the "delimiter" in the heredoc redirection affects whether its contents are subject to expansions. The quoted variety is bash's only context in which an arbitrary string may be used with no special interpretation except for lines beginning with the delimiter, which marks the end of the heredoc.

{ printf '%s\n' before "$(</dev/stdin)" after; } <<\EOF
# Anything but "EOF" can go here
EOF

See also


CategoryShell

Quotes (last edited 2022-11-06 13:28:25 by ormaaj)