Shell Quoting – Why Quote Variable for If but Not for Echo?

quotingshell

I've read that you need double quotes for expanding variables, e.g.

if [ -n "$test" ]; then echo '$test ok'; else echo '$test null'; fi

will work as expected, while

if [ -n $test ]; then echo '$test ok'; else echo '$test null'; fi

will always say $test ok even if $test is null.

but then why don't we need quotes in echo $test?

Best Answer

You always need quotes around variables in all list contexts, that is everywhere the variable may be expanded to multiple values unless you do want the 3 side effects of leaving a variable unquoted.

list contexts include arguments to simple commands like [ or echo, the for i in <here>, assignments to arrays... There are other contexts where variables also need to be quoted. Best is to always quote variables unless you've got a very good reason not to.

Think of the absence of quotes (in list contexts) as the split+glob operator.

As if echo $test was echo glob(split("$test")).

The shell behaviour is confusing to most people because in most other languages, you put quotes around fixed strings, like puts("foo"), and not around variables (like puts(var)) while in shell it's the other way round: everything is string in shell, so putting quotes around everything would be cumbersome, you echo test, you don't need to "echo" "test". In shell, quotes are used for something else: prevent some special meaning of some characters and/or affect the behaviour of some expansions.

In [ -n $test ] or echo $test, the shell will split $test (on blanks by default), and then perform filename generation (expand all the *, '?'... patterns to the list of matching files), and then pass that list of arguments to the [ or echo commands.

Again, think of it as "[" "-n" glob(split("$test")) "]". If $test is empty or contains only blanks (spc, tab, nl), then the split+glob operator will return an empty list, so the [ -n $test ] will be "[" "-n" "]", which is a test to check wheter "-n" is the empty string or not. But imagine what would have happened if $test was "*" or "= foo"...

In [ -n "$test" ], [ is passed the four arguments "[", "-n", "" and "]" (without the quotes), which is what we want.

Whether it's echo or [ makes no difference, it's just that echo outputs the same thing whether it's passed an empty argument or no argument at all.

See also this answer to a similar question for more details on the [ command and the [[...]] construct.

Related Solutions

Bash String Concatenation – Build Parameter List

There is a difference between:

PARMS+="... --exclude='.git'"

and

... --exclude='.git'

In the first, the single quotes are inside quotes themselves, so they are literally present in the substituted text given to rsync as arguments. rsync gets an argument whose value is --exclude='.git'. In the second, the single quotes are interpreted by the shell at the time they're written, because they aren't inside quotes themselves, and rsync gets to see --exclude=.git.

In this case, you don't need the single quotes at all — .git is a perfectly valid shell word on its own, with no special characters, so you can use it literally in the command.

Better for this kind of thing, though, is an array:

PARMS=(-rvu)
PARMS+=(--delete --exclude='.git')
rsync "${PARMS[@]}"

This builds up your command as separate words, with whatever quoting you want interpreted at the time you write the array line. "${PARMS[@]}" expands to each entry in the array as a separate argument, even if the argument itself has special characters or spaces in it, so rsync sees what you wrote as you meant it.

Bash – Why do options in a quoted variable fail, but work when unquoted

Basically, you should double quote variable expansions to protect them from word splitting (and filename generation). However, in your example,

wget_options='--mirror --no-host-directories'
wget $wget_options --directory_prefix="$local_root" "$remote_root$relative_path"

word splitting is exactly what you want.

With "$wget_options" (quoted), wget doesn't know what to do with the single argument --mirror --no-host-directories and complains

wget: unknown option -- mirror --no-host-directories

For wget to see the two options --mirror and --no-host-directories as separate, word splitting has to occur.

There are more robust ways of doing this. If you are using bash or any other shell that uses arrays like bash do, see glenn jackman's answer. Gilles' answer additionally describes an alternative solution for plainer shells such as the standard /bin/sh. Both essentially store each option as a separate element in an array.

Related question with good answers: Why does my shell script choke on whitespace or other special characters?

Double quoting variable expansions is a good rule of thumb. Do that. Then be aware of the very few cases where you shouldn't do that. These will present themselves to you through diagnostic messages, such as the above error message.

There are also a few cases where you don't need to quote variable expansions. But it's easier to continue using double quotes anyway as it doesn't make much difference. One such case is

variable=$other_variable

Another one is

case $variable in
    ...) ... ;;
esac

Best Answer

Related Solutions

Bash String Concatenation – Build Parameter List

Bash – Why do options in a quoted variable fail, but work when unquoted

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