{"id":576,"date":"2017-04-22T20:43:42","date_gmt":"2017-04-22T20:43:42","guid":{"rendered":"http:\/\/www.vidarholen.net\/contents\/blog\/?p=576"},"modified":"2017-04-22T20:43:42","modified_gmt":"2017-04-22T20:43:42","slug":"z-var-works-unreasonably-well","status":"publish","type":"post","link":"https:\/\/www.vidarholen.net\/contents\/blog\/?p=576","title":{"rendered":"[ -z $var ]<\/code> works unreasonably well"},"content":{"rendered":"

There is a subreddit \/r\/nononoyes<\/a> for videos of things that look like they’ll go horribly wrong, but amazingly turn out ok.<\/p>\n

[ -z $var ]<\/code> would belong there.<\/p>\n

It’s a bash statement that tries to check whether the variable is empty, but it’s missing quotes. Most of the time, when dealing with variables that can be empty, this is a disaster. <\/p>\n

Consider its opposite, [ -n $var ]<\/code>, for checking whether the variable is non-empty. With the same quoting bug, it becomes completely unusable:<\/p>\n\n\n\n\n\n
Input<\/th>\nExpected<\/th>\n[ -n $var ]<\/code><\/th>\n<\/tr>\n
“”<\/td>\nFalse<\/td>\nTrue!<\/strong><\/td>\n<\/tr>\n
“foo”<\/td>\nTrue<\/td>\nTrue<\/strong><\/td>\n<\/tr>\n
“foo bar”<\/td>\nTrue<\/td>\nFalse!<\/strong><\/td>\n<\/tr>\n<\/table>\n

These issues are due to a combination of word splitting and the fact that [<\/code> is not shell syntax but traditionally just an external binary with a funny name. See my previous post Why Bash is like that: Pseudo-syntax<\/a> for more on that.<\/p>\n

The evaluation of [<\/code> is defined in terms of the number of argument. The argument values have much less to do with it. Ignoring negation, here’s a simplified excerpt from POSIX test<\/a>:<\/p>\n\n\n\n\n\n\n
# Arguments<\/th>\nAction<\/th>\nTypical example<\/th>\n<\/tr>\n
0<\/td>\nFalse<\/td>\n[ ]<\/code><\/td>\n<\/tr>\n
1<\/td>\nTrue if $1 is non-empty<\/td>\n[ \"$var\" ]<\/code><\/td>\n<\/tr>\n
2<\/td>\nApply unary operator $1 to $2<\/td>\n[ -x \"\/bin\/ls\" ]<\/code><\/td>\n<\/tr>\n
3<\/td>\nApply binary operator $2 to $1 and $3<\/td>\n[ 1 -lt 2 ]<\/code><\/td>\n<\/tr>\n<\/table>\n

Now we can see why [ -n $var ]<\/code> fails in two cases:<\/p>\n

When the variable is empty and unquoted, it’s removed, and we pass 1 argument: the literal string “-n”. Since “-n” is not an empty string, it evaluates to true when it should be false.<\/p>\n

When the variable contains foo bar<\/code> and is unquoted, it’s split into two arguments, and so we pass 3: “-n”, “foo” and “bar”. Since “foo” is not a binary operator, it evaluates to false (with an error message) when it should be true.<\/p>\n

Now let’s have a look at [ -z $var ]<\/code>:<\/p>\n\n\n\n\n\n
Input<\/th>\nExpected<\/th>\n[ -z $var ]<\/code><\/th>\nActual test<\/th>\n<\/tr>\n
“”<\/td>\nTrue: is empty<\/td>\nTrue<\/strong><\/td>\n1 arg: is “-z” non-empty<\/td>\n<\/tr>\n
“foo”<\/td>\nFalse: not empty<\/td>\nFalse<\/strong><\/td>\n2 args: apply -z<\/code> to foo<\/td>\n<\/tr>\n
“foo bar”<\/td>\nFalse: not empty<\/td>\nFalse<\/strong> (error)<\/td>\n3 args: apply “foo’ to -z and bar<\/td>\n<\/tr>\n<\/table>\n

It performs a completely wrong and unexpected action for both empty strings and multiple arguments. However, both cases fail in exactly the right way!<\/p>\n

In other words, [ -z $var ]<\/code> works way better than it has any possible business doing.<\/p>\n

This is not to say you can skip quoting of course. For “foo bar”, [ -z $var ]<\/code> in bash will return the correct exit code, but prints an ugly error in the process. For ” ” (a string with only spaces), it returns true when it should be false, because the argument is removed as if empty. Bash will also incorrectly pass var=\"foo -o x\"<\/code> because it ends up being a valid test through code injection. <\/p>\n

The moral of the story? Same as always: quote, quote quote. Even when things appear to work. <\/p>\n

ShellCheck is aware of this difference, and you can check the code used here online<\/a>. [ -n $var ]<\/code> gets an angry red message, while [ -z $var ]<\/code> merely gets a generic green quoting warning.<\/p>\n","protected":false},"excerpt":{"rendered":"

There is a subreddit \/r\/nononoyes for videos of things that look like they’ll go horribly wrong, but amazingly turn out ok. [ -z $var ] would belong there. It’s a bash statement that tries to check whether the variable is empty, but it’s missing quotes. Most of the time, when dealing with variables that can … Continue reading “[ -z $var ]<\/code> works unreasonably well”<\/span><\/a><\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"open","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"footnotes":"","jetpack_publicize_message":"","jetpack_is_tweetstorm":false,"jetpack_publicize_feature_enabled":true},"categories":[6,4,23],"tags":[11,21],"class_list":["post-576","post","type-post","status-publish","format-standard","hentry","category-basic-linux","category-linux","category-programming","tag-bash","tag-shell-script"],"jetpack_featured_media_url":"","_links":{"self":[{"href":"https:\/\/www.vidarholen.net\/contents\/blog\/index.php?rest_route=\/wp\/v2\/posts\/576","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/www.vidarholen.net\/contents\/blog\/index.php?rest_route=\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/www.vidarholen.net\/contents\/blog\/index.php?rest_route=\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/www.vidarholen.net\/contents\/blog\/index.php?rest_route=\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/www.vidarholen.net\/contents\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fcomments&post=576"}],"version-history":[{"count":36,"href":"https:\/\/www.vidarholen.net\/contents\/blog\/index.php?rest_route=\/wp\/v2\/posts\/576\/revisions"}],"predecessor-version":[{"id":661,"href":"https:\/\/www.vidarholen.net\/contents\/blog\/index.php?rest_route=\/wp\/v2\/posts\/576\/revisions\/661"}],"wp:attachment":[{"href":"https:\/\/www.vidarholen.net\/contents\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=576"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.vidarholen.net\/contents\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=576"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.vidarholen.net\/contents\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=576"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}