{"id":96735,"date":"2017-08-02T07:00:00","date_gmt":"2017-08-02T21:00:00","guid":{"rendered":"https:\/\/blogs.msdn.microsoft.com\/oldnewthing\/?p=96735"},"modified":"2019-03-13T01:14:46","modified_gmt":"2019-03-13T08:14:46","slug":"20170802-00","status":"publish","type":"post","link":"https:\/\/devblogs.microsoft.com\/oldnewthing\/20170802-00\/?p=96735","title":{"rendered":"Why does attempting to echo an undefined environment variable result in the message “ECHO is on”?"},"content":{"rendered":"

From a batch file, suppose you try to echo the contents of a variable that hasn’t been defined: <\/p>\n

\nset AWESOME=1\necho %AWESUM%\n<\/pre>\n
This prints ECHO is off<\/code>. What’s up with that? <\/p>\n
The first half of the puzzle is knowing that if a variable is not defined, then it expands to an empty string. Therefore, after expension, the command <\/p>\n
\necho %AWESUM%\n<\/pre>\n
becomes simply <\/p>\n
\necho\n<\/pre>\n
The second half of the puzzle is knowing that the ECHO<\/code> command, if run with no command line, reports the current echo state, printing either ECHO is on<\/code> or ECHO is off<\/code>, accordingly. <\/p>\n
Putting it all together: If you try to echo the value of an undefined variable, and the variable is not defined, then the echo command gets a blank command line, at which point it reports its current state. <\/p>\n
So how do you echo the value of a variable that might be undefined? <\/p>\n
One way is to put something else on the command line. <\/p>\n
\necho AWESUM=%AWESUM%\n<\/pre>\n
If AWESUM<\/code> is not defined, this will expand to <\/p>\n
\necho AWESUM=\n<\/pre>\n
and that prints AWESUM=<\/code>. <\/p>\n
Okay, but what if you really want to echo the value on a line all by itself? <\/p>\n
You can exploit a quirk of the ECHO<\/code> command: If you follow the word ECHO<\/code> immediately with a period (no space), then the period is not echoed, but the presence of the period suppresses the “no command line” behavior, and you get a blank line. <\/p>\n
\nECHO.\n<\/pre>\n
That prints a blank line. <\/p>\n
If you want to print a variable that might not be defined, you would say <\/p>\n
\nECHO.%AWESUM%\n<\/pre>\n
This quirk of the ECHO<\/code> command was originally a bug in the command line parser, but once people discovered it, the bug became elevated to a feature. (The phenomenon of a bug elevated to a feature happens<\/a> a lot<\/a>. Obligatory XKCD<\/a>.) <\/p>\n
Exercise<\/b>: Explain the output of this batch file, and how would you fix it so it prints \/?<\/code>? <\/p>\n
\nset message=\/?\necho %message%\n<\/pre>\n","protected":false},"excerpt":{"rendered":"
Because that’s what happens when you echo nothing.<\/p>\n","protected":false},"author":1069,"featured_media":111744,"comment_status":"open","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"_acf_changed":false,"footnotes":""},"categories":[1],"tags":[25],"class_list":["post-96735","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-oldnewthing","tag-code"],"acf":[],"blog_post_summary":"
Because that’s what happens when you echo nothing.<\/p>\n","_links":{"self":[{"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/posts\/96735","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/users\/1069"}],"replies":[{"embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/comments?post=96735"}],"version-history":[{"count":0,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/posts\/96735\/revisions"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/media\/111744"}],"wp:attachment":[{"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/media?parent=96735"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/categories?post=96735"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/tags?post=96735"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}