\nIf you want to put the string “Meet at 2” into the file “schedule”,\nyou might be tempted to use\n<\/p>\n
\necho Meet at 2>schedule\n<\/pre>\n\nIf you try this, however, you’ll see the string “Meet at” on the\nscreen and the “schedule” file will be blank.\n[Typo fixed, 10am]\n<\/p>\n
\nWhat happened?\n<\/p>\n
\nA digit immediately before a redirection operator modifies\nwhich stream the redirection operator applies to.\nIf you’re going to redirected an alternate output stream,\nit’ll nearly always be the standard error stream, or stream 2.\nTo put the error output into a file, you would write\nsomething like this:\n<\/p>\n
\nsort \/invalidswitch 2>errorfile\n<\/pre>\n\nThere is also the operator “>&” that reopens a stream\nas another stream.\nThe idiom\n<\/p>\n
\nsome-command >output 2>&1\n<\/pre>\n\nsays, “Put the normal output into the file
output<\/code>,\nand then change the error output stream (2) to refer to the\nnormal output stream (1).”\nThe result is that both the regular output and error output\nend up in theoutput<\/code> file.\n<\/p>\n\nBut what if you really want to put the string “Meet at 2” into\nthe file “schedule”?\n<\/p>\n
\nYou can insert a space between the “2” and the “>”.\nThis works for most programs since they ignore trailing spaces\non their command line, but this was a trick question:\nThe
echo<\/code> command is one of the few commands\nthat actually pays attention to trailing spaces.\nAs a result, the contents of the “schedule” file is\n“Meet at 2<space><cr><lf>”.\nMaybe this is close enough for you, in which case you can skip\nthe next paragraph.\n<\/p>\n\nBut what if you don’t want that trailing space?\nFor that, you can use the metacharacter escape character, the ^<\/tt>:\n<\/p>\n
\necho Meet at ^2>schedule\n<\/pre>\n\nThe last gotcha is that the pesky “2” might come from environment\nvariable expansion.\n<\/p>\n
\nset message=Meet at 2\necho %message%>schedule\n<\/pre>\n\nThe trailing “2” in
%message%<\/code> interacts with the\ngreater-than sign, leading to an unintended redirection.\nFor this, you can insert a space before the greater-than sign,\nassuming you are in a scenario where that space is not going to\ncause you any problems.\n(And if you’re in a scenario where that space will cause a problem,\nyou can use a trick we’ll look at next time.)\n<\/p>\n\nMind you, if you’re going to take an environment variable\nwhose contents you do not control and expand it\nonto your command line unquoted, you have much worse problems\nthan a trailing digit messing up your file redirection.\nSomebody might have decided that the message\nshould be “
&format C: \/y<\/code>“. Inserting this into the command\nline unquoted would yield “echo &format C: \/y>schedule<\/code>”\nwhich is a pretty good way to ruin somebody’s day.\n(Well, okay, you can’t format a drive with an active pagefile,\nbut you get the idea.)<\/p>\n","protected":false},"excerpt":{"rendered":"If you want to put the string “Meet at 2” into the file “schedule”, you might be tempted to use echo Meet at 2>schedule If you try this, however, you’ll see the string “Meet at” on the screen and the “schedule” file will be blank. [Typo fixed, 10am] What happened? A digit immediately before a […]<\/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":[104],"class_list":["post-31173","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-oldnewthing","tag-tipssupport"],"acf":[],"blog_post_summary":"
If you want to put the string “Meet at 2” into the file “schedule”, you might be tempted to use echo Meet at 2>schedule If you try this, however, you’ll see the string “Meet at” on the screen and the “schedule” file will be blank. [Typo fixed, 10am] What happened? A digit immediately before a […]<\/p>\n","_links":{"self":[{"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/posts\/31173","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=31173"}],"version-history":[{"count":0,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/posts\/31173\/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=31173"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/categories?post=31173"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/tags?post=31173"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}