{"id":4801,"date":"2021-06-12T00:01:15","date_gmt":"2021-06-12T07:01:15","guid":{"rendered":"https:\/\/c-for-dummies.com\/blog\/?p=4801"},"modified":"2021-06-26T06:42:45","modified_gmt":"2021-06-26T13:42:45","slug":"reading-wildcards-from-the-command-line","status":"publish","type":"post","link":"https:\/\/c-for-dummies.com\/blog\/?p=4801","title":{"rendered":"Reading Wildcards from the Command Line"},"content":{"rendered":"

Back in May, I wondered how command line input could be processed when a wildcard is present (Lesson link<\/a>). My research lead me to the glob()<\/em> function, but you don’t use this function to process a command line wildcard argument. The reason is that these wildcards are handled by the shell; your code has no direct way to determine when a wildcard is present as a command line argument.
\n
\nMy glob Lesson<\/a> helped present the globbing concept. It’s useful, but when it comes to processing a wildcard in a command line argument, things work differently — though the principles of glob remain in effect.<\/p>\n

When your code attempts to read a wildcard argument, only the first matching file is returned. Why is that? And why isn’t the wildcard returned directly?<\/p>\n

The answer deals with how the shell processes wildcard command line arguments: They are expanded from a single argument into multiple arguments, one for each matching filename.<\/p>\n

For example, if you have 14 C source code files in a directory, and you have a command line with a single *.c<\/code> wildcard as its argument, the operating system creates 15 command line arguments: the program name plus the 14 matching C source code files. The value of argc<\/code> in the main()<\/em> function reflects this condition:<\/p>\n

2021_06_12-Lesson-b.c<\/a><\/h3>\n
\r\n#include <stdio.h>\r\n#include <stdlib.h>\r\n\r\nint main(int argc, char *argv[])\r\n{\r\n    int x;\r\n\r\n    for( x=0; x<argc; x++ )\r\n        printf(\"%d = %s\\n\",x,argv[x]);\r\n\r\n    return(0);\r\n}<\/pre>\n
In the above code, each command line argument is output. A for<\/em> loop at Line 8 outputs each argument and its argv[]<\/code> element number, starting with zero. Here is the output when only the program name is present:<\/p>\n
$ .\/a.out
\n0 = .\/a.out<\/code><\/p>\n
And here is the program’s output with a wildcard argument:<\/p>\n
$ .\/a.out *.c
\n0 = .\/a.out
\n1 = 0424a.c
\n2 = 0424b.c
\n3 = 0529.c<\/code><\/p>\n
Though only two arguments are specified, the operating system generates four argv[]<\/code> elements: the program filename and the three matching C source code files.<\/p>\n
It’s important to remember that if your code requires several command line arguments, you must properly parse them and not rely upon their discrete positions. I’ll cover a technique for processing command line switches in a future Lesson<\/a>.<\/p>\n
For next week’s Lesson<\/a>, I present a function similar to glob()<\/em>, which also uses wildcards.<\/p>\n","protected":false},"excerpt":{"rendered":"
You can’t detect when a wildcard is present as a command line argument, but your code can properly process such an argument. Continue reading →<\/span><\/a><\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"open","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[2],"tags":[],"class_list":["post-4801","post","type-post","status-publish","format-standard","hentry","category-main"],"_links":{"self":[{"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=\/wp\/v2\/posts\/4801","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fcomments&post=4801"}],"version-history":[{"count":9,"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=\/wp\/v2\/posts\/4801\/revisions"}],"predecessor-version":[{"id":4850,"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=\/wp\/v2\/posts\/4801\/revisions\/4850"}],"wp:attachment":[{"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=4801"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=4801"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=4801"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}