I am no shell scripting expert, but I have written a lot of bash code in my day. Most of my shell scripting know-how has come from trial-and-error and desperate googling. This has (slowly) taught me a few nifty tricks and some of the more interesting details of how bash actually “works” (for lack of a better word). I’d like to share one small tidbit with you now.<\/p>\n
The “[<\/b>” program.<\/p>\n
No, that’s not a mistake. There actually is<\/b> a program on UNIX systems called “[“, and it generally lives in \/usr\/bin. Don’t believe me? Check out this which<\/b> command:<\/p>\n Isn’t that odd? On some systems (namely Fedora-variants) the “[” program is actually a symbolic link (“symlink” among friends) to a program called “test<\/b>“:<\/p>\n You see, every good UNIX-like OS ships with a handy utility called test (man 1 test<\/tt>). It’s a simple program that can answer questions like “is variable $foo greater than 42?”, like this:<\/p>\n It can do a whole slew of other comparisons too, including math, string comparisons, file operations (does file $foo exist?), and a bunch of other stuff (man 1 test<\/tt> for full details)<\/p>\n The “test” program gives output solely by its exit status (that’s what $? stores after you run a program in bash). That way, you can use it in bash if statements, like this:<\/p>\n It turns out that our good UNIX forebears hated typing, so they invented the “[” program, which does exactly the same thing, but it looks<\/b> a heck of a lot like a C conditional when used in shell scripts, like this:<\/p>\n The only difference between “test” and “[” is that “[” requires you to have a matching “]” as its last argument or it complains on stderr. Isn’t that nifty? I think so. This explains why, by the way, you have to have spaces before and after the “[” and “]”. It’s because “[” is actually a program name and “]” is just a command line argument like any other argument.<\/p>\n Now get back to that shell script you were hacking.<\/p>\n","protected":false},"excerpt":{"rendered":" I am no shell scripting expert, but I have written a lot of bash code in my day. Most of my shell scripting know-how has come from trial-and-error and desperate googling. This has (slowly) taught me a few nifty tricks and some of the more interesting details of how bash actually “works” (for lack of […]<\/p>\n","protected":false},"author":2,"featured_media":0,"comment_status":"closed","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[2],"tags":[],"class_list":["post-188","post","type-post","status-publish","format-standard","hentry","category-code-and-cruft"],"_links":{"self":[{"href":"https:\/\/thesmithfam.org\/blog\/wp-json\/wp\/v2\/posts\/188","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/thesmithfam.org\/blog\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/thesmithfam.org\/blog\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/thesmithfam.org\/blog\/wp-json\/wp\/v2\/users\/2"}],"replies":[{"embeddable":true,"href":"https:\/\/thesmithfam.org\/blog\/wp-json\/wp\/v2\/comments?post=188"}],"version-history":[{"count":21,"href":"https:\/\/thesmithfam.org\/blog\/wp-json\/wp\/v2\/posts\/188\/revisions"}],"predecessor-version":[{"id":1547,"href":"https:\/\/thesmithfam.org\/blog\/wp-json\/wp\/v2\/posts\/188\/revisions\/1547"}],"wp:attachment":[{"href":"https:\/\/thesmithfam.org\/blog\/wp-json\/wp\/v2\/media?parent=188"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/thesmithfam.org\/blog\/wp-json\/wp\/v2\/categories?post=188"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/thesmithfam.org\/blog\/wp-json\/wp\/v2\/tags?post=188"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}# which [
\n\/usr\/bin\/[
\n<\/code><\/p><\/blockquote>\n# ls -l \/usr\/bin\/[
\nlrwxrwxrwx root \/usr\/bin\/[ -> test<\/code><\/p><\/blockquote>\n# foo=43
\n# test $foo -gt 42
\n# echo $?
\n0
\n# test $foo -lt 42
\n# echo $?
\n1<\/code><\/p><\/blockquote>\nfoo=43
\nif test $foo -gt 42; then
\n echo $foo is greater than 42
\nelse
\n echo $foo is NOT greater than 42
\nfi<\/code><\/p><\/blockquote>\nfoo=43
\nif [ $foo -gt 42 ]; then
\n echo $foo is greater than 42
\nelse
\n echo $foo is NOT greater than 42
\nfi<\/code><\/p><\/blockquote>\n