I have a command which outputs groups I'm a member of as text:
$ groups
dbadmin hdpcl3 stargroup9 jorgesgroup ...I would like to check if this output contains a group I have in mind. I tried:
$ groups > mygroups.txt
$ grep -R "stargroup9" mygroups.txt
stargroup9This works, but I'd like to do it in one line without creating a temporary file. One way that would be neat, as it would generalize, would be to know how to:
$ groups | grep -R "stargroup9" .Where '.' means put the input here, like in R's dplyr/magrittr syntax:
> my_species %>% filter(iris, Species == .)1 Answer
To make grep process its stdin you should invoke it without file (or directory) names as operands. This is a straightforward, although not the best solution:
groups | grep "stargroup9"
This is way better:
groups | grep -E '(^| )stargroup9( |$)'where (^| ) matches the beginning of the line or a space character, ( | $) matches a space character or the end of the line; so e.g. a group named stargroup98765 is not a match.
-E uses extended regular expressions. As a basic regular expression (without -E) the pattern should look like this: \(^\| \)stargroup9\( \|$\) (and it should be single-quoted). I used -E because the pattern is more readable this way.
You may encounter an advice with a pattern like \bstargroup9\b, where \b anchors at word boundary (or grep -w stargroup9 which is eqivalent). Do not use this approach here. The problem is group names can contain - but this character (as far as \b is concerned) separates words. In effect the pattern can match e.g. a group named stargroup9-123foo. If you are in this group but not in stargroup9, the pattern with \b will mislead you (and the same applies to grep -w).
In a script you may want to check if you're a member without printing any output from grep. Use -q to make the command quiet. Its exit status is enough. Examples:
groups | grep -qE '(^| )stargroup9( |$)' && do_something
if groups | grep -qE '(^| )stargroup9( |$)'; then …What is wrong with your approach?
grep -R reads files under specified directories recursively. The option changes nothing if you specify only file(s) (i.e. non-directory).
If you want grep to read from its stdin (like in groups | grep …), you cannot specify any operands (files or directories). Then you cannot use -R because without operands the option makes grep process the current directory, not stdin.
Side note: -R is not specified by POSIX, therefore not portable.
I think groups may produce output in a form of
username : group1 group2 …especially if you specify the user name (groups username). If so, you may want to remove everything excessive first. Since : cannot appear anywhere else (it's not a valid character in user/group name), the following command should work regardless of whether groups generates output starting with username : or not:
groups username | sed 's/.*: //' | grep -E '(^| )groupname( |$)'
