This one return abcc because there is a in the front and 2 c at the tail. Update based on a comment: in a regular expression is not exactly the. He took the name from the ed command string g/re/p, which translates as 'global regular expression search. If you want to just match abc, you could just say grep abc myFile. Thompson spent about an hour that evening making his tool a general utility that could be used by others and renamed it as grep. He needed a tool that could search for phrases and strings within text files. McMahon was trying to identify the authors of the Federalist papers through textual analysis. His department head at Bell Labs, Doug Mcilroy, approached Thompson and described the problem one of his colleagues, Lee McMahon, was facing.

Ken Thompson had extracted the regular expression search capabilities from the ed editor ( pronounced ee-dee) and created a little program - for his own use - to search through text files. The first two are bang on the third is slightly off.

grep (value TRUE) returns a character vector containing the selected elements of x (after coercion, preserving names but. (For information about regular expression matching, see Regular expressions. egrep works in a similar way, but uses extended regular expression matching. It does not use regular expressions instead, it uses fixed string comparisons to find matching lines of text in the input. 10 In a bash script, I have a list of lines in a file I wish to grep and then display on standard out, which is easiest done with a while read: grep 'regex' 'filepath' while read line do printf 'line ' done However, I would like to inform the user if no lines were matched by the grep. This will be an integer vector unless the input is a long vector, when it will be a double vector. fgrep searches files for one or more pattern arguments. Thirdly, it was written overnight to satisfy a particular need. grep (value FALSE) returns a vector of the indices of the elements of x that yielded a match (or not, for invert TRUE ). Secondly, the wealth of options can be overwhelming. The grep command is famous in Linux and Unix circles for three reasons. grep -R -exclude-dir/sys -exclude-dir/proc -exclude-dir. It also has an -exclude-dir option to 'Exclude directories matching the pattern DIR from recursive searches.' So you could write your search as. It also works with piped output from other commands. GNU grep, which is what you're using if you're running Linux, will do recursive searches by itself, without the need for find and xargs. I confirm your result with grep 2.5.4, but all four of your greps do work when using grep 2.6.3 (Ubuntu 10.10). The Linux grep command is a string and pattern matching utility that displays matching lines from multiple files. This looks like a behavior difference in the handling of s between grep 2.5 and newer versions (a bug in old grep).

Grep pattern not matching