My hostname details are as below after showing command of hostname in linux
my-host-test-db-10001.dns.biz.xyz.com
my-host-test2-db-10002.dns.biz.xyz.com
my-host-test3-db-10003.dns.biz.xyz.com
I want to fetch the 3rd string from these above (test/test2/test3). how can I achieve it?
In addition to the simpler solution using cut, you can use more flexible grep:
hostname | grep -Po '^[^-]+-[^-]+-\K[^-]+'
For example:
grep -Po '^[^-]+-[^-]+-\K[^-]+' <<< 'my-host-test2-db-10002.dns.biz.xyz.com'
Output:
test2
Here, GNU grep uses the following options:
-P : Use Perl regexes.
-o : Print the matches only (1 match per line), not the entire lines.
\K : Cause the regex engine to "keep" everything it had matched prior to the \K and not include it in the match. Specifically, ignore the preceding part of the regex when printing the match.
SEE ALSO:
perlre - Perl regular expressions
I am trying to extract the version from a colon delimited list. The value I want is for foo, however there is another value in the list called foo-bar causing both values to return. This is what I am doing:
LIST="foo:1.0.0
foo-bar:1.0.1"
VERSION=$(echo "${LIST}" | grep "\bfoo\b" | cut -s -d':' -f2)
echo -e "VERSION: ${VERSION}"
Output:
VERSION: 1.0.0
1.0.1
NOTE: Sometimes LIST will look like the following, which should result in version being empty (this is expected).
LIST="foo
foo-bar:1.0.1"
You may use a PCRE regex enabled with -P option and use a (?!-) negative lookahead that will fail the match in case there is a - after a whole word foo:
grep -P "\bfoo\b(?!-)"
See online demo
This regex should extract any number and optional dots at the end of each line. If the line ends with a colon, then it won't match.
grep -oE '(([[:digit:]]+[.]*)+)$
I want to match tags in files (with optional brackets) ... easy one would think ... the regex is something like ^\[?MyTag\]?. But ... Grep doesn't like it. None of the lines that would be valid matches are actually matched.
The interesting part is: if I replace the ? with a * (so zero to infinite matches, not zero or one) it matches everything like it should, but really that would mean the feature is broken and I don't believe that.
Any input?
Using grep (GNU grep) 2.22 on Windows.
PS: so grep is like this ...
grep -e "^\[?MyTag\]?" file.txt
and my test file is like this
[MyTag] hello
NotMyTag ugly
[NotMyTag] dumb
MyTag world
which obviously should result in 1st and 4th line showing but shows nothing.
First off, ? is not supported in vanilla grep, so you need to use the -E flag to enable extended regex. You can easily verify this by running grep '?' <<< 'a' and grep -E '?' <<< 'a'. Only the latter will match. -e just explicitly indicates what your regex is. It is not the same as -E.
Your initial command works fine if you change the -e to upper case:
grep -E '^\[?MyTag\]?'
Example:
$ grep -E '^\[?MyTag\]?' <<< '[MyTag] hello
> NotMyTag ugly
> [NotMyTag] dumb
> MyTag world'
Output:
[MyTag] hello
MyTag world
Credit goes to the answers of this question on SuperUser.
? is not part of the basic regular expressions, which grep supports. GNU grep supports them as an extension, but you have to escape them:
$ grep '^\[\?MyTag\]\?' file.txt
[MyTag] hello
MyTag world
Or, as pointed out, use grep -E to enable extended regular expressions.
For GNU grep, the only difference between grep and grep -E, i.e., using basic and extended regular expressions, is what you have to escape and what not.
Basic regular expressions
Capture groups and quantifying have to be escaped: \( \) and \{ \}
Zero or one (?), one or more (+) and alternation (|) are not part of BRE, but supported by GNU grep as an extension (but need to be escaped: \? \+ \|)
Extended regular expressions
Capture groups and quantifying don't have to be escaped: ( ) and { }
?, + and | are supported and don't need be be escaped
I am having problems with using grep along with a pipe. The scenario is as follows:
I am running a python script that outputs (using print) to the screen debug messages. I use ./prog | grep "\[\[\[\[" to catch the strings with "[[[[" in them. It returns few matching results but not others (Another observation: results found by grep come before the results not found by grep in the file). I have ran the ./prog without pipe and grep and it outputs all the strings with "[[[[" pattern.
The problem is that the left square bracket is a special character in regular expressions. "grep" is not just a string matcher. Regular expressions are an involved language that let you describe patterns of text. Grep is trying to interpret [[[[ as a regular expression, not just a string.
As your question subject suggests, you can usually escape special characters with a backslash. So the following might work:
./prog | grep '\[\[\[\['
You can also "escape" square brackets by putting them inside square brackets. Thus, [[][[][[][[] or [[]{4} if your version of grep handles it.
You also need to determine whether your program, ./prog, is sending output to "standard output" or "standard error". You can put all your stderr through the pipe with:
./proc 2>&1 | egrep '[[]{4}'
UPDATE:
[ghoti#pc ~]$ printf '[[[[\n[[[\n[[[[\n[[[[[\n[[\n' | grep '\[\[\[\['
[[[[
[[[[
[[[[[
[ghoti#pc ~]$ printf '[[[[\n[[[\n[[[[\n[[[[[\n[[\n' | egrep '[[]{4}'
[[[[
[[[[
[[[[[
[ghoti#pc ~]$
Obviously, my results do not match yours. If you can provide more details as to the data you're processing, it will be helpful in trying to duplicate your results.
Error messages are usually sent to stderr, not stdout; your pipe is filtering stdout. (Your "another observation" hints at this.) You can redirect stderr along with stdout to the pipe:
./prog 2>&1 | grep '\[\[\[\['
Is there a way to make grep output "words" from files that match the search expression?
If I want to find all the instances of, say, "th" in a number of files, I can do:
grep "th" *
but the output will be something like (bold is by me);
some-text-file : the cat sat on the mat
some-other-text-file : the quick brown fox
yet-another-text-file : i hope this explains it thoroughly
What I want it to output, using the same search, is:
the
the
the
this
thoroughly
Is this possible using grep? Or using another combination of tools?
Try grep -o:
grep -oh "\w*th\w*" *
Edit: matching from Phil's comment.
From the docs:
-h, --no-filename
Suppress the prefixing of file names on output. This is the default
when there is only one file (or only standard input) to search.
-o, --only-matching
Print only the matched (non-empty) parts of a matching line,
with each such part on a separate output line.
Cross distribution safe answer (including windows minGW?)
grep -h "[[:alpha:]]*th[[:alpha:]]*" 'filename' | tr ' ' '\n' | grep -h "[[:alpha:]]*th[[:alpha:]]*"
If you're using older versions of grep (like 2.4.2) which do not include the -o option, then use the above. Else use the simpler to maintain version below.
Linux cross distribution safe answer
grep -oh "[[:alpha:]]*th[[:alpha:]]*" 'filename'
To summarize: -oh outputs the regular expression matches to the file content (and not its filename), just like how you would expect a regular expression to work in vim/etc... What word or regular expression you would be searching for then, is up to you! As long as you remain with POSIX and not perl syntax (refer below)
More from the manual for grep
-o Print each match, but only the match, not the entire line.
-h Never print filename headers (i.e. filenames) with output lines.
-w The expression is searched for as a word (as if surrounded by
`[[:<:]]' and `[[:>:]]';
The reason why the original answer does not work for everyone
The usage of \w varies from platform to platform, as it's an extended "perl" syntax. As such, those grep installations that are limited to work with POSIX character classes use [[:alpha:]] and not its perl equivalent of \w. See the Wikipedia page on regular expression for more
Ultimately, the POSIX answer above will be a lot more reliable regardless of platform (being the original) for grep
As for support of grep without -o option, the first grep outputs the relevant lines, the tr splits the spaces to new lines, the final grep filters only for the respective lines.
(PS: I know most platforms by now would have been patched for \w.... but there are always those that lag behind)
Credit for the "-o" workaround from #AdamRosenfield answer
It's more simple than you think. Try this:
egrep -wo 'th.[a-z]*' filename.txt #### (Case Sensitive)
egrep -iwo 'th.[a-z]*' filename.txt ### (Case Insensitive)
Where,
egrep: Grep will work with extended regular expression.
w : Matches only word/words instead of substring.
o : Display only matched pattern instead of whole line.
i : If u want to ignore case sensitivity.
You could translate spaces to newlines and then grep, e.g.:
cat * | tr ' ' '\n' | grep th
Just awk, no need combination of tools.
# awk '{for(i=1;i<=NF;i++){if($i~/^th/){print $i}}}' file
the
the
the
this
thoroughly
grep command for only matching and perl
grep -o -P 'th.*? ' filename
I was unsatisfied with awk's hard to remember syntax but I liked the idea of using one utility to do this.
It seems like ack (or ack-grep if you use Ubuntu) can do this easily:
# ack-grep -ho "\bth.*?\b" *
the
the
the
this
thoroughly
If you omit the -h flag you get:
# ack-grep -o "\bth.*?\b" *
some-other-text-file
1:the
some-text-file
1:the
the
yet-another-text-file
1:this
thoroughly
As a bonus, you can use the --output flag to do this for more complex searches with just about the easiest syntax I've found:
# echo "bug: 1, id: 5, time: 12/27/2010" > test-file
# ack-grep -ho "bug: (\d*), id: (\d*), time: (.*)" --output '$1, $2, $3' test-file
1, 5, 12/27/2010
cat *-text-file | grep -Eio "th[a-z]+"
You can also try pcregrep. There is also a -w option in grep, but in some cases it doesn't work as expected.
From Wikipedia:
cat fruitlist.txt
apple
apples
pineapple
apple-
apple-fruit
fruit-apple
grep -w apple fruitlist.txt
apple
apple-
apple-fruit
fruit-apple
I had a similar problem, looking for grep/pattern regex and the "matched pattern found" as output.
At the end I used egrep (same regex on grep -e or -G didn't give me the same result of egrep) with the option -o
so, I think that could be something similar to (I'm NOT a regex Master) :
egrep -o "the*|this{1}|thoroughly{1}" filename
To search all the words with start with "icon-" the following command works perfect. I am using Ack here which is similar to grep but with better options and nice formatting.
ack -oh --type=html "\w*icon-\w*" | sort | uniq
You could pipe your grep output into Perl like this:
grep "th" * | perl -n -e'while(/(\w*th\w*)/g) {print "$1\n"}'
grep --color -o -E "Begin.{0,}?End" file.txt
? - Match as few as possible until the End
Tested on macos terminal
$ grep -w
Excerpt from grep man page:
-w: Select only those lines containing matches that form whole words. The test is that the matching substring must either be at the beginning of the line, or preceded by a non-word constituent character.
ripgrep
Here are the example using ripgrep:
rg -o "(\w+)?th(\w+)?"
It'll match all words matching th.