linux - extract first instance per line (maybe grep?) -

i want extract first instance of string per line in linux. trying grep yields instances per line. below want strings (numbers , letters) after "tn="...but first set per line. actual characters combination of numbers or letters. , there space after them. there space before tn=

given following file:

hello name dog tn=12g3 fun 23k3 hello tn=1d3i9 cheese 234kd dks2 tn=6k4k ksk  1263 chairs tn=k38493kd cars run vroom it95958 tn=k22djd fair gold tn=293838 tounge 

desired output:

12g3  k38493 

here's 1 way can if have gnu grep, (mostly) supports perl compatible regular expressions -p. also, non-standard switch -o used print part matching pattern, rather whole line:

grep -po '^.*?tn=\k\s+' file 

the pattern matches start of line ^, followed characters .*?, ? makes match non-greedy. after first match of tn=, \k "kills" previous part you're left bit you're interested in: 1 or more non-space characters \s+.

as in ed's answer, may wish add space before tn avoid accidentally matching footn=.... might prefer use \w match "word" characters (equivalent [[:alnum:]_]).