Demystifying grep: A Comprehensive Guide

1. Unix Talk #2 grep/egrep/fgrep

2. grep • Where does the name come from? • :g/RE/p • globally regular expression print • How it works? • grep looks inside file(s) and returns any line that contains the string or expression • prints lines matching a pattern to STDOUT

3. grep examples • grep Pattern • grep Pattern filename • Ex. grep ‘permission’ thisFile • grep pattern file1 file2 • Ex. grep ‘permission’ file1 file2 • file1: • file2: • If grep cannot find a line in any of the specified files that contain the requested pattern, no output is produced.

4. grep Exit Status • $? is set to 0 • if grep finds something • $? is set to 1 • if grep finds nothing • $? is set to 2 • if one of the input file cannot be found. • sed and awk do not use the exit status to indicated the success or failure of locating a pattern. They report failure only if there is a syntax error in a command.

5. grep continue • The general format: • grep [options] PATTERN [FILE…] • Reading from files • grep re * • Reading from STDIN • grep ‘pattern’ • cat file | grep ‘pattern’ • ls –l | grep ‘Jan’

6. More examples A=‘This is it’ echo $A | grep ‘This’ A=`grep ‘tborrelli’ /etc/passwd` if [[ -z $A ]] ; then echo “tborrelli Not found” else echo “tborrelli found” fi

7. grep Options • grep • -i: ignore case • grep –i unix filename • -n: list line numbers along with the matching line • [File:]lineNumber:theLine • -v: invert match • grep –v ‘#’ /etc/hosts • Produces a list of all the lines in /etc/hosts that do not contain the # character. • ps –ef | grep tborrelli | grep –v grep

8. grep Options cont’ • -l: only listing the filenames • grep –l delete projects/* • Pqops.c • Pqops.h • Scheduler.c • -w: matches the whole word • grep –w wordonly FILE • grep –w north datafile • -c: Print the number of lines where the pattern was found • grep –c ‘^no[a-z]*’ datafile • -A [B] num: Print num of lines after [before] match lines • grep –A 1 ‘^south[a-z]’ datafile

9. The grep family • grep • grep ‘^[0-9][0-9]*$’ • Try grep ‘^[0-9]+$’ • Try grep ‘^[0-9]\+$’ • egrep (grep –E) • Extended/Enhanced grep with more RE metacharacters • egrep ‘^[0-9]+$’ • fgrep (grep –F) • Fixed/fast grep, treats all characters as literals • fgrep ‘^[0-9]+$’ • rgrep (grep –r/R) • Recursive grep

10. fgrep • Special case: fgrep fgrep STRING file1 file2 … • Same as grep -F STRING file1 file2 … • The STRING is a fixed string, not a REGEXP • All metacharacters are disabled – i.e. they are treated as themselves

11. egrep • Special case: egrep egrep REGEXP file1 file2 … • Same as egrep REGEXP file1 file2 … • The meta characters ?, +, {, |, (, and ) have their special meanings • In grep (as opposed to egrep) these have no special meaning unless escaped with a backslash \

12. egrep • Metacharacter +, ?, a|b, ( ) • Examples (copy the datafile from /home/fac/pub) • egrep ‘NW|EA’ datafile • egrep ‘3+’ datafile • egrep ‘2\.?[0-9]’ datefile • egrep ‘(no)+’ datafile • egrep ‘S(h|u)’ datafile • egrep ‘Sh|u’ datafile • egrep ‘[[:space:]]\.[[:digit:]][[:space:]]’ datafile

13. egrep (Con’t) • egrep ‘^$’ file • egrep ‘fun\.$’ file • egrep ‘[A-Z]…[0-9]’ file • egrep –v ‘[tT]est’ file • egrep –i ‘sam’ file • egrep –l ‘Dear Boss’ * • egrep –n “$name” file

14. The grep family (Con’t) • GNU grep • grep • egrep or grep –E • fgrep or grep –F • More metacharacters • \w same as [a-zA-Z0-9_] • \W same as [^a-zA-Z0-9_]