Linux administration

• grep prints lines from files which match a pattern
• For example, to find an entry in the password file /etc/passwd relating to the user nancy:

$ grep nancy /etc/passwd

• grep has a few useful options:
  • -i makes the matching case-insensitive
  • -r searches through files in specified directories, recursively
  • -l prints just the names of files which contain matching lines
  • -c prints the count of matches in each file
  • -n numbers the matching lines in the output
  • -v rev erses the test, printing lines which don't match

• Use grep to find patterns, as well as simple strings
• Patterns are expressed as regular expressions
• Certain punctuation characters have special meanings
• For example this might be a better way to search for Nancy's entry in the password file:

$ grep 'ˆnancy' /etc/passwd

• The caret (ˆ) anchors the pattern to the start of the line
  • In the same way, $ acts as an anchor when it appears at the end of a string, making the pattern match only at the end of a line

• Some regexp special characters are also special to the shell, and so need to be protected with quotes or backslashes

• We can match a repeating pattern by adding a modifier:

$ grep -i 'continued\.*'

• Dot (.) on its own would match any character, so to match an actual dot we escape it with \
• The * modifier matches the preceding character zero or more times
• Similarly, the \+ modifier matches one or more times

• Multiple subpatterns can be provided as alternatives, separated with \|, for example:

$ grep 'fish\|chips\|pies' food.txt # first method
$ grep -E 'fish|chips|pies' food.txt    # second method

• The previous command finds lines which match at least one of the words
• Use \(...\) to enforce precedence:

$ grep -i '\(cream\|fish\|birthday\) cakes' delicacies.txt

• Use square brackets to build a character class:

$ grep '[Jj]oe [Bb]loggs' staff.txt

• Any single character from the class matches; and ranges of characters can be expressed as 'a-z'

• egrep runs grep in a different mode
  • Same as grep -E
• Special characters don't have to be marked with \
  • So \+ is written +, \(...\) is written (...), etc
  • In extended regexps, \+ is a literal +

• sed reads input lines, runs editing-style commands on them, and writes them to stdout
• sed uses regular expressions as patterns in substitutions. sed regular expressions use the same syntax as grep
• For example, to used sed to put # at the start of each line:

$ sed -e 's/ˆ/#/' < input.txt > output.txt

• sed has simple substitution and translation facilities, but can also be used like a programming language

• man 7 regex
• Book: Sed and Awk, 2nd edition, by Dale Dougherty and Arnold Robbins, 1997
• The Sed FAQ, http://www.dbnet.ece.ntua.gr/˜george/sed/sedfaq.html
• The original Sed user manual (1978), http://www.urc.bl.ac.yu/manuals/progunix/sed.txt

• a. Use grep to find information about the HTTP protocol in the file /etc/services.
• b. Usually this file contains some comments, starting with the # symbol. Use grep with the -v option to ignore lines starting with # and look at the rest of the file in less.
• c. Add another use of grep -v to your pipeline to remove blank lines (which match the pattern ˆ$).
• d. Use sed (also in the same pipeline) to remove the information after the / symbol on each line, leaving just the names of the protocols and their port numbers.