Evaluations, separators and grep

Evaluations and separators: ; && ||

  • With ; you can put several commands on the same line.

Chaining: command_a ; command_b: always runs both commands.

Remember exit codes? In shell, 0=success and anything 1-255=failure. Note that this is opposite of normal Boolean logic!

The && and || are short-circuit (lazy) boolean operators. They can be used for quick conditionals.

  • command_a && command_b

    • If command_a is successful, also run command_b

    • final exit code is last evaluated one, which has the role of Boolean and.

  • command_a || command_b

    • If command_a is not successful, also run command_b

    • final exit code is that of the last evaluated command, which has the role of Boolean or.

Hint command_a && command_b || command_c

Try: cd /nonexistent_dir && ls /nonexistent_dir compare with cd /nonexistent_dir; ls /nonexistent_dir

Try: ping -c 1 8.8.8.8 > /dev/null && echo online || echo offline

grep

Later on you’ll find out that grep is one of the most useful commands you ever discover on Linux (except for all the other most useful commands ever)

grep <pattern> <filename>  # grep lines that match <pattern>
 -or-
command | grep <pattern>  # grep lines from stdin

# search all the files in the dir/ and its subdirs, to match the word 'is', case insensitive
grep -R -iw 'is' dir/

# grep all lines from *command* output, except those that have 'comment' in it
*command* | grep -v comment

# displaying 2 extra lines before and after the match (-A just after, -B just before)
grep -C 2 'search word' file

# counts the number of matches
grep -c <pattern> file(s)

# shows only the matched part of the string (by default grep shows whole line)
grep -o <pattern> file(s)

# accepts way more advanced regular expressions as a search pattern
grep -E <extended_regexpr> file(s)

For details on what <pattern> could be, look for REGULAR EXPRESSIONS at man grep. Some examples:

# grep emails to a list
grep -Eio "\b[a-z0-9._%+-]+@[a-z0-9.-]+\.[a-z]{2,6}\b" file.txt

# grep currently running firefox processes
ps auxw | grep firefox

# grep H1 and H2 header lines out of HTML file
grep "<[Hh][12]>" file.html

Exercise 1.4

Exercise

  • make a pipe that counts number of files/directories (including dot files) in your directory

  • grep directories out of ls -l

  • grep all but blank lines of the ‘man cut | grep …’

  • Using pipes and commands echo/tr/uniq, find doubled words out of My Do Do list: Find a a Doubled Word.

  • If you are on a multiuser system, count unique logged in users. Tip: w or users gives you a list of all currently login users, many of them have several sessions open. Commands to discover: cut / sort / wc

  • (*) Play with the commands grep, cut: find at least two ways to extract IP addresses out of /etc/hosts. Tip: grep has -o option, thus one can build a regular expression that will grab exactly what you need.