techne/bash.org

#+title: Bash
#+setupfile: ../org-templates/page.org

** Split large text file into smaller files with equal number of lines
#+begin_src shell
split -l 60 bigfile.txt prefix-
#+end_src

** Loop through lines of file
#+begin_src shell
while read line; do
    echo "$line";
done </path/to/file.txt
#+end_src

** Use grep to find URLs from HTML file

#+begin_src shell
cat urls.html | grep -Eo "(http|https)://[a-zA-Z0-9./?=_%:-]*"
#+end_src

- ~grep -E~: egrep
- ~grep -o~: only output what has been grepped
- ~(http|https)~: either http OR https
- ~a-zA-Z0-9~: match all lowercase, uppercase, and digits
- ~.~: match period
- ~/~: match slash
- ~?~: match ?
- ~=~: match =
- ~_~: match underscore
- ~%~: match percent
- ~:~: match colon
- ~-~: match dash
- ~*~: repeat the [...] group any number of times

** Use Awk to print the first line of ~ps aux~ output followed by each grepped line

To find all cron processes with ~ps aux~.

#+BEGIN_SRC bash
ps aux | awk 'NR<2{print $0;next}{print $0 | grep "cron"}' | grep -v "awk"
#+END_SRC

- ~ps aux~ : equivalent to ~ps -aux~. ~-a~ displays info about other users processes besides to current user. ~-u~ displays info associated with keywords ~user~, ~pid~, ~%cpu~, ~%mem~, ~vsz~, ~rss~, ~tt~, ~state~, ~start~, ~time~, and ~command~. ~-x~ includes processes which do not have a controlling terminal. See ~man 1 ps~.
- ~awk 'NR<2{print $0;next}{print $0 | "grep cron"}' | grep -v "awk"~ : For number of input records (~NR~) less than 2, ~print~ the input record (~$0~), go to the next input record and repeat the ~{print $0}~ pattern until the end is reached, then execute the END rule. The End rule in this case is ~{print $0 | "grep cron"}~, it prints the remaining input records after piping them through the ~"grep cron"~ command. This allows printing the first line of the ~ps aux~ output, which consists of the column labels, and filters out everything besides what you want to grep for (e.g. "cron" processes).
- ~grep -v "awk"~ : avoids printing the line containing this command.