adds some exercise ideas

essential/introduction_to_the_commandline.md

@@ -1235,6 +1235,9 @@ Adding the `--color` argument to `grep` will make the matched patterns jump out
 
 Both are quite related and for simplicities sake you can view regular expressions as wildcards on steroids.
 A more detailed explaination can be found [here](https://unix.stackexchange.com/questions/57957/how-do-regular-expressions-differ-from-wildcards-used-to-filter-files).
+
+### Wildcards
+
 The syntax for wildcards is rater simple and also goes by the name of [globbing](https://en.wikipedia.org/wiki/Globbing) of [filename expansion](https://www.gnu.org/software/bash/manual/html_node/Filename-Expansion.html).
 The most important rules to remember are the following.
 
@@ -1284,6 +1287,9 @@ Can you count the occurences?
 
 * [exercise from Linux long](https://gitea.86thumbs.net/waldek/linux_course_doc/src/branch/master/modules/qualifying/learning_regex.md)
 
+### Regular expressions
+
+TODO
 
 # Pipes and redirects
 
@@ -1567,6 +1573,12 @@ Pipes are a very powerful tool to do text manipulations!
 
 # Text manipulation
 
+Below is a list of some of the essential tools to perform text manipulation.
+I highly recommend reading the man pages for each of these programs.
+Some of them have a myriad of options, others are more limited but they all have their purpose.
+This might a good time to read up a bit on the fundamental [philosophy](https://en.wikipedia.org/wiki/Unix_philosophy) of Linux and Unix, especial the idea to create small programs that do one thing well.
+
+
 | command | description |
 | --- | --- |
 | cut | remove sections from each line of files |
@@ -1583,5 +1595,57 @@ Pipes are a very powerful tool to do text manipulations!
 | tac | concatenate and print files in reverse |
 | comm | compare two sorted files line by line |
 | shuf | generate random permutations |
+| paste | merge lines of file |
+| join | join lines of two files on a common field |
+| patch | apply a diff file to an original |
+| aspell | interactive spell checker |
 
+Below are some examples to illustrate the power of pipes and text manipulation programs.
+You can copy/paste the command in your terminal and remove each pipe, one by one, to discover it's effect.
+Most of these pipes are not actually *useful* but I hope they illustrate the flexibility of the concept.
 
+1. sort by size 
+	```
+	waldek@debian:~$ ls -la | sort --key 5 -g -r | grep -v -E "total|root" | nl
+	     1	drwxr-xr-x 2 waldek waldek 4096 May 10 15:10 .
+	     2	-rw-r--r-- 1 waldek waldek 3526 May  9 11:59 .bashrc
+	     3	-rw-r--r-- 1 waldek waldek  807 May  9 11:59 .profile
+	     4	-rw-r--r-- 1 waldek waldek  722 May 10 12:45 files_with_my_name
+	     5	-rw-r--r-- 1 waldek waldek  220 May  9 11:59 .bash_logout
+	     6	-rw-r--r-- 1 waldek waldek   74 May  9 16:06 .selected_editor
+	     7	-rw------- 1 waldek waldek   49 May 10 15:10 .lesshst
+	waldek@debian:~$
+	```
+1. number of users with `bash` as login shell
+	```
+	waldek@debian:~$ cat /etc/passwd | grep bash | cut -d ":" -f 1 | wc -l
+	2
+	waldek@debian:~$
+	```
+1. files in `/etc` that contain my username
+	```
+	waldek@debian:~$ grep -R $USER /etc 2> /dev/null | cut -d ":" -f 1 | sort | uniq
+	/etc/group
+	/etc/group-
+	/etc/passwd
+	/etc/passwd-
+	/etc/subgid
+	/etc/subuid
+	waldek@debian:~$ 
+	```
+1. files in `/etc` that contain the world *password* but listed by number of occurrences and only the three most populated ones are shown
+	```
+	waldek@debian:~$ grep -R password /etc 2> /dev/null | cut -d ":" -f 1 | sort | uniq -c | sort -g | tail -3
+	      7 /etc/pam.d/common-password
+	      9 /etc/debconf.conf
+	     12 /etc/login.defs
+	waldek@debian:~$
+	```
+
+# Ideas
+
+* find unique phone numbers
+* find valid mobile phone numbers [based on](https://en.wikipedia.org/wiki/Telephone_numbers_in_Belgium#Mobile_numbers)
+* find valid IP addresses
+* find all ugly filenames and replace with better names (replace with \_)
+*