====== Gnu tools ======

===== tail =====

Display last 20 lines of a file:
<code>
tail -20 myfile.txt
</code>

===== awk =====


===== head =====

Display first line :
<code>
head -1
</code>

Display two first lines :
<code>
head -2
</code>

===== sed =====

remove last "character" found in a line (here : ,):
<code>
sed 's/\(.*\),/\1/' 
</code>

Remove blanck lines :
<code>
sed '/^$/d'
</code>

Replace a value after a pattern :
For example, to replace the integer after the string my_string in file myfile :
<code bash myfile>
my_string : 40
</code>
Here no -i, only to test and show what will be modified. Note that \s+ means "space" or "multiple spaces", in case of user add a space somewhere.
<code>
mynewvalue=10
sed -re "s/(my_string\s+:\s+)[^=]*$/\1 $mynewvalue/" myfile
</code>
Add -i to apply directly in the file :
<code>
mynewvalue=10
sed -i -re "s/(my_string\s+:\s+)[^=]*$/\1 $mynewvalue/" myfile
</code>
From http://stackoverflow.com/questions/19737398/sed-how-to-replace-a-string-found-after-a-specific-pattern-is-located-in-a-file