# while read line; do echo "$line"; done < file.txt

Is pretty easy done

# cat /path/to/file.txt|tr '[:upper:]' '[:lower:]'

or

# echo "LadiDA"|tr '[:upper:]' '[:lower:]'

Delete a line with a string in it. For example old ssh-keys:

# sed -i "/myuser@old.laptop/d"  .ssh/authorized_keys

You can use the expr

# expr 1 + 2

and you will get 3 as result.

Also some more fancy stuff is possible. For example in a for-loop:

for i in {1..9}; do echo "10.20.30.$(expr $i + 40)   servername-$i"; done

stdout to file

# command > file.txt

stderr to file

# command 2> /dev/null

stdout AND stderr to file

# command &> file.txt

stdout to file AND stderr to file

# command > file.txt 2> /dev/null

stdout to stderr

# command 1>&2

stderr to stdout

# command 2>&1