sed

The sed command replaces strings. The result is shown to the screen.
It uses vi commands for editing. More on that later!
The string used for a search can also be a regular expression.
Any line of input that matches the search string that sed is checking for will be processed.

The basic syntax for calling sed is:
```
 sed -e'editing instruction' file
```
If you do not pass in a file name, it gets its input from standard input.
Some useful options:

-e followed by the editing command
-n don't print all the lines
-E use extended regular expressions

Note you can use multiple -e flags with sed.

Here are some examples


sed -e '/^[    ]*$/d' file1 # deletes all the lines that are, or look blank
                             # (that's a space and a tab inside the square
                             # brackets).

sed -e 's/[Dd]og/cat/g' file1   # changes the word dog or Dog 
                                # to cat across the whole line


cat file1 | sed 's/\\/\//g'         # what do these do?

cat file1 | sed -n '2,4p'
cat file1 | sed -n '75,$p'

ls *.JPG | sed 's/JPG/jpg/g'

Remember that the output shows up on the screen; the file is NOT changed.