6. Editing on the fly

The tr program is used to transliterate characters:
```
echo "lowercase letters" | tr a-z A-Z
```
Multiple characters can be converted to a single character:
```
echo "lowercase letters" | tr [:lower:] A
```
With the option -d it can delete characters:
```
echo "lowercase letters" | tr -d e
```
With the option -s it can squeeze repeated characters:
```
echo "aaabbbccc" | tr -s ab
```
```
echo "abcabcabc" | tr -s ab
```
As another example, let's use tr to implement ROT13 encoding (where each character is moved 13 places up the alphabet):
```
echo "secret text" | tr a-zA-Z n-za-mN-ZA-M
```
To decode, perform the same translation a second time:
```
echo "frperg grkg" | tr a-zA-Z n-za-mN-ZA-M
```
The program sed means stream editor.
```
echo "front" | sed 's/front/back/'
```
The command s stands for substitute.
```
echo "front" | sed 's_front_back_'
```
The character immediately after s becomes the delimiter.

The commands in sed can be preceded by an address:
```
echo "front" | sed '1s/front/back/'
```
```
echo "front" | sed '2s/front/back/'
```
Let's try some more examples on distros.txt:
```
sed -n '1,5p' distros.txt
```
The option -n does not print lines by default, and the p command prints only the lines in the given range.
```
sed -n '/SUSE/p' distros.txt
```
Prints only the lines that match the given regular expression.
```
sed -n '/SUSE/!p' distros.txt
```
Prints only the lines that do not match.
The command s substitutes by default only the first occurrence on a matching line:
```
echo "aaabbbccc" | sed 's/b/B/'
```
We can append the modifier g (global) to replace all the occurrences:
```
echo "aaabbbccc" | sed 's/b/B/g'
```
Let's change the date format from MM/DD/YYYY to YYYY-MM-DD on distros.txt:
```
sed -E 's#([0-9]{2})/([0-9]{2})/([0-9]{4})$#\3-\1-\2#' distros.txt
```
First, we are using the option -E, --regexp-extended because there are lots of special characters like (, ), {, } in the regular expression and escaping all of them by a \ would make it really messy and unreadable.

Then, we are enclosing in parentheses the parts of the regexp that match the month ([0-9]{2}), the day ([0-9]{2}) and the year ([0-9]{4}).

The strings that are matched by a subexpression can be used in the replacement like this: \n, where n is the number of the matching subexpression (pair of parentheses).
It is possible to give several commands to the same sed program, like this:
```
echo "aaabbbccc" | sed -e 's/b/B/g' -e 's/a/A/g'
```
However, sometimes it is preferable to list these commands in a sed script file, and call this script instead. For example:
```
cat <<'EOF' > distros.sed
#sed script to produce a distro report

1 i\
\
Linux Distribution Report\

s#([0-9]{2})/([0-9]{2})/([0-9]{4})$#\3-\1-\2#
y/abcdefghijklmnopqrstuvwxyz/ABCDEFGHIJKLMNOPQRSTUVWXYZ/
EOF
```
download
wget https://linux-cli.fs.al/examples/lesson07/distros.sed
```
cat distros.sed
```
```
sed -E -f distros.sed distros.txt
```
The first line is a comment.

Then, the i command inserts something before the first line.

The s command changes the date format.

Finally, the y command transliterates the lower case characters to upper case, similar to the command tr. However, unlike tr, it does not recognize character ranges or character classes, so we have to list all the letters of the alphabet.

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