1. sort

Let's try and compare these commands:
```
du -s /usr/share/* | less
```
```
du -s /usr/share/* | sort | less
```
```
du -s /usr/share/* | sort -r | less
```
```
du -s /usr/share/* | sort -nr | less
```
```
du -s /usr/share/* | sort -nr | head
```
The command du gets the size (disk usage) of the files and directories of /usr/share, and head filters the top 10 results.

Then we try to sort them with sort and sort -r (reverse), but it does not seem to work as expected (sorting results by the size). This is because sort by default sorts the first column alphabetically, so 2 is bigger than 10 (because 2 comes after 1 on the character set).

With the option -n we tell sort to do a numerical sort. So, the last command returns the top 10 biggest files and directories on /usr/share.
This example works because the numerical values happen to be on the first column of the output. What if we want to sort a list based on another column? For example the result of ls -l:
```
ls -l /usr/bin | head
```
Ignoring for the moment that ls can sort its results by size, we could use sort to sort them like this:
```
ls -l /usr/bin | sort -nr -k 5 | head
```
The option -k5 tells sort to use the fifth field as the key for sorting. By the way, ls like most of the commands, separates the fields of its output by a TAB.
For testing we are going to use the file distros.txt, which is like a history of some Linux distributions (containing their versions and release dates).
```
wget https://linux-cli.fs.al/examples/lesson07/distros.txt
cat distros.txt
```
```
cat -A distros.txt
```
The option -A makes it show any special characters. The tab character is represented by ^I, and the $ shows the end of line.
Let's try to sort it:
```
sort distros.txt
```
The result is almost correct, but Fedora version numbers are not in the correct order (since 1 comes before 5 in the character set).

To fix this we are going to sort on multiple keys. We want an alphabetic sort on the first field and a numeric sort on the second field:
```
sort --key=1,1 --key=2n distros.txt
```
```
sort -k 1,1 -k 2n distros.txt
```
```
sort -k1,1 -k2n distros.txt
```
Notice that if we don't use a range of fields (like 1,1, which means start at field 1 and end at field 1), it is not going to work as expected:
```
sort -k 1 -k 2n distros.txt
```
This is because in this case it starts at field 1 and goes up to the end of the line, ignoring thus the second key.

The modifier n stands for numerical sort. Other modifiers are r for reverse, b for ignore blanks, etc.
Suppose that we want to sort the list in reverse chronological order (by release date). We can do it like this:
```
sort -k 3.7nbr -k 3.1nbr -k 3.4nbr distros.txt
```
The --key option allows specification of offsets within fields. So 3.7 means start sorting from the 7-th character of the 3-rd field, which is the year. The modifier n makes it a numerical sort, r does reverse sorting, and with b we are suppressing any leading spaces of the third field.

In a similar way, the second sort key 3.1 sorts by the month, and the third key 3.4 sorts by day.
Some files don't use tabs and spaces as delimiters, for example the file /etc/passwd:
```
head /etc/passwd
```
In this case we can use the option -t to define the field separator character. For example to sort /etc/passwd on the seventh field (the account's default shell), we could do this:
```
sort -t ':' -k 7 /etc/passwd | head
```

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