What can this sort command do?

[Rashid Ali]

I saw a book cited an example as shown below,
#sort -t: +2n -3 /etc/passwd

Can someone tell me how to interpret this?

Thanks in advance!

[Stefan Farrelly]

This sort command basically displays the passwd file sorted by UID (the 3rd field).

The -t: command means use the colan as a field separator (instead of a space or tab), then the normal sort notation of +X -Y means to sort by fields X to Y, in this case 2 to 3 (the UID is the 3rd field in the passwd file after userid and password), so sort after 2 and until the 3rd field. The n on +2n means sort numerically (as opposed to alphabetically) as the UID field are numbers.

See man on sort for more detailed info.

[federico_3 · Accepted solution]

this command use with the t option as field separator ": ". The option +2n causes sort to skip the first 2 colomns and start to sort at the third field of each /etc/passwd line ( ID number) . -3 says that in case of equal third field, to put the first fields ( unix user ) in the right order.

federico

[Rashid Ali]

What's the difference between the previous command and "sort -t: +2n /etc/passwd" ?
I did a test and it seems getting the same result.

[Stefan Farrelly]

there is no difference between +2n -3 or just using +2n. Just using +2n means do the sort calculation until the next field separator, which in this case is a colan so the result is the same. However, good practice is to use the full sort command format, field +X to -Y, if you use the sort command to sort other files you may need the -Y so good practicse is to use it.

[Rashid Ali]

So sort -t: +3n -4 /etc/passwd will sort the file by Group id,right? But if I want to sort firstly by Group ID (primary key) and then User id (secondary key), should I use
sort -t: -k 4n,4 -k 3n,3 /etc/passwd ?

[federico_3]

Yes, it's ok.


federico