Two files with a common field - is it possible to sort one based on the other?

Question

Lets say I got two files with a common field. I want to sort the first one based on a field (eg. a numeric field) found in this file... but I want this re-ordering to apply to the other file as well - through the common field.

To take an example -- completely grabbed out of thin air ;-) -- lets take the /etc/passwd and /etc/shadow:

/etc/passwd:
(...)
sshd:x:124:65534::/var/run/sshd:/usr/sbin/nologin
sndiod:x:999:29::/var/lib/sndiod:/usr/sbin/nologin
dictd:x:125:135:Dictd Server,,,:/var/lib/dictd:/bin/false
postgres:x:126:136:PostgreSQL administrator,,,:/var/lib/postgresql:/bin/bash
(...)

/etc/shadow:
(...)
sshd:*:17055:0:99999:7:::
sndiod:!:17055::::::
dictd:*:17055:0:99999:7:::
postgres:*:17055:0:99999:7:::
(...)

Sorting /etc/passwd numerically on UID is easy enough:

sort -n -t: -k3 /etc/passwd > pw

Which with the lines above yields:

/etc/passwd:
(...)
sshd:x:124:65534::/var/run/sshd:/usr/sbin/nologin
dictd:x:125:135:Dictd Server,,,:/var/lib/dictd:/bin/false
postgres:x:126:136:PostgreSQL administrator,,,:/var/lib/postgresql:/bin/bash
sndiod:x:999:29::/var/lib/sndiod:/usr/sbin/nologin
(...)

However, /etc/shadow does not have a numeric UID-field I can sort after... It does however -- like /etc/passwd -- got a username-field...

So is there a way to re-order the lines in /etc/shadow so that the username-field had the same order as the username-field in the numerically sorted /etc/passwd?

• Is there a command -- sort or something similar -- which can sort file A after some key in that file, and simultaneously sort file B on a common field shared by both files?
• Alternatively, is there a suitable two-step-process -- a command, a sed/AWK/Perl-script, or something -- that can use the sorted file A to re-order file B after their common field?

Answer 1

Join the files, sort the combined file, and strip out the columns you don't want.

In this case, field 1 of /etc/passwd is to be joined with field 1 of /etc/shadow, with : as the field separator.

join -t : -1 1 -2 1 /etc/passwd /etc/shadow |
sort -t : -k 3,3n |
cut -d : -f 1,8-

Answer 2

There may be a more elegant way, but here's a two-step method:

for user in $(sort -n -t: -k3 /etc/passwd | cut -d: -f1)
do 
  grep ^${user}: /etc/shadow
done > /tmp/shadow.new

It reads /etc/shadow once for every entry in /etc/passwd.

Answer 3

I need sudo to read /etc/shadow on my machine:

sudo awk -F: 'NR==FNR {uid[$1]=$3; next} {print uid[$1], $0}' /etc/passwd /etc/shadow |
 sort -n |
 cut -d" " -f2-

The awk script prints the uid from the passwd file corresponding to the username in the shadow file, separated by a space. It's then sorted numerically and then the uid is dropped by the cut command.