How do I get a list of users not present in a CSV file from /etc/passwd?

Question

I need to compare the user list (/etc/passwd) to a .csv file of account and get the account from /etc/passwd that are not in my csv file.

/etc/passwd shows:

root:x:0:0:root:/root:/bin/bash
albert:x:521:521:auto:/home/albert:/bin/bash
alfred:x:521:521:auto:/home/alfred:/bin/bash

.csv file shows:

albert,abc123

Expected output is:

root
alfred

I tried for 2 days with awk and grep but the result are not what I need..

Answer 1

A combination of grep, tr, cut and process substitution:

cut -d: -f1 /etc/passwd | grep -xvFf <(tr ',' '\n'  < file.csv)

The cut command gets the usernames from /etc/passwd (the first :-delimited field).

Then, the tr command converts your CSV fields to one username per line.

The grep command then looks for entire lines (-x) which don't match (-v) any fixed patterns (-F) from a file containing patterns, one per line (-f). We use process substitution (<()) to use tr's output for these patterns.

Answer 2

Not as smooth as muru's solution, but another way using comm, tr and cut and sort:

comm -13 <(<file tr ',' '\n' | sort) <(cut -d ':' -f 1 /etc/passwd | sort)

• comm -13 <(<file tr ',' '\n' | sort) <(cut -d ':' -f 1 /etc/passwd | sort): compares the lines in the output of <file tr ',' '\n' | sort and cut -d ':' -f 1 /etc/passwd | sort and prints only the lines unique to cut -d ':' -f 1 /etc/passwd | sort;
• <file tr ',' '\n' | sort: reads STDIN from file, translates commas to newlines and sorts the output file;
• cut -d ':' -f 1 /etc/passwd | sort: prints the first colon-separated field of each line in /etc/passwd and sorts the output file;

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And another way using combine (in the moreutils package: sudo add-apt-repository universe && sudo apt-get update && sudo apt-get install moreutils), which allows to save a couple of sort commands (though most likely combine sorts the files internally anyway):

combine <(cut -d ':' -f 1 /etc/passwd) NOT <(<file tr ',' '\n')

• combine <(cut -d ':' -f 1 /etc/passwd) NOT <(<file tr ',' '\n'): compares the lines in the output of cut -d ':' -f 1 /etc/passwd and <file tr ',' '\n' and prints only the lines unique to cut -d ':' -f 1 /etc/passwd;
• <file tr ',' '\n': reads STDIN from file and translates commas to newlines;
• cut -d ':' -f 1 /etc/passwd: prints the first colon-separated field of each line in /etc/passwd;