Ubuntu – list all local/system users

$ cut -d: -f1 /etc/passwd

Explanation

cut OPTION... [FILE]... – print selected parts of lines from each FILE to standard output. With no FILE, or when FILE is -, read standard input. Arguments:

• -d, --delimiter=DELIM – use DELIM instead of TAB for field delimiter. In our case the delimiter is :.
• -f, --fields=LIST – select only these fields; also print any line that contains no delimiter character, unless the -s option is specified. In our case the field list contains only one field, 1, meaning that the cut command will print the first field resulted after splitting the line by the delimiter. If we need more fields printed we just list them all separated by comma e.g. cut -d: -f1,5,6 /etc/passwd.