cut(1) -f1 -d: /etc/passwd
remove sections from each line of files
-f, --fields=LIST
       select only these fields;  also print any line that contains no delimiter character, unless the -s
       option is specified
-d, --delimiter=DELIM
       use DELIM instead of TAB for field delimiter
With no FILE, or when FILE is -, read standard input.
source manpages: cut