Fixed width data
How many households had just 1 person? Referring to the file layout, we see
that the 106th and 107th characters of a household record indicate the number
of people in the household. We can use the cut command to pull out just that
bit of data from each record. The argument -c106-107 instructs cut to print
the 106th through 107th characters of each line. The head command prints just
the first few lines of a file (or its standard input).
$ census_data>grep "^H" pums_53.dat | cut -c106-107 | head -5
03
02
03
02
02
You can give cut a comma separated list to pull out multiple ranges. To see
the household id along with the number of occupants of the household:
$ census_data>grep "^H" pums_53.dat | cut -c2-8,106-107 | head -5
000011703
000024602
000231203
000242102
000250202
The -c argument is used for working with so called "fixed-width" data. Data
where the columns of a record are found at certain offset in bytes from the
beginning of a record. Fixed width data abounds on a Unix system. ls -l
writes its output in a fixed width format:
$ ls -l /etc | head -5
total 6548
-rw-r--r-- 1 root root 46 Dec 4 12:23 adjtime
drwxr-xr-x 4 root root 4096 Oct 8 2003 alchemist
-rw-r--r-- 1 root root 1048 Aug 31 2001 aliases
-rw-r--r-- 1 root root 12288 Oct 8 2003 aliases.db
As does ps:
$ ps -u'
USER PID %CPU %MEM VSZ RSS TTY STAT START TIME COMMAND
jrauser 26870 0.0 0.1 2576 1388 pts/0 S 09:45 0:00 /bin/bash
jrauser 8943 0.0 0.0 2820 880 pts/0 R 12:58 0:00 ps -u
Returning to the question of how many 1 person households are there in Washington:
$ grep "^H" pums_53.dat | cut -c106-107 | grep -c 01
7192
7,192, or about 28% of households have only one occupant.
Delimited data
In delimited data, elements of a record are separated by a special delimiter character. In the password file, fields are delimited by colons:
$ head -5 /etc/passwd
root:x:0:0:root:/:/bin/bash
bin:x:1:1:bin:/bin:/sbin/nologin
daemon:x:2:2:daemon:/sbin:/sbin/nologin
adm:x:3:4:adm:/var/adm:/sbin/nologin
lp:x:4:7:lp:/var/spool/lpd:/sbin/nologin
The 7th column of the password file is the user's login shell. How many people use bash as their shell?
$ cut -d: -f7 /etc/passwd | grep -c /bin/bash
170
You can give either -c or -f a comma separated list, so to see a few users
that use tcsh as their shell:
$ cut -d: -f1,7 /etc/passwd | grep /bin/tcsh | head -5
iglass:/bin/tcsh
svowell:/bin/tcsh
dsedaris:/bin/tcsh
skine:/bin/tcsh
jhitt:/bin/tcsh
Tricky delimiters
The space character is a common delimiter. Unfortunately, your shell probably
discards all extra whitespace on the command line. You can sneak a space
character past your shell by wrapping it in quotes, like: cut -d" " -f 5.
The tab character is another common delimiter. It can be hard to spot, because
on the screen it just looks like any other white space. The od (octal dump)
command can give you insight into the precise formatting of a file. For
instance I have a file which maps first names to genders (with 95%
probability). When casually inspected, it looks like fixed width data:
$ head -5 gender.txt
AARON M
ABBEY F
ABBIE F
ABBY F
ABDUL M
But on closer inspection there are tab characters delimiting the columns:
$ od -bc gender.txt | head
0000000 101 101 122 117 116 040 040 040 040 040 040 011 115 012 101 102
A A R O N \t M \n A B
0000020 102 105 131 040 040 040 040 040 040 011 106 012 101 102 102 111
B E Y \t F \n A B B I
0000040 105 040 040 040 040 040 040 011 106 012 101 102 102 131 040 040
E \t F \n A B B Y
0000060 040 040 040 040 040 011 106 012 101 102 104 125 114 040 040 040
\t F \n A B D U L
0000100 040 040 040 011 115 012 101 102 105 040 040 040 040 040 040 040
\t M \n A B E
The first thing to do is read your system's manpage on cut: it may already
delimit by tab by default. If not, it requires a bit of trickery to get a tab
character past your shell to the cut command. First, many shells have a
feature called tab completion; when you hit tab they don't actually insert a
tab, instead they attempt to figure out which file, directory or command you
want and type that instead. In many shells you can overcome this special
functionality by typing a control-v first. Whatever character you type after
the control-v is literally inserted. Like a space character, you need to
protect the tab character with quotes or the shell will discard it like any
other white space separating pieces of the command line.
So to get the ratio of male first names to female first names I might run the
following commands. Between the double quotes I typed control-v and then hit
tab.
$ wc -l gender.txt
5017 gender.txt
$ cut -d" " -f2 gender.txt | grep M | wc -l
1051
$ cut -d" " -f2 gender.txt | grep F | wc -l
3966
Apparently there's much more variation in female names than male names.
If your system's cut command delimits on tab, the above command becomes
simply cut -f2 gender.txt.