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

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

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'
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

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

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 

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

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 
$ cut -d" " -f2 gender.txt | grep F | wc -l 

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.