CSC 209 extra problems regarding software tools

1. Write a C program which acts as a standard unix tool (i.e. takes file name arguments or stdin if no args; outputs to stdout; etc) which turns all funny characters in its input to visible sequences as follows:

characters less than ' ' (space) are represented as '^' and the character they're the control character of, e.g. the byte 1 is represented as "^A". 64 is the number to add; 'A' is 1 + 64.
the character 127 is represented as "^?".
characters greater than or equal to 128 are represented as "M-" (for "meta") and then the character minus 128. Note that one of the above two points might then apply.

solution

2. Write a C program to calculate byte frequencies of the input, like this:

% echo helloooooo | bfreq
012: 1
e: 1
h: 1
l: 2
o: 6
%

Use isprint() to make the decision as to whether to use %c or 0%o before the colon.

solution

2b. Write a command-line which sorts the occurrences into frequency order using sort. (Be sure that a frequency of 15 is sorted after a frequency of 7, despite occurring earlier in alphabetical order.)

solution

3. Command-line arguments are file names. Stat them and decrease their mtime by one hour using utimes(). Example session:

% ls -l foo
-rw-r--r--    1 ajr      instrs       1759 Apr 14 11:29 foo
% back1 foo
% ls -l foo
-rw-r--r--    1 ajr      instrs       1759 Apr 14 10:29 foo
%

solution

4. Write the "nth" command, which outputs the nth field of each line (one-origin), based on a strtok-like delimitation of fields. It prints a blank output line when the input line has fewer than the specified number of fields.

Similar to grep, the first non-option argument is the n, and subsequent non-option arguments, if any, are the names of files to be processed. If there are no file names listed, the standard input is processed.

Example:
If the input consists of the two lines

    Hello, world, how are you
    today?  Myself, I am a C program with no feelings.

then "nth 4" would produce the output

    are
    am

and "nth 7" would produce a blank line and then the line "program".

[solution]

5. Write simple versions of any of the following unix tools. That is, implement a minimum of the command-line arguments or other special processing; mostly, just do the simple case, and you can impose arbitrary limits if necessary.

cat [solution]
echo (include -n option; get all spacing and newline exactly right) [solution without -n]
ls (then add the -l option, although you will want to cheat on some fields)
cmp
cmp with a -r option for "recursive"
tr
fgrep (just grep for a fixed string)
sort (just sort whole lines in strict lexical order)
mkdir, including the -p option