Programming Assignment #2 Solution

Unix utilities  tend  to produce  output which has been described  as “compact” by fans of Unix and as “user-unfriendly” by its detractors. This assignment will have you make the output of a common systems utility  more palatable to a user and to add value to the utility  itself.

 

grep is a most useful utility  which scans specified files for lines which contain  a specified pattern. Do a “man  grep”  to see the  on-line manual  page.  You are to develop a program  called rgpp (for recursive grep  post-processor) which behaves roughly as follows:

 

• It takes the output of a run of grep -r -H -n -s -I -i pattern directory on its standard input.  We assume that the pattern is just a string (not a regular expression).  We can describe the semantics  of this command  argument by argument.

 

–  -r: grep will be run  recursively  against  all files in the  subtree  of the  filesystem rooted at the designated  directory;

 

–  -H: file names will be output for each matching  line;

 

–  -n: line numbers  where matches  occur will be produced;

 

–  -s: error messages about  inaccessible or unreadable files will not be produced;

 

–  -I: binary  files are not searched (treat binary  files as if there  were no matches  in them);

 

–  -i: the pattern will be matched  in case-insensitive  mode;

 

• rgpp will produce,  on its standard output:

 

–  A “banner  line” with a suitable  message and a count of the number  of lines in the grep- searched  files which matched  the  specified pattern, followed by a listing  of each file in which a match  occurred  with each line optionally  numbered  and all lines matching  the grep search string  clearly tagged  (perhaps  preceded  by a “−− ” or similar string).

 

–  A header line should be output before the listing of a file in which there is a match.  That header  line should  include  the  name  of the  file.  An example  of an appropriate header line can be seen in the examples linked from the assignments  web page.

 

Having  given you a rough  idea of the  operation  of rgpp, the  following is a statement of its func- tionality relative  to command  line arguments which control  rgpp’s operation.  The  rgpp program is invoked as:

 

rgpp [-l | -w word] {-n} {-b}

The  first  command  argument  must  be a -l or a -w.  A -l means  that rgpp is operating  in “line mode,” behaving much as described above.  Lines from files which correspond  to line hits from grep will be tagged.  The -n is optional:  if present,  the  file listing will be line-numbered;  if absent,  the lines will be output without  a leading line number.  The -b is optional:  if present,  the banner  line stating  the  number  of lines hit  by grep will be produced  before the  listing of the  files.  If there  is no -b then  there  is no banner  line.

 

Note that the optional  -b and -n may appear  together, and in either  order.  Your program  should not impose a required  order.

 

If rgpp is invoked with -w as the first command  line argument, it operates  in “word mode.”  In word mode, the  banner,  if requested,  states  the  number  of non-overlapping  instances  of the  designated pattern in the  files before emitting  the  numbered, tagged,  listings.   On tagged  lines (lines where a match  to  the  pattern must  be found),  the  non-overlapping instance(s) of the  pattern must  be highlighted. Under the assumption that rgpp will be run within an xterm,  the string  \e[7m sent to the  standard output will switch it into  inverse video mode.  The  string  \e[0m switches the  xterm back to normal  (non-inverted) video mode.  A pattern is “highlighted” by rendering  it in inverse video.   The  output is optionally  line-numbered   as  above,  depending  on the  presence  of the  -n switch.

 

Bear in mind that this code is to be written  as a system  utility,  and,  as such, should “handle”  all possible errors  (e.g., it is possible that what  is believed to be grep output contains  line numbers which  are  non-numeric  or beyond  the  highest  numbered  line in the  program,  an  unreadable file containing  grep’s output may be specified by an idiot user, etc.).  You are responsible  for produc- ing code which  is robust  in the  presence  of ignorant, stupid,  or even malicious,  use by the  user community. Your program  will be tested  against  bizarre  input  when it is graded.  Please note that there will be some stupid  uses of rgpp with which you cannot  reasonably  deal.  For example,

 

% grep -r -H -s -I -i xxx . | rgpp -l -b

 

violates  the  restriction that grep must  be invoked with  the  command  line arguments as specified above (-n is missing in this case);

 

% grep -r -H -n -s -I -i xxx . grepout; cd somewhere; rgpp -l < grepout

 

is even more difficult to handle (given the way that grep embeds file names in its output), and

 

% grep -r -H -n -s -I -i xxx . | gpp -w yyy -b

 

is mass confusion.  Do the best that you can.

 

The operative rule is that rgpp should not dump core, but it is not expected to produce good output when  invoked  incorrectly.    Finally,  generate  informative   error  messages  when  doing  so doesn’t require  an inordinate amount of work.

 

Your  implementation of rgpp should  not  impose limits  on the  number  or size of the  files or the number  of lines on the standard input.  In operational terms,  this means that you must dynamically allocate  some of the major data  structures required  for the implementation of rgpp.