UNIX Processes & Redirection

Last time: Using UNIX

Prof. Brian D. Davison

Computer Science & Engineering, Lehigh University

Additional Credits

Announcements

• HW#2 due today
• Why are man and info better than other alternatives?
• Comments about labs? How many finished with a working mycat that handles multiple files?

C Arrays

• Assuming we know the number of elements, we can easily create and manipulate arrays of items.

  int i[5];
  
  i[0] = i[1] = 1;
  i[2] = 2;
  i[i[2] + 1] = i[2] + 2;
  i[i[2] + i[2]] = i[3] + i[2];
  

• Unfortunately, we cannot set all values of an array at once, nor assign one array to another. Instead, we must iterate through each item.

  int i, a[10];
  
  // clear or copy all values
  for ( i = 0; i < 10; i++ ) {
     a[i] = 0;  or   a[i] = x[i];
  }
  

Array Initialization

• We can, however, initialize the elements of an array when declared.
• All of these are equivalent:
  
  

  unsigned short int a[10] = {1, 2, 3, 4, 5, 6, 0, 0, 0, 0};
  unsigned short int a[10] = {1, 2, 3, 4, 5, 6};
  unsigned short int a[] = {1, 2, 3, 4, 5, 6, 0, 0, 0, 0};
  unsigned short int a[] = {[0]=1, [1]=2, [2]=3, [3]=4, [4]=5,
                             [5]=6, [9]=0};
  

  
  
• The number of elements can be inferred from the initializers, and missing initializers are always set to 0.

Multidimensional Arrays

• You can have arrays of any type, including arrays of arrays.

  int i, j;
  float a[3][10] = { {14.2, 1, 0, 9.4},
                     {0, 144, 7, 2.5},
                     {2, 3, 109, 42.0} };
  

• Could print out contents:

  for (j = 0; j < 10; j = j + 1)
     printf("\t%d:", j);
  printf("\n");
  
  for (i = 0; i < 3; i = i + 1) {
     printf("%d:", i);
     for (j = 0; j < 10; j = j + 1)
          printf("\t%f", a[i][j]);
     printf("\n");
  }
  

• Note that we could have gone in the opposite order (2nd subscript first).

Character Arrays

• An array of characters works the same. We can create and initialize them with the same syntax.

  char hello[] = {'h', 'e', 'l', 'l', 'o', '!'};
  

• There are many characters that are not letters, numbers, or keyboard symbols. Such characters are represented by an escape sequence using the backslash.
• Common characters include:

    \n    a "newline" character (linefeed, ctrl-j)
    \b    a backspace
    \r    a carriage return (without a line feed, ctrl-m)
    \'    a single quote (e.g., in a character constant)
    \"    a double quote (e.g., in a string constant)
    \\    a single backslash
  

• Character constants always use single quotes.

Processes

• A process is the memory, open files, and other system resources needed while a program runs. The OS (kernel) creates a process when you run a program.
• For example, try:
  io:~% more /etc/termcap (e.g., view a large file with more)
  
  Then, press CTRL-Z to suspend the process.
  [1]+ Stopped more /etc/termcap
  io:~%
  SysV (Solaris) ps has different options than BSD-derived ps (e.g., Linux). Use whereis to find the other.

Process information

• ps reports information about processes (owner, cpu/memory usage, etc.)
  [1]+ Stopped more /etc/termcap
  io:~% ps
     PID TTY   TIME CMD
    2556 pts/7 0:00 more
    2540 pts/7 0:00 bash
  io:~%

  By default, it only shows the processes associated with the current terminal.

• When installed, the top command can also be used to see what programs are running, their memory and cpu usage, etc.

Managing Processes

• Normally, running a program in a shell will stop the shell while the other program runs.
• To allow both to run, start the program and put it into the background.
io:~% top &
io:~%
• Pressing CTRL-Z in a running program will suspend that process.

Managing Processes

• The jobs command will show you what processes are running or stopped from this shell:
  io:~% jobs
  [1]+ Stopped more /etc/termcap
  
• The fg and bg commands move the currently stopped program into the foreground or background (and change status of other jobs currently stopped or running).

Killing Processes

• Sometimes, you get a program that you can't stop with CTRL-C.
• To kill your process that won't otherwise stop, type
  kill <processid>

  or if needed,
  kill -9 <processid>

• The killall command can also be used to kill a process using its program name (instead of process identifier).

Redirection

• One of the great features of UNIX is the ability to connect multiple processes that read from stdin and stdout.
• cat, for example, writes to standard out
• In UNIX, stdin and stdout (and stderr) can be redirected.
  • "ls -l > ls-l.txt" will take the output of "ls -l" and write it to a new file called ls-l.txt
  • "more < file1" will use file1 as the standard input to more
  • "prog1 >> file1" will append prog1 stdout to file1 (not replace)
  • "prog1 2> errorfile" will only write stderr messages to errorfile
  • "prog1 &> bothfile" will write both stdout and stderr to bothfile
  • "sort < numfile > sortednums" redirects both stdin and stdout

Pipes

• Programs can be chained together using "pipes"
  • "cat longfile | wc" sends the output of "cat longfile" to the input of "wc"
  • "cat numfile | sort > sortednums" sends the output of "cat numfile" to the input of "sort", whose output is written to "sortednums"
  (wc is a program that counts the number of lines, words, and characters sent to it.)

Example of Pipes

• A more complex (realistic) example:

  cat airweb-access_log airweb-access_log.2005Jan* | grep -v airweb | 
     cut -d' ' -f11 | sort | uniq -c | sort -nr | more

  takes web server logs with entries such as:

  192.114.107.4 - - [23/Jan/2005:09:20:10 -0500] "GET / HTTP/1.1" 200 3515 
  "http://www2005.org/tutorials/accept.html" "Mozilla/4.0 (compatible; MSIE 
  6.0; Windows NT 5.0; .NET CLR 1.0.3705)"

  and generates reports like:
  

     1410 "-"
       24 "http://www.cse.lehigh.edu/~brian/"
       21 "http://www2005.org/tutorials/"
       18 "http://timconverse.com/blog/"
       17 "http://www2005.org/tutorials/workshops.html"
       16 "http://nike.psu.edu/dbevents/"
        8 "http://www.kdnuggets.com/news/2004/n24/28i.html"
        8 "http://blog.searchenginewatch.com/blog/050121-124259"