A list of useful commands

• ls: what files were updated last.
  ls -ltu
  sorts by "time of last access".
  You can use it to discover unsuspected dependencies by trying out bits of code and then seeing what got accessed.
• tar: create an archive file, e.g. for backup
  cd parentdir; tar cvhf subdir.tar ./subdir
  tars the subdirectory subdir
  cd to parent directory to ensure the tar is a local subdirectory. Never use absolute path name for the tarred directory. c create, v verbose, f file, h follow links as if standard files, p preserve permissions. More on file transfer and backup.
• diff: find common files in two directory trees
  diff -r -s jag jag.old | grep identical > junk.jagident &
  lists all files that are identical in two directory trees jag, jag.old.
  This perl script removes all identical files from jag.old:

        #!/usr/bin/perl
        #diff -r -s jag jag.old | grep identical > junk.jagident &
        open(FILE,"< junk.jagident");
        while () {
          $line = $_;
          $line =~ s/\bFiles\b//g;
          $line =~ s/\band\b//g;
          $line =~ s/\bare identical\b//g;
          $line =~ s/\bjag\/\S*\b//g;
          `/bin/rm -f $line`;
  	print $line; 
        }
        

  ls -lR jag.old
  lists all files that are not identical.
• find: Find files
  find looks at each downstream file and applies set of specified tests.
  find dir -size +10000 -print
  Locates very large files.
  find dir -size +5000 -print
  Locates large files.
  find dir -size +1000 \( -mtime +30 -o -atime +730 \)\ -exec ls -l {}\;
  The -size argument finds files in access (+) of 1000 512 blocks.
  The -o enables logical or between the two time arguments.
  The atime specifies files last accessd more (+) than 2 years (730 days) ago.
  ctime refers to last file or file attribute change.
  mtime refers to last file modification date. (contents)
  Note how () and * are quoted.
  find dir \( -name core -o '*.o' \)\ -print
  The name arguments pass files named core or having suffix .o.
  find . -perm 644 -print
  finds files with permission set according to octal 644.
  find . -type f -size +1048575c\ -atime +730 -print
  c\ after size prevents from measuring 512-byte blocks.
  find . -name '*.tex' -exec grep clone {} ';' -print
  greps the string "clone" in all files with suffix ".tex" in the current directory and its subdirectories.
  rm -f 'find . -type f -name 'tmp*' -print'
  removes all files (type f) with name tmp by returning list of these files (result find command in backquotes) to rm command. find . -type f -name 'temp*' -mtime +1 -exec rm -f {} ';'
  finds all files named temp older than 1 day, and removes them. The ; signals the end of the -exec command. Note how the semicolon has to be quoted.
  find . -type f -name 'temp*' -mtime +1 -ok rm -f {} ';'
  Same as before but asks interactively before removing the file.
  
• Check the machine status
  df lists free disk space.
  du -s dir states disk usage: size of dir trees.
  du dir | sort -nr lists all files and dirs by size.
  /etc/pstat -T disk space sorted by job, e.g. swap, files.
  top displays running processes dynamically.
  vmstat states memory usage by process.
  dmesg prints kernel messages (e.g. bootup).
  arp you better look that one up.
• Machine reports about itself
  UNIX does not define commands for such meta information. But here is the pseudo standard. There are tools such as GNU's configure, or PPM's architecture command. Here is the emerging pseudo standard UNIX command:
  arch returns application architecture.
  arch -k returns kernel architecture.
  
• Security
  pwck verifies the password file integrity.
  grep ':00*:' /etc/passwd finds accounts without userID 0.
  grep '^[^:]*::' /etc/passwd finds accounts without password.
  ls -l /etc/passwd checks owner passwd file.
  find / \( -perm -2000 -o -perm -4000 \) -type f -print list files with SUID SGID access mode.
  ~matt/usr/bin/permch checks set permissions.