Lecture III. Highlights and more

---

More on "ls"

---

• Typical options
• ls <filename>
  simply spits <filename> back on screen
• ls <directoryname>
  lists the contents of the directory
• ls -d <directoryname>
  simply spits <directoryname> back on screen
  
• ls -d
  simply spits everything, files or directories
  
  
• Regular expressions
• ls *R
  lists files and (contents of) directories whose names end in R
  "*" matches anything, of any length, including nothing!
• ls R*
  lists files and (contents of) directories whose names begin in R
  
• ls *R*
  lists files and (contents of) directories whose names contain R
  
• ls ???R??
  lists files and (contents of) directories whose names have six
  characters, fourth of which is an "R"; "?" matches exactly one character.
  
• ls [123]R
  lists files and (contents of) directories whose names are either
  1R, 2R, or 3R. Enclose a "choice" construct inside square brackets.
  
  


• Note the "escape" character "\". When placed in front of the symbol it makes sure that its literal meaning is used and not the expression meaning. (i.e. \$ means use the $ , not match everything preceding the $)


• Miscellaneous
• To list only names of directories, not files, use:
  "ls -d */"
  
• Can also use:
  "ls -d */." and "ls -d */.." (why?)
  
• "ls -a" lists all files and directories, including those that start with ".".
  
• "ls -R" does a recursive listing

---

More on regular expressions

---

• Contexts
• can use them anywhere a name is expected, e.g.
  cd C* will try to cd into a directory that starts with a C. If more than one such directory exists, UNIX will complain. Even if only one thing exists, and that one thing is a file, UNIX will complain because you cannot cd into a file.
  
• On the other hand cat C* will cheerfully concatenate all files that fit the pattern (here, you will hear complaints if the pattern matches a directory).

---

Deciphering the "ls -l" output

---

• A longish output display

total 28
drwxr-xr-x  3 onufriev onufriev 4096 Jan 17 21:09 Desktop/
drwxr-x--x  3 onufriev onufriev 4096 Feb  3 16:52 PAPERS/
drwxr-x--x  2 onufriev onufriev 4096 Jan 30 01:24 STRUCTURES/
-rw-r-----  1 onufriev onufriev    0 Feb  5 20:26 test

• First column lists permissions.
  
• Second column lists the number of hard links to that entry.
  
• Third column lists the owner.
  
• Fourth column lists the group the file/directory is assigned to.
  
• Fifth column lists the size (bytes. Use ls -lh to get human readable format).
  
• Sixth column etc., list timestamp and name.
  

---

Changing permissions

---

• What permissions do you have?
• Look at the first column of "ls -l" output
  
• The first character indicates the type of file:

d = directory; - = plain file; l = link

• Characters 2 . . . 10 give the file access permissions:

r = read; w = write; x = execute.

• Characters 2, 3, 4 give the owner's permissions, followed by group and other user's permissions. A - denotes absence of the permission.
• Updating permissions can be done:
  • for "u" (user), "g" (group), or "o" (others)
  • towards three types of accesses: "r" (read), "w" (write), and "x" (execute files; also used for cd access to directories).
  • "+" gives permission; "-" takes away permission.
  • Use chmod command, e.g.,
    chmod u-rw README
    takes away ("-") read ("r") and write ("w") permission for user ("u") toward README. Cannot read and cat this file anymore.
    
  • chmod u+r README now allows the user to cat the file. chmod a+r README The change is for all users. But still cannot edit (read AND write)
    
  • chmod u+w README gives back the writing capabilities as well.

---

The second column of "ls -l"

---

• lists the number of hard (not soft) links to the file.
  • Files usually have "1" listed unless there are new hard links created using the "ln" command.
    
  • Directories have different numbers listed. In the above "ls -l" output, the directory STRUCTURES has "2" listed because it has two hard links, one is itself, and the other is to a file in it.
    
  • The PAPERS directory has 1 subdirectory and 1 file in it. Now this subdirectory may have their own subdirectories but we cannot infer that from just this output.

---

I/O Redirection

---

• UNIX commands receive their input from the standard input (stdin) and send their output to the standard output (stdout), by default these files are the console or terminal.
• Shells allow input/output (I/O) to be redirected from/to other devices, thus UNIX commands are unaware from what device their input may originate or to what device their output may be sent.

command name [args] > filename

• The output redirection symbol, > , sends the command's output to the specified file instead of the console/terminal screen.

ls -al > ls.out

• The file ls.out is created if it does not exist (or emptied and overwritten if it exists prior to command execution) and sent the directory listing.
• Programs which send their output to the console/terminal may also be redirected.

head -5 README > head

• Note that this redirects the output to a file called "head" (not to be confused with the head command).
• The input redirection symbol, < , sends the command input from the specified file instead of the console/terminal keyboard.
  

cat < README > README2

• This reads input from README, passes it to cat, and then sends the output to README2.
• Pipes | are the logical extension of I/O redirection.
• Pipes allow the stdout of one program to become the stdin of another program.

ls -R | more

• The above command allows the viewing of the long recursive listing of a large directory one screen at a time; Type man more to see what more does. Also see less.
• Can create your own customized "pipes". e.g., here is how you can spit out just lines three to five of a given file.
  

  cat README | head -5 | tail -3