Viewing, searching

• ls is the flexible command allowing you to view the contents of your directory. In what context have we used this command before? This is the first example of a command which takes switches, that is, extra letters preceded by a `-', enabling the utility to do different things. The most common switches for ls follow:
  1. -a: list all entries, including files starting with `.'
  2. -l: list entries, one per line.
  3. -r: list all files in reverse alphabetical order
  4. -u: list files, show when file was last modified, rather than last created
  The various switches can be combined in any order behind one `-'. The one I use the most, for instance, is

  % ls -al.

  $\circ$ Exercise 5: Try each of the above ways to list directory info. Try the directory /home/vastro/tutorial. Which do you prefer? What situations do you think are better suited to the different flavors of ls? First try just ls -l and then ls -al on the directory /home/vastro/tutorial/texts. Did you see anything more with the latter command than the former?

• file filename returns the type of filename.

  For example, in the directory /home/vastro/tutorial/examples,

  % file Uhuru.txt
  Uhuru.txt: ASCII text
  % file unixt.ps
  unixt.ps: PostScript document
  % file xboard-3.6.2.tar.Z
  xboard-3.6.2.tar.Z: compressed data block compressed 16 bits
  % file w3browse-16280.tar
  w3browse-16280.tar: USTAR tar archive
  %

• cat filename dumps the contents of the file to screen. This is okay if the file is less than one screen page long.

  For example, in the directory /home/vastro/tutorial/texts,

  % cat riddle.txt
  At night they come without being fetched,
  And by day they are lost without being stolen.
  What are they?
  %

• more filename dumps the contents of filename to the screen one page at a time. Hit the space bar to go from page to page, and return to step forward line by line.

• tail filename dumps the final 10 lines of a file to screen. tail -n dumps the last n lines of the file to screen.

  For example

  % tail -2 riddle.txt
  And by day they are lost without being stolen.
  What are they?
  %

  $\circ$ Exercise 6: Use each of cat, more and tail to view all the text files in the directory texts. Which was more useful for which files?

  $\bullet$ Question 6: What is the difference between the commands more , cat, and tail?

• grep string filename searches for string in the file filename. The line(s) containing the string is/are returned.

  For example,

  % grep stolen riddle.txt
  And by day they are lost without being stolen.
  %

  $\circ$ Exercise 7: Use grep to find the word `planet' in drake.txt. This file is located in the diretory tutorial/texts. Is `planet' found in any of the other files in this directory?

• To save space during transport of files over the internet, these files are usually compressed. There are several ways to compress files, but the one we will use most often is compress. The switch `-v' displays how efficient the compression was.

  For example, (you can find lightcurve.ps in the directory
  /home/vastro/tutorial/ps.)

  % compress -v lightcurve.ps
  lightcurve.ps: Compression: 71.74%

• uncompress filename.Z decompresses the file
• tar is usually used to construct tape archives for uploading to or downloading from some magnetic medium like tape. It is convenient because one can upload an entire directory as one archive entry, which eases bookkeeping vastly. Tar has several switches, the most common of which are:
  1. -x: extract archive
  2. -v: verbose listing of archive contents
  3. -f: use `tar' argument as name of archive
  4. -c: create archive

  For example (w3browse-16280.tar can be found in the directory
  /home/vastro/tutorial/examples.)

  % tar -xvf w3browse-16280.tar
  x me/rates/d/d45390.lc.Z, 5728 bytes, 12 tape blocks
  x me/rates/d/d45397.lc.Z, 6048 bytes, 12 tape blocks
  x me/rates/d/d45404.lc.Z, 6455 bytes, 13 tape blocks
  x me/rates/d/d45413.lc.Z, 7381 bytes, 15 tape blocks
  %
  creates the subdirectory tree me/rates/d, and place all the files ending with .lc.Z in subdirectory d.

  Figure: Sample session showing the mechanics of extracting and uncompressing an archive's files. Note the use of the wildcard (*), which allowed multiple files to be uncompressed at once. (See section 8 for details). Also, note that the prompt has been changed back to % to minimize clutter in this example.

  

  $\bullet$ Question 7: What are the fundamental uses of compressing and taring?

  $\circ$ Exercise 8: Remember the file .objects.tar you found in the directory
  /home/vastro/tutorial/texts? Go to that directory and use tar to extract the contents of this file. How many directories and files did you extract? What can you say about each of the files you extracted?