Some of these commands work only in the csh or tcsh shells. However, those shells are the default on the Math Department computers, and most likely you are set up to use these shells. You can check this by doing "finger login", where "login" is your login name.

Shell shortcuts

• !!
  Repeats the most recent command.
• !t
  Repeats the most recent command beginning with "t". Similarly for other letters, or strings of initial letters in a command. This is very handy when going through a compiling/running/editing cycle. For example, if you work on a latex document, after giving the full command names "latex file", "xdvi file", "dvips file", during the first cycle, you can then repeat the steps using the shortcuts "!l", "!x", "$d". (Be careful, though; if you type "!r" instead of "!t", you may inadvertently repeat the last delete command.)
• dvips !$
  This executes dvips with the last argument of the most recent command. For example, after doing "xdvi verylongfilename", the above command will run the file "verylongfilename" through dvips. Similarly, after editing a file with "vi verylongfilename", you can print out the file by saying "lpr verylongfilename".
• h
  Gives a list of the most recent commands. Useful if you want to repeat a command, but don't remember the exact form you used.
• !196
  Repeats the command numbered 196. The command numbers can be displayed with the "h" command.
• Up/down arrows As an alternative to the exclamation mark commands, you can use the up/down arrows to move back and forth in the command history and repeat/edit a command.

Pipes and Redirection

Redirection of output is a very simple concept; instead of displaying the output on the screen the program dumps its output onto a file. A Unix pipe works much like a plumber's pipe: it takes the output of the command to the left of the pipe symbol (|) and uses this as the input of the command to the right of the pipe symbol. Multiple pipes can be stacked together. Also, you can combine history shortcuts with pipes. For example, if, after displaying a sorted file with the "sort" command, you find that the file is too big to fit on one screen, the command "!! | less" will redisplay tthe file, one screen at a time.

The following examples illustrate the use of pipes. Most are self-explanatory; "file" stands for a generic filename.

• sort file | less
  Sort the file and display the sorted file, one page at a time. (If you prefer the standard command "more", you can use this instead of "less". "less" is an enhanced version of "more" - for example, it allows you to move backwards and forwards in the file; to exit "less" , use "q".)
• sort file | lpr
  Sort the file and send the sorted file to the printer.
• sort file > file.sorted
  Sort the file and create a new file, "file.sorted", with the contents of the sorted file.

Tools for analyzing data files

• sort: Sort files.
• grep: Extract lines matching a given pattern from a file.
• awk: Extract specific fields, columns, from a file (plus much more).
• perl: The ultimate geek tool. Can do all of the above, plus much more.

While learning the full power of these tools takes time and effort (years in the case of perl), it is easy to learn enough to be able to use these utilities on the command line for simple data analysis tasks. To illustrate this, assume you have a file of numbers, three per line, separated by blanks (this is the most convenient format for the above utilities), like the following:

123  398  17359
317  19  2909
 39  -399  -5789
 49  33   200
255 33   -378

• sort -n file
  Sort the file by first column. The -n option ensures numeric (as opposed to lexicographic) sort.
• sort +1 -n file
  Sort the file by second column. The "+1" here denotes the column used as sort key relative to the first column, i.e., +1 sorts by column 2, +2 sorts by column 3, etc.
• grep '33' file
  Extract all lines containing the string "33" (in the above example, lines 4 and 5).
• grep '-' file
  Extract all lines containing a minus sign (in the above example, lines 3 and 5).
• grep -c '-' file
  Same, but display only the number of matching lines (2 in the example), not the lines themselves.
• grep -c '-' *.out
  Same command, but applied to all files in the current directory matching "*.out". For each file there is an output line of the form "Filename: x", where "x" is the number of matching lines in the file.
• sort -n file | cat -n
  Sort the file, then prepend line numbers to each line. This results in the following:

       1	 39  -399  -5789
       2	 49  33   200
       3	123  398  17359
       4	255 33   -378
       5	317  19  2909
  

  This can be useful if you want to count the number of lines in which the first entry is in a given range: simply subtract the line numbers corresponding to the beginning and end of the range.
• sort -n file | uniq -c | sort -n
  This counts the number of occurrences of each line, then displays the distinct lines of the file, along with the corresponding counts in increasing order. This is useful for files containing multiple identical lines. For example, suppose a file consists of lines each containing a single number from a given set, say {0,1,2}. Then above command would show how many times each of the numbers 0,1,2 in the set occurs.
• awk '{print $2}' file
  Print second column of the file.
• awk '{print $3, ",", $2}' file
  Print the second and third columns of the file in reverse order, separated by a comma. (Something like this may be useful to generate a file that can be imported to gnuplot, gp, etc.)
• awk '{print $NF}' file
  Print the last column of the file.
• perl -pe 's/3/1/g' file
  Replace 3 in the file by 1. (The "g" modifier stands for a global substitute/replace operation. Without it, only the first occurrence would be substituted.)
• perl -lne 'print scalar reverse' file
  Prints the file with all lines reversed by character (i.e., a line "4 15 02" becomes "20 51 4").
• perl -lne 'print if "$_" eq reverse' file
  Finds all palindromic lines in file. In particular, if each line contains a single number, it displays all palindromes among these numbers. If applied to a dictionary file (such as /usr/share/lib/dict/words), with one word per line, it displays the palindromes among the words.
• perl -e '$n=100; $n2=300; while ($n++ < $n2) {print "\n$n" if ((("p" x $n) !~ /^((p)\2+)\1+$/)}'
  Find all primes between 100 and 300.
• perl -e '$n=1; while ($n++) {print "\n$n" if ("p" x $n) !~ /^((p)\2+)\1+$/ and "$n" eq reverse "$n"}'
  Find palindromic primes.

Last modified: Mon 16 Dec 2002 08:33:36 AM CST A.J. Hildebrand