CS 246 Winter Tutorial 2

Transcription

1 CS 246 Winter Tutorial 2 Detailed Version January 14, Summary git Stuff File Properties Regular Expressions Output Redirection Piping Commands Basic Scripting Persistent Data Password-less Login 2 git Stuff git clone Creates a clone of the latest version of a given repository git pull Updates a local repository to the latest version of the remote repository 1

2 3 File Properties ls -l prints a detailed list Types of users Owner - owner of the file Group - arbitrary name associated with a set of userids Other - any other user Separate permissions for each type of user Read - allows specified user to read a file/directory Write - allows specified user to write a file/directory Execute file : execute as a command directory : traverse through directory node but not read (cannot read file names) The mode of a file is the collection of permissions assigned to Owner, Group and Other. File Size, Modification date, File name Every directory contains the directory. and.. which represents the current directory and parent directory respectively. 2

3 4 Regular Expressions Two main types of pattern matching you will be using in this course. Globbing expressions are used match filenames with a specified pattern Regular expressions are used by commands including egrep and sed Some useful regular expression operators are: ^ - matches the beginning of the line $ - matches the end of the line. - matches any single character? - the preceding item can be matched 0 or 1 times * - the preceding item can be matched 0 or more times + - the preceding item can be matched 1 or more times [...] - match one of the characters in the set [^...] - match one character not in the set expr1 expr2 - match expr1 or expr2 We will use egrep to exemplify regular expressions Example1: Find all lines in wordcollection that contains a z egrep "z" wordcollection Example2: Find all lines in wordcollection that starts with a and ends with z egrep "^a.*z$" wordcollection Example3: Find all lines in wordcollection that do not contain a or z egrep "^[^az]*$" wordcollection Example4. Find all lines in wordcollection that starts with either a or bl and end with a z egrep "^(a (bl)).*z$" wordcollection 3

4 5 Output Redirection Suppose the program printer prints odd numbers to stdout (standard output) and even numbers to stderr (standard error). Redirect each stream to a separate file./printer > print.out 2> print.err Redirect both streams to the same file. We need to tie the two streams together../printer &> print.both redirects both stdout and stderr to print.both./printer > print.out 2>&1 redirects stdout to print.out and redirects stderr to where stdout is going./printer 2> print.err 1>&2 /dev/null redirects stderr to print.err and redirects stdout to where stderr is going Think of /dev/null as a black hole Any output redirected to this file is discarded. What is the difference between./printer 2>&1 > out and./printer > out 2>&1? Order matters!./printer 2>&1 > print.out To begin both stdout and stderr are printing to the terminal stderr is redirected to where stdout is currently going (terminal) stdout is redirected to the file print.out./printer > print.out 2>&1 To begin both stdout and stderr are printing to the terminal stdout is redirected to the file print.out stderr is redirected to where stdout is currently going (print.out) 4

5 6 Piping Commands Piping commands together allows us to redirect the output of one command into the input of another. Create a pipe that prints the 10 most commonly occurring words in the file wordcollection and prints the results to the file top10. Idea: Use some combination of sort, uniq, and tail. But how? Probably need uniq -c (count occurrences) and sort -n (numeric-sort). Try: uniq -c wordcollection sort -n But what s the problem? uniq only removes consecutive duplicates! We need to first sort the list. Try: sort wordcollection uniq -c sort -n We still need to print the top 10 most common words to the file top10. Try: sort wordcollection uniq -c sort -n tail > top10 7 Basic Scripting Each script starts with a shebang, in our case that will be #! /bin/bash simple.bash To make more complex shell scripts, we likely want to accept arguments from the command line. Recall the following special shell variables: ${#} provides the number of arguments, excluding ${0} ${0} contains the name of the script ${1}, ${2}, ${3},... refer to the arguments by position (not name) ${@} all arguments (except ${0}) supplied to the currently running script as separate strings Write a script named split that takes in two filenames as arguments. split should print the first 5 lines of the first file to stdout and the last 5 lines of the second file to stderr. 5

6 8 Persistent Data How can we create aliases and variables that persist through multiple sessions?.aliases One option is to add aliases and variables to a file like.profile that gets run whenever a new session begins. Like.profile,.aliases also gets run at the beginning of a new sessions Convenient location to store all of your aliases. What if we wanted to call a script to define all of our variables and aliases? source export When we execute a shell script a new process is created to run that script, a subshell. When the script finishes we exit the subshell and return to our original shell. As a result all definitions created in the subshell are lost. Try the following:./definitions.bash date Executes the provided script within the current shell (no new process is created). Try the following: source definitions.bash date Observe the change in the output of ps and that we have assigned a value to QVAR. Makes the provided variable an environment variable. When a subshell is created it is given a copy of each environment variable. Try the following: SVAR= Bird./export-test.bash export SVAR 6

7 ./export-test.bash source export-test.bash 9 Password-less Login You can use RSA public-key encryption to securely SSH into your undergraduate environment without having to provide a password. Only setup on a trusted machine Setup: On your local machine you will first need to generate the RSA keys. ssh-keygen -b t rsa Next you will need to provide the server with your public key. cat /.ssh/id rsa.pub ssh userid@linux.student.cs.uwaterloo.ca mkdir -p.ssh && cat >>.ssh/authorized keys" Note that the private key should only be known and accessible by you. 7