Description
Configuration Files The two most important configuration files are .bashrc and .bash_profile The file .bashrc is executed every time you start a subshell. The file .bash_profile is executed every time you log in. Environment Variables Here are some common environment variables: Variable Meaning EDITOR User’s preferred editor. HOME User’s home (login) directory. HOSTNAME Name of the host machine. LD_LIBRARY_PATH Path to search for dynamically loadable libraries. LESS Flags to provide to the GNU “less” pager. MAIL Location of incoming mail. MANPATH Path to search for manual pages. MORE Flags to provide to the “more” pager. PAGER User’s preferred pagination (terminal file display) program. PATH The sequence of directory prefixes that bash applies in searching for a file known by an incomplete path name. PWD User’s current working directory (yes, it’s PWD, not CWD). SHELL The file name of the user’s login shell. TERM Terminal type for which output is to be prepared. USER The login name of the user. Task #1: 1. Write down the value of your PS1 environment variable as well as the file in which it was set. 2. Write down the values of the above environment variables on your Linux machine. Indicate any that have no value. Setting the prompt There are a lot of escape sequences offered by the Bash shell for insertion in the prompt. From the Bash 2.02 man page: COSC350 SYSTEM SOFTWARE, FALL 2020 2 When executing interactively, bash displays the primary prompt PS1 when it is ready to read a command, and the secondary prompt PS2 when it needs more input to complete a command. Bash allows these prompt strings to be customized by inserting a number of backslash-escaped special characters that are decoded as follows: String Meaning \a ASCII bell character \d date in “Thu Mar 17” format \e ASCII escape character \H hostname \n newline \r carriage return \s name of the shell \t current time in 24-hour HH:MM:SS format \T current time in 12-hour HH:MM:SS format \@ current time in 12-hour am/pm format \u username of current user \v bash version (short form) \V bash version (long form) \w current working directory \W basename of current working directory \! history number of this command (starting at historic time) \# command number of this command (starting at shell creation time) \$ if effective UID is 0 (root), a #, otherwise a $ \nnn character corresponding to octal number nnn \\ a backslash \[ begin a sequence of non-printing characters. Could be used to embed a terminal or control sequence into the prompt. \] end a sequence of non-printing characters Task #2: 1. Save your prompt. Write down exactly how you did it. 2. Change your prompt so it looks like [COSC350 basecwd]: where “basecwd” means the basename of the current working directory. Write down exactly how you did it. 3. Change your prompt to its previous value. Write down exactly how you did it. COSC350 SYSTEM SOFTWARE, FALL 2020 3 Command-line completion in Linux You can save a lot of typing by getting bash to complete your commands and file or directory names. Do this by pressing the tab key after enough of the word you are trying to complete has been typed in. If bash can figure out the rest, it will complete the word for you. Otherwise, it’s possible that after hitting tab there are multiple possibilities for the completion. Press tab again and it will list the possibilities. Task #3: 1. Try out command-line completion. Just type the beginning few letters of something and hit the “Tab” key. (Suggestion: from your home directory, type ls -l D, then hit Tab.) Play around a little with this feature. Nothing to write down. Redirecting Standard Input and Output There are always three default “files” open, stdin (directed by default from the keyboard), stdout (directed by default to the screen), and stderr (error messages; also directed by default to the screen). These, and any other open files, can be redirected. Redirection simply means changing where the output or input is directed. Redirection Format Meaning Example COMMAND_OUTPUT > FILENAME Output of the command is redirected to the file. Creates the file if not present, otherwise overwrites it. ls -l > foo Writes a long listing to the file named foo, creating the file if necessary. COMMAND_OUTPUT >> FILENAME Output of the command is redirected to the file. Creates the file if not present, otherwise appends to it. ls -l >> foo Appends a long listing to the file foo, creating the file if necessary. COMMAND_OUTPUT 2> FILENAME Error output of command is redirected to the file. Normal output of the command is not redirected. nosuchcommand 2> foo Redirects the error output to the file foo. 2>&1 Redirects stderr to get sent to same place as stdout. cmd > foo 2>&1 Redirects normal output to the file foo and redirects error output to the same place. COMMAND < FILENAME stdin is redirected from the file to the command. sort < foo Sends the contents of file foo as input to the sort command. Task #4: 1. Invoke ls with a non-existent filename. You should see the error output on the screen. Do it again, but redirect the error output to a file named bar. Write down exactly how you did this. 2. Do it again, but redirect the error output to the “gone forever” file /dev/null. Write down exactly how you did this. 3. Create a file named foo by echo-ing the numbers 3,5,2,1 into it, one number per line. Write down exactly how you did this. 4. Create a file named bar by cat-ing foo into it. Write down exactly how you did this. (Yes, cp would also work, but this lab exercise is about redirection.) COSC350 SYSTEM SOFTWARE, FALL 2020 4 5. Redirect input from foo (it contains numbers, right?) to the sort function. You should see the sorted numbers on the screen. Write down exactly how you did this. Did the numbers turn out sorted numerically? If not, explain how the sort was done. 6. Do it again, but redirect the output from the screen into the file bar. Write down exactly how you did this. Piping Bash allows processes to be connected to other processes. When we connect the output of one process to the input of another, we say we are piping the first process into the second. You can think of a water pipe connecting the first process to the second; output of the first “travels” through the pipe into the second process. Any number of processes can be piped together. The pipe symbol is | (found on most keyboards above the backslash character). The format for using pipes is CMD1 | CMD2. For example: ls | less pipes the output of the command ls into the command less. Another (but silly) example is ls | cat | less. Here we have three processes piped together. The output of ls is piped into cat which merely copies it and pipes its output into less. It’s important to realize that all of the processes in the pipe chain run concurrently. As soon as one process starts producing output, the next process starts working on it. Task #5: 1. Create a file named numbs that contains the integers 1 through 100, one integer per line. The file will have 100 lines. Write down a short description of how you did this. (You can do it any way you want, including dumb brute force. You might also want to consider the bash for loop or a small C++ program.) 2. Run wc on the file numbs. Write down the output and your explanation of what it means. Check the man page for wc if you’re not sure. 3. Use pipes and redirection to produce a second file named somenumbs that contains lines 25 through 38 of numbs. Write down exactly what you did. 4. Run wc on the file somenumbs. Write down the output and your explanation of what it means. COSC350 SYSTEM SOFTWARE, FALL 2020 5 Task #6: Bash shell script (use one of loop statements) Write shell script using for loop to print the following patterns on screen a) task6a.sh 1 22 333 4444 55555 b) task6b.sh * ** *** **** ***** **** *** ** * c) task6c.sh: your program ask a number between 5 to 9. If an input is not between 5 and 9, display error message and ask again. 1 2 2 3 3 3 4 4 4 4 5 5 5 5 5 6 6 6 6 6 6 7 7 7 7 7 7 7 8 8 8 8 8 8 8 8 9 9 9 9 9 9 9 9 9 Task #7: Bash shell scripts Write a script named task7.sh to calculating factorial of given number by using while loop. The shell accepts one integer argument as a parameter and calculates factorial and display the result. Task #8: Bash shell scripts Write a script named “task8.sh” to print given numbers sum of all digit. The shell accept one integer argument as a parameter. Your program must check number of argument is one. If number of argument is not one, your program must display error message and exit. COSC350 SYSTEM SOFTWARE, FALL 2020 6 For example) [separk@sejong] ./sumdigit.sh 345 Sum of digit for number is 12 [separk@sejong] ./sumdigit.sh You need pass one numerical argument Task #9: Bash shell scripts Write a Bash shell script named task9.sh that searches for a word in a file as follows: 1. Asks user for a directory in which to find the file. o If not a valid directory, quits with appropriate error message. 2. Asks user for the name of a readable file in that directory, giving the user three attempts to name one. o If no readable file is named, quits with appropriate error message. 3. Asks user for a word to find in the file. o If word is in the file, FOUND! is printed o Otherwise, NOT FOUND is printed 4. Exit codes are to be: 0 success 1 no such directory 2 no such file (after three attempts) 3 file is not readable 4 word not found in the file What to Hand In Please organize your materials in the order given below. Clearly identify your materials by task. Write your name on every page. Staple the pages neatly. Task #1: Value of your PS1 Values of the given environment variables. Task #2: Command to save prompt. Command to change prompt. Command to change prompt to its previous value. Task #3: Nothing required. Task #4: Command to redirect error output to bar.this. Command to redirect error output to /dev/null. Command(s) to create foo containing numbers. Command(s) to cat foo into bar. Command to redirect foo into sort Answer to numeric sort question with explanation. Command to redirect above into bar. Task #5: Description of how you created numbs. Output of wc on numbs and your explanation of what it means. Command(s) using pipes and redirection to produce somenumbs. Output of wc on somenumbs and explanation. Task #6, 7, 8, 9: submit by email: cosc350fall20@gmail.com
