bash scripting continued; remote X windows; unix tidbits

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1 bash scripting continued; remote X windows; unix tidbits
CSE 391 Lecture 6 bash scripting continued; remote X windows; unix tidbits slides created by Marty Stepp, modified by Jessica Miller and Ruth Anderson

2 Lecture summary more shell scripting various new Unix/Linux commands
if/else while/until select/case advanced: arrays and functions various new Unix/Linux commands newlines in Unix vs Windows Remote X windows file archiving and compression

3 Exit Status Every Linux command returns an integer code when it finishes, called its “exit status” 0 usually* denotes success, or an OK exit status Anything other than 0 (1 to 255) usually denotes an error You can return an exit status explicitly using the exit statement You can check the status of the last command executed in the variable $? $ cat someFileThatDoesNotExist.txt $ echo $? 1 # “Failure” $ ls 0 # “Success” * One example exception: diff returns “0” for no differences, “1” if differences found, “2” for an error such as invalid filename argument

4 if/else if [ condition ]; then # basic if commands fi
if [ condition ]; then # if / else if / else commands1 elif [ condition ]; then commands2 else commands3 The [ ] syntax is actually shorthand for a shell command called “test” (Try: “man test”) there MUST be spaces as shown: if space [ space condition space ] include the semi-colon after ] (or put “then” on the next line)

5 The test command $ test 10 –lt 5 $ echo $? 1 # “False”, “Failure” $ test 10 –gt 5 0 # “True”, “Success” Another syntax for the test command: Don’t forget the space after [ and before ] $ [ 10 –lt 5 ] $ [ 10 –gt 5 ]

6 <, <=, ==, >, >=, !=
test operators comparison operator description =, !=, \<, \> compares two string variables -z, -n tests if a string is empty (zero-length) or not empty (nonzero-length) -lt, -le, -eq, -gt, -ge, -ne compares numbers; equivalent to Java's <, <=, ==, >, >=, != -e, -f, -d tests whether a given file or directory exists -r, -w, -x tests whether a file exists and is readable/writable/executable if [ $USER = “husky14" ]; then echo ‘Woof! Go Huskies!’ fi LOGINS=$(w -h | wc -l) if [ $LOGINS -gt 10 ]; then echo ‘attu is very busy right now!’ *Note: man test will show other operators.

7 compound comparison operators
More if testing # alert user if running >= 10 processes when # attu is busy (>= 5 users logged in) LOGINS=$(w -h | wc -l) PROCESSES=$(ps -u $USER | wc -l) if [ $LOGINS -ge 5 -a $PROCESSES -gt 10 ]; then echo "Quit hogging the server!" fi compound comparison operators description if [ expr1 -a expr2 ]; then ... if [ expr1 ] && [ expr2 ]; then ... and if [ expr1 -o expr2 ]; then ... if [ expr1 ] || [ expr2 ]; then ... or if [ ! expr ]; then ... not -a does NOT short circuit As seen previously, && short circuits || short circuits

8 safecopy Exercise Write a script called safecopy that will mimic the behavior of cp –i where from is a filename and to is a filename: $ cp –i from.txt to.txt Do you want to overwrite to.txt? (yes/no) $ ./safecopy.sh from.txt to.txt

9 safecopy Exercise Solution
#!/bin/bash FROM=$1 TO=$2 if [ -e $TO ]; then read -p "Do you want to overwrite $TO?" ANSWER if [ $ANSWER = "yes" ]; then cp $FROM $TO fi else HIDE

10 BMI Exercise Write a program that computes the user's body mass index (BMI) to the nearest integer, as well as the user's weight class: $ ./bmi.sh Usage: ./bmi.sh weight height $ ./bmi.sh Your Body Mass Index (BMI) is 15 Here is a sandwich; please eat. $ ./bmi.sh Your Body Mass Index (BMI) is 32 There is more of you to love. BMI Weight class  18 underweight normal overweight  30 obese

11 BMI Exercise solution #!/bin/bash # Body Mass Index (BMI) calculator
if [ $# -lt 2 ]; then echo "Usage: $0 weight height" exit 1 # 1 indicates failure, 0 for success fi let H2=“$2 * $2” let BMI="703 * $1 / $H2" echo "Your Body Mass Index (BMI) is $BMI" if [ $BMI -le 18 ]; then echo "Here is a sandwich; please eat." elif [ $BMI -le 24 ]; then echo "You're in normal weight range." elif [ $BMI -le 29 ]; then echo "You could stand to lose a few." else echo "There is more of you to love." HIDE

12 Common errors [: -eq: unary operator expected [: too many arguments
you used an undefined variable in an if test [: too many arguments you tried to use a variable with a large, complex value (such as multi-line output from a program) as though it were a simple int or string let: syntax error: operand expected (error token is " ") you used an undefined variable in a let mathematical expression

13 while and until loops while [ condition ]; do # go while condition is true commands done until [ condition ]; do # go while condition is false

14 While exercise Prompt the user for what they would like to do. While their answer is “open the pod bay doors” tell them that you cannot do that and prompt for another action.

15 While Exercise solution
#!/bin/bash # What would you like to do? read -p "What would you like me to do? " ACTION echo "You said: $ACTION" while [ "$ACTION" = "open the pod bay doors" ]; do echo "I'm sorry Dave, I'm afraid I can't do that." done echo "Bye" HIDE The quotes around “$ACTION” are important here, try removing them and see what happens.

16 select and case Bash Select statement:
PS3=prompt # Special variable* for the select prompt select choice in choices; do commands break # Break, otherwise endless loop done Bash Case statement: case EXPRESSION in  CASE1) COMMAND-LIST;;  CASE2) COMMAND-LIST;; ...  CASEN) COMMAND-LIST;; esac *see lecture 5

17 Select Example PS3="What is your favorite food? " # Goes with the select stmt echo "Welcome to the select example!" echo "It prints out a list of choices" echo "but does nothing interesting with the answer." select CHOICE in "pizza" "sushi" "oatmeal" "broccoli"; do echo "You picked $CHOICE" break done echo “For the select statement, you pick a number as your choice."

18 Case Example echo "Welcome to the case example!" echo "Without a select statement, you must get the spelling/case exact." read -p "What format do you prefer? (tape/cd/mp3/lp) " FORMAT echo "You said $FORMAT" case "$FORMAT" in "tape") echo "no random access!";; "cd") echo "old school";; "mp3") echo "how modern";; "lp") echo "total retro";; esac

19 select/case Exercise Have the user select their favorite kind of music, and output a message based on their choice

20 select/case Exercise Solution
PS3="What is your favorite kind of music? " select CHOICE in "rock" "pop" "dance" "reggae"; do case "$CHOICE" in "rock") echo "Rock on, dude.";; "pop") echo "Top 100 is called that for a reason.";; "dance") echo "Let's lay down the Persian!";; "reggae") echo "Takin' it easy...";; * ) echo "come on...you gotta like something!";; esac break done HIDE

21 Arrays name=(element1 element2 ... elementN)
name[index]=value # set an element $name # get first element ${name[index]} # get an element ${name[*]} # elements sep.by spaces ${#name[*]} # array's length arrays don't have a fixed length; they can grow as necessary if you go out of bounds, shell will silently give you an empty string you don't need to use arrays in assignments in this course Indexing starts counting at zero.

22 Functions function name() { # declaration commands # ()’s are optional
} name # call functions are called simply by writing their name (no parens) parameters can be passed and accessed as $1, $2, etc. (icky) you don't need to use functions in assignments in this course The function definition must precede the first call to it. Can do recursion – but please don’t!

23 Other useful tidbits

24 Newlines in Windows/Unix
Early printers had two different command characters: Carriage return (\r) – move the print head back to the left margin Line feed (\n) – move the paper to the next line Both occurred when you wanted a “newline” As time went on, various combos were used to signify a “newline” Windows typically uses the (\r\n) version MacOS uses (\r) Unix uses (\n) Can cause problems when displaying text files created on one system on another system Most modern text editors recognize both and do the right thing Can convert if needed: dos2unix and unix2dos commands

25 ssh -X attu.cs.washington.edu
Remote X display Normally, you can’t run graphical programs on remote servers (e.g. attu) however, if you connect your SSH with the -X parameter, you can! the X-Windows protocol is capable of displaying programs remotely ssh -X attu.cs.washington.edu Other options (-Y for “Trusted” mode, -C for compressed, see online) Then try: xeyes, xterm, xclock

26 Tar slides – from lecture 4

27 tar files To create a single file from multiple files:
description tar create or extract .tar archives (combines multiple files into one .tar file) Originally used to create “tape archive” files Combines multiple files into a single .tar file You probably always want to use –f option and IT SHOULD COME LAST To create a single file from multiple files: $ tar -cf filename.tar stuff_to_archive -c creates an archive -f read to/from a file stuff_to_archive - can be a list of filenames or a directory To extract files from an archive: $ tar -xf filename.tar -x extracts files from an archive

28 Compressed files To compress a file: $ gzip filename produces: filename.gz To uncompress a file: $ gunzip filename.gz produces: filename Similar for zip, bzip2. See man pages for more details. command description zip, unzip create or extract .zip compressed archives gzip, gunzip GNU free compression programs (single-file) bzip2, bunzip2 slower, optimized compression program (single-file)

29 .tar.gz archives Many Linux programs are distributed as .tar.gz archives (sometimes called .tgz) You could unpack this in two steps: gzip foo.tar.gz produces: foo.tar tar –xf foo.tar extracts individual files You can also use the tar command to create/extract compressed archive files all in one step: $ tar -xzf filename.tar.gz -x extracts files from an archive -z filter the archive through gzip (compress/uncompress it) -f read to/from a file Handy tip: You can use the “file” command to see what type a file is, just changing the file extension on a file does not change its type.

30 tar examples You can combine options (-v, -z, etc.) various ways:
Create a single .tar archive file from multiple files (without compression): $ tar -cvf filename.tar stuff_to_archive -c creates an archive file called filename.tar -v verbosely list the files processed -f read to/from a file (as opposed to a tape archive) stuff_to_archive - can be filenames or a directory Add –z option and use filename.tar.gz to use compression: $ tar -cvzf filename.tar.gz stuff_to_archive

31 tar