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Intro to shell scripting

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1 Intro to shell scripting
CSE 391 Lecture 5 Intro to shell scripting slides created by Marty Stepp, modified by Jessica Miller & Ruth Anderson

2 Lecture summary basic script syntax and running scripts
shell variables and types control statements: the for loop

3 Shell scripts script: A short program meant to perform a targeted task. a series of commands combined into one executable file shell script: A script that is executed by a command-line shell. bash (like most shells) has syntax for writing script programs if your script becomes > ~ lines, switch to a real language To write a bash script (in brief): type one or more commands into a file; save it type a special header in the file to identify it as a script (next slide) enable execute permission on the file run it!

4 Basic script syntax #!interpreter
written as the first line of an executable script; causes a file to be treated as a script to be run by the given interpreter (we will use /bin/bash as our interpreter) Example: A script that removes some files and then lists all files: #!/bin/bash rm output*.txt ls -l

5 Running a shell script by making it executable (most common; recommended): chmod u+x myscript.sh ./myscript.sh fork a process and run commands in myscript.sh and exit by launching a new shell : bash myscript.sh advantage: can run without execute permission (still need read permission) by running it within the current shell: source myscript.sh advantage: any variables defined by the script remain in this shell (more on variables later)

6 echo Example: A script that prints your current directory.
#!/bin/bash echo "This is my amazing script!" echo "Your current dir is: `pwd`" Exercise : Write a script that when run on attu does the following: clears the screen displays the current date/time Shows who is currently logged on & info about processor command description echo produces its parameter(s) as output (the println of shell scripting) -n flag to remove newline (print vs println)

7 Script example #!/bin/bash
clear # please do not use clear in your hw scripts! echo "Today's date is `date`" echo echo "These users are currently connected:" w -h | sort echo "This is `uname -s` on a `uname -m` processor." echo "This is the uptime information:" uptime echo "That's all folks!"

8 Comments # comment text Example:
bash has only single-line comments; there is no /* ... */ equivalent Example: #!/bin/bash # Leonard's first script ever # by Leonard Linux echo "This is my amazing script!" echo "The time is: `date`" # This is the part where I print my current directory echo “Current dir is: `pwd`"

9 Shell variables name=value (declaration) $name (usage)
must be written EXACTLY as shown; no spaces allowed often given all-uppercase names by convention once set, the variable is in scope until unset (within the current shell) AGE=64 NAME="Michael Young" $name (usage) echo "$NAME is $AGE years old" Produces: Michael Young is 64 years old

10 Common errors if you misspell a variable's name, a new variable is created NAME=Ruth ... Name=Rob # oops; meant to change NAME if you use an undeclared variable, an empty value is used echo "Welcome, $name" # Welcome, when storing a multi-word string, must use quotes NAME=Ruth Anderson # Won’t work NAME=“Ruth Anderson" # $NAME is Ruth Anderson

11 More Errors… Using $ during assignment or reassignment
$mystring=“Hi there” # error mystring2=“Hello” $mystring2=“Goodbye” # error Forgetting echo to display a variable $name echo $name

12 Capture command output
variable=`command` captures the output of command into the given variable Note – this is `back ticks` (not 'single quotes' – see next slide) Simple Example: FILE=`ls *.txt` echo $FILE More Complex Example: FILE=`ls -1 *.txt | sort | tail –n 1` echo "Your last text file is: $FILE" What if we use double quotes instead?

13 Double vs. Single quotes
Double quotes - Variable names are expanded & Back ticks work NAME="Bugs Bunny" echo "Hi $NAME! Today is `date`" Produces: Hi Bugs Bunny! Today is Tues Apr 26 13:37:45 PDT 2016 Single quotes – don’t expand variables or execute commands in Back ticks echo 'Hi $NAME! Today is `date`' Hi $NAME! Today is `date` Tricky Example: STAR=* echo "You are a $STAR" echo 'You are a $STAR' echo You are a $STAR Lesson: When referencing a variable, it is good practice to put it in double quotes.

14 Types and integers most variables are stored as strings
operations on variables are done as string operations, not numeric to instead perform integer operations: x=42 y=15 let z="$x + $y" # 57 integer operators: + - * / % bc command can do more complex expressions if a non-numeric variable is used in numeric context, you'll get 0

15 Bash vs. Java x=3 x vs. $x vs. "$x" vs. '$x' vs. \'$x\' vs. 'x' Java
String s = "hello"; s=hello System.out.println("s"); echo s System.out.println(s); echo $s s = s + "s"; // "hellos" s=${s}s String s2 = "25"; String s3 = "42"; String s4 = s2 + s3; // "2542" int n = Integer.parseInt(s2) + Integer.parseInt(s3); // 67 s2=25 s3=42 s4=$s2$s3 let n="$s2 + $s3"

16 Special variables these are automatically defined for you in every bash session Exercise : Change your attu prompt to look like this: See man bash for more info (search on PROMPTING) variable description $DISPLAY where to display graphical X-windows output $HOSTNAME name of computer you are using $HOME your home directory $PATH list of directories holding commands to execute $PS1 the shell's command prompt string $PWD your current directory $SHELL full path to your shell program $USER your user name

17 $PATH When you run a command, the shell looks for that program in all the directories defined in $PATH Useful to add commonly used programs to the $PATH Exercise: modify the $PATH so that we can directly run our shell script from anywhere echo $PATH PATH=$PATH:/homes/iws/rea What happens if we clear the $PATH variable?

18 set, unset, and export typing set or export with no parameters lists all variables Exercise: set a local variable, and launch a new bash shell Can the new shell see the variable? Now go back and export and launch a shell again. Can you see it now? shell command description set sets the value of a variable (not usually needed; can just use x=3 syntax) unset deletes a variable and its value export sets a variable and makes it visible to any programs launched by this shell readonly sets a variable to be read-only (so that programs launched by this shell cannot change its value)

19 Console I/O Example: variables read from console are stored as strings
#!/bin/bash read -p "What is your name? " name read -p "How old are you? " age printf "%10s is %4s years old" $name $age shell command description read reads value from console and stores it into a variable echo prints output to console printf prints complex formatted output to console

20 Command-line arguments
Example.sh: #!/bin/bash echo “Name of script is $0” echo “Command line argument 1 is $1” echo “there are $# command line arguments: Example.sh argument1 argument2 argument3 variable description $0 name of this script $1, $2, $3, ... command-line arguments $# number of arguments array of all arguments

21 for loops for name in value1 value2 ... valueN; do commands done
Note the semi-colon after the values! the pattern after in can be: a hard-coded set of values you write in the script a set of file names produced as output from some command command line arguments: Exercise: create a script that loops over every .txt file in the directory, renaming the file to .txt2 for file in *.txt; do mv $file ${file}2

22 for loop examples for val in red blue green; do echo "val is: $val"
done for val in do for val in `seq 4`; do command description seq outputs a sequence of numbers

23 Exercise Write a script createhw.sh that creates directories named hw1, hw2, ... up to a maximum passed as a command-line argument. $ ./createhw.sh 8 Copy criteria.txt into each assignment i as criteria(2*i).txt Copy script.sh into each, and run it. output: Script running on hw3 with criteria6.txt ...

24 Exercise solution #!/bin/bash
# Creates directories for a given number of assignments. for num in `seq $1`; do let CRITNUM="2 * $num" mkdir "hw$num" cp script.sh "hw$num/" cp criteria.txt "hw$num/criteria$CRITNUM.txt" echo "Created hw$num." cd "hw$num/" bash ./script.sh cd .. done


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