ops102_8_2.pdf

Bash Scripting Part 2 OPS102 Week 8 Class 2 Chris Tyler/John Sellens July 2, 2024 Seneca Polytechnic Outline Recap From Last Class The Read Command Command Capture (Substitution) Shell Arithmetic Exit Status and Conditionals Summary OPS102 W8C2 - Bash Scripting...

Bash Scripting Part 2 OPS102 Week 8 Class 2 Chris Tyler/John Sellens July 2, 2024 Seneca Polytechnic Outline Recap From Last Class The Read Command Command Capture (Substitution) Shell Arithmetic Exit Status and Conditionals Summary OPS102 W8C2 - Bash Scripting Part 2 1/19 Recap From Last Class Recap From Last Class Scripts are handy, and easy to create – yay text files! The “shebang” line is a “magic number” that guides the kernel on how to execute a script. Variables are handy – local and environment. Quoting and backslash escaping hide special characters and whitespace. OPS102 W8C2 - Bash Scripting Part 2 2/19 The Read Command Reading a Variable Value from Stdin: read You can read from standard input into a variable with the read command: read variable For example: $ read course Seneca OPS102 $ echo $course Seneca OPS102 If you give read multiple variables, it will tokenize the input. $ read first last restofline Chris Tyler likes using Linux $ echo $restofline likes using Linux OPS102 W8C2 - Bash Scripting Part 2 3/19 Using read with a Prompt String You can display a message to the user when reading from stdin by using the –p (prompt) option to read: $ read -p "Please enter a course code: " ccode Please enter a course code: OPS102 $ echo "The selected course is $ccode" The selected course is OPS102 Of course, you can also use a separate echo command instead! Which has a -n option to suppress the trailing newline. OPS102 W8C2 - Bash Scripting Part 2 4/19 Demo: Variables in a Script #!/bin/bash read -p "Please enter your name: " name echo "Pleased to meet you, $name" read -p "Please enter a filename: " file echo "Saving your name into the file..." echo "yourname=$name" >>$file echo "Done." OPS102 W8C2 - Bash Scripting Part 2 5/19 Command Capture (Substitution) Command Capture (Substitution) You can capture the standard output (stdout) of a command as a string using the notation $(command) $ echo "The current date and time is: $(date)" The current date and time is: Mon 19 Jun 2034 12:02:11 AM EDT $ files="$(ls|wc -l)" $ echo "There are $files files in the current directory $(pwd)" There are 2938 files in the current directory /bin It’s also called “command substitution” since the output of the command is substituted for what was on the command line. OPS102 W8C2 - Bash Scripting Part 2 6/19 Command Capture: Avoid Backticks You may see old scripts that use backticks (reverse single quotes) for command capture: $ files=`ls` Don’t do this! This is an archaic syntax which is deprecated. Some fonts make it hard to distinguish between backticks and single quotes, and nesting backticks is difficult. Unless you’re writing code that needs to be portable to non-bash systems. OPS102 W8C2 - Bash Scripting Part 2 7/19 Shell Arithmetic Arithmetic! Bash can do integer arithmetic To evaluate a arithmetic expression and return a value, use $(( )) To evaluate a arithmetic expression without returning a value, use (( )) Dollar-sign prefixes for variables are not required inside $(( )) or (( )) $ echo $((a*b)) $ ((a++)) 1200 $ echo $a $ a=100 $ echo $((b++)) 101 $ b=12 12 $ ((c=a*b*2)) $ echo $b $ echo "The answer is $c" 13 The answer is 2626 OPS102 W8C2 - Bash Scripting Part 2 8/19 Old Style Arithmetic The expr command evalutes expressions. Can be used in command substitution (or output redirection). Less convenient than bash arithmetic, but more portable. OPS102 W8C2 - Bash Scripting Part 2 9/19 Exit Status and Conditionals Exit Status When a program runs, it exits with a numeric value. This goes by any of several names: exit status, exit code, status code, error code Usually, an exit status of zero means that no errors were encountered, and a non-zero status means that something went wrong. Alternately, program authors can use this value as they see fit, so the exit status may indicate something else, like the number of data items processed. In C programs the exit status is from exit(): exit( 3 ); A shell script exits with the exit command: exit 0 Integer argument optional, defaults to 0 OPS102 W8C2 - Bash Scripting Part 2 10/19 Exit Status: $? The special variable $? can be used to find out the exit status of the last command executed: $ ls /foo/bar/baz ls: cannot access '/foo/bar/baz': No such file or directory $ echo $? 2 $ ls /usr/bin/bash /usr/bin/bash $ echo $? 0 OPS102 W8C2 - Bash Scripting Part 2 11/19 Exit Status: Why do we care? In a script you may want to notice if a command fails. And the exit statuses of commands are the key to conditional logic (if statements) and looping (for/while/until) in bash scripts. A C program exits by calling exit( 0 ); A shell script exits by running exit 0 Or falling off the end of the script OPS102 W8C2 - Bash Scripting Part 2 12/19 Conditional logic: if / then / elif / else / fi The if command takes two or more lists of commands, and uses the result of one list to control the execution of the other. if cmdlist1 # if the exit status is 0 then cmdlist2 # then run these commands fi OPS102 W8C2 - Bash Scripting Part 2 13/19 Conditional logic: if / then / elif / else / fi if grep –q "OPS102" testfile then echo "The course is mentioned in the file" fi The shell runs the grep command, and if the string is found in the file, grep exits 0, which indicates “true”, and so the command(s) in the “then” of the “if” are run. OPS102 W8C2 - Bash Scripting Part 2 14/19 Conditional logic: if / then / elif / else / fi There are else and elif (else-if) keywords too: if cmdlist1 # If the exit status is success then cmdlist2 # then run this elif cmdlist3 # else if this exits with success then cmdlist4 # then do this else cmdlist5 # otherwise do this. fi OPS102 W8C2 - Bash Scripting Part 2 15/19 Conditional logic: if / then / elif / else / fi if grep –q "OPS102" testfile then echo "The course is mentioned in the file" else echo "The file does not mention OPS102" fi OPS102 W8C2 - Bash Scripting Part 2 16/19 Conditional logic: if / then / elif / else / fi if grep –q "OPS102" testfile then echo "The course is mentioned in the file" elif grep –q "ULI101" testfile then echo "The old ULI101 course is in the file" else echo "The file does not mention OPS102 or ULI101" fi OPS102 W8C2 - Bash Scripting Part 2 17/19 More on if Statements The command lists can be one of more commands, separated by newlines or semi-colons. Common formatting style: if grep –q "OPS102" testfile ; then echo "The course is mentioned in the file" elif grep –q "ULI101" testfile ; then echo "The old ULI101 course is in the file" else echo "The file does not mention OPS102 or ULI101" fi OPS102 W8C2 - Bash Scripting Part 2 18/19 Summary Summary Reading input Command capture / command substitution Integer arithmetic Exit status if statements Next class? test, parameters, while, until, for OPS102 W8C2 - Bash Scripting Part 2 19/19