cse 374 - week 3 (mon) scripting...cse 374 - week 3 (mon) scripting instructor: andrew hu tas: jim...

CSE 374 - Summer 2020

CSE 374 - Week 3 (Mon)Scripting

Instructor: Andrew Hu

TAs: Jim Limprasert, Kaelin Laundry, Keith Jiang, Nick Durand, Simon McFarlane, Yitian Hao

1


Administrivia

● Ex 5 is released today, first coding assignment

○ Start early!

● Spec is linked on the course website ("Ex 5 due" button)

● Autograded on Gradescope

● Any content that we don't cover today, we'll do on Wed

● No exercise for Wed

● HW 1 also released today, spec linked on course website2


Demo: Autograder Feedback

3


What is a Scripting Language?

● A language which automates the execution of tasks which

could alternatively be done manually, one at a time

● Bash is a shell, but also a scripting language

● Other languages used for scripting: Python, R, etc.

4


Simple Bash Script

● Simply put several bash commands in a text file

○ Convention is to end these files with .sh

● Execute by running sh myscript.sh

5


Interpreting Scripts (Sha-bang)

● Bash can run your scripts as if they were executables

○ Instead of sh myscript.sh

○ Just ./myscript.sh

● Add "#!" and the path to the interpreter as first line of script

○ #! /bin/sh (at the top of shell scripts)

○ #! /bin/python3 (at the top of a Python 3 script)

● # by itself starts a comment line

○ # This line will be ignored by Bash 6


Bash Permissions

● A file cannot be executed unless it has execute permissions

● All files support read, write, and execute permissions

● These permissions are different for the file's owner, group,

and others

● Change these permissions (modes) using chmod

○ e.g. chmod a+x myscript.sh "Let all users execute myscript.sh"

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Demo: Straight Line Script, Permissions

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Script Arguments

● Scripts can take arguments just like any other program

● Arguments are automatically named $1, $2, …

○ $1 is the first argument after the filename

● The script can accept any number of arguments (no limit)

9


Dealing with All Arguments

● Use $@ to get a space-separated list of all the arguments

● Use "$@"with quotes to make each argument quoted

○ Prevents args which were originally quoted from being read as multiple args

● To remove the first argument from $@, use shift

○ $1 goes away, $2 becomes $1, $3 becomes $2, etc.

○ shift n will apply shift n times

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Demo: arglist, allargs, shift

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Poll Question: PollEv.com/andrewhu

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http://pollev.com/andrewhu


Poll Question (PollEv.com/andrewhu)

What does ./script.sh foo "hear me out" output?

A. 1

B. 2

C. 3

D. 4

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#! /bin/bash

shift

numargs $@



Control Flow

● It is possible to do conditionals and loops in bash

● However, the syntax is different!

● Beware of common pitfalls

○ Whitespace matters

○ if then fi

○ for in do done

○ while do done

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If Statement

if ./myprogram args; then

command1 arg1 arg2 ...

command2 arg1 arg2 ...

...

fi

● Executes body if ./myprogram succeeds

○ i.e. if ./myprogram returns exit code 0

● The semicolon denotes the end of one command15


if grep -q -E 'myregex' file.txt; then

echo "Found it!"

fi

● Use the quiet (-q) option so your script doesn't print the

matches

If File Contains

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Test Command

● test is a command that compares two arguments

● Check string equality○ test "$str1" == "$str2"

● Check if file exists○ test -f result.txt

● Check integer equality○ test $num -eq 0

● And many more17


Testing Conditions with If

● We can combine test with if

○ if test -f result.txt; then … fi

● Bash has handy syntax so we don't have to write out test

● Use [where you would say test

○ if [ -f result.txt ]; then … fi

● Be aware of the spaces around the brackets and the

semicolon

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Demo: If Syntax

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Which of these if statements has correct syntax?

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if ("hello world" == $msg) then

echo "what a day"

fi

if "hello world" == "$msg"; then

echo "what a day"

fi

if ["hello world" == $msg]; then

echo "what a day"

fi

if [ "hello world" == "$msg" ]; then

echo "what a day"

fi

A. B.

D.C.


Bash Utility of the Day: wc

● Word Count

● Prints lines, words, and bytes in a file, along with the filename

● -c to print just the number of bytes (characters)

● If it reads from stdin, doesn't print filename

○ e.g. wc -c < file.txt outputs just "945"

● Aside: command substitution is allowed in double quotes

○ e.g. echo "$(wc -c < file.txt) bytes in this file"21


For Loop

● Bash for loops iterate over a list of strings

● Outputs:

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for word in list of words

do

echo $word

done

list

of

words


Iterate Over Files

● Print every file in the current directory

for file in $(ls)

do

if [ -f $file ]; then

echo "$(wc -c < $file) bytes in $file"

fi

done

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Demo: Looping Over Args

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Aside: alias

● Create a globally visible one line command

○ alias cleantmp='rm -rf /tmp/*'

● To add them permanently, add that line to ~/.bashrc

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Poll Question (PollEv.com/andrewhu)

How was the pace of this lecture?

A. Too fast

B. A little fast

C. About right

D. A little slow

E. Too slow

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cse 374 - week 3 (mon) scripting...cse 374 - week 3 (mon) scripting instructor: andrew hu tas: jim...

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