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Shell Scripting

Ian McDonald

Sign In Link: decal.ocf.berkeley.edu/signin

Magic Word:

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the motivation

You’re a sysadmin

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the motivation

You’re a sysadmin
You have to run some commands all the time

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the motivation

You’re a sysadmin
You have to run some commands all the time
But you you wanna stay DRY

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the motivation

You’re a sysadmin
You have to run some commands all the time
But you you wanna stay DRY
Describe your task as a step by step set of instructions so that a computer can do the dirty work for you

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with great power comes…

xkcd.com/974

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Bash Is...

A shell...

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Bash Is...

A shell…
A programming language!

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shell variables and types

NAME=”value”

Whitespace matters!

echo “$NAME”

Variable interpolation (value of a variable vs. string literal)

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shell variables and types

Types? What types?
Bash variables are untyped
Arithmetic and other operations are contextual

Does this text contain a digit?

FOO=1
$FOO + 1

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shell variables and types

FOO=1
$FOO + 1

bash: 1: command not found

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shell variables and types

FOO=1
$FOO + 1

bash: 1: command not found

expr $FOO + 1

2

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test

test evaluates an expression

synonymous with [ ]

Sets exit status to 0 (true) or 1 (false)
(Yes, 0 is true and 1 is false)

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test - integer comparison

$ test 0 -eq 0; echo $? # exit code 0 means true�0�$ test 0 -eq 1; echo $? # exit code 1 means false�1

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test - string comparison

$ test zero = zero; echo $? # exit code 0 means true�0�$ test zero = one; echo $? # exit code 1 means false�1

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test - integer comparison

$ [ 0 -eq 0 ]; echo $? # exit code 0 means true�0�$ [ 0 -eq 1 ]; echo $? # exit code 1 means false�1

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test - string comparison

$ [ zero = zero ]; echo $? # exit code 0 means true�0�$ [ zero = one ]; echo $? # exit code 1 means false�1

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if-then

if TEST-COMMANDS; then�� CONSEQUENT-COMMANDS��elif MORE-TEST-COMMANDS; then�� MORE-CONSEQUENT-COMMANDS��else �� ALTERNATE-CONSEQUENT-COMMANDS;��fi

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if-then

#!/bin/bash�# contents of awesome_shell_script��if [ “$1” -eq “$2” ]; then� echo “args are equal”�else� echo “args are not equal”�fi

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if-then

$ ./awesome_shell_script 0 0�args are equal�$ ./awesome_shell_script 0 1�args are not equal

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while

while TEST-COMMANDS; do�� CONSEQUENT-COMMANDS��done

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while

#!/bin/bash�# contents of awesome_shell_script��n=”$1”�while [ “$n” -gt 0 ]; do� echo “$n”� let n=”$n-1”�done

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while

$ ./awesome_shell_script 5�5�4�3�2�1

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command substitution

$ A=`expr 1 + 1`

$ echo $A

2

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functions

name_of_function() {�� FUNCTION_BODY��}

name_of_function $arg1 $arg2 ... $argN

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functions

#!/bin/bash�# contents of awesome_shell_script��foo() {� echo hello “$1”�}��foo “$1”

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functions

$ ./awesome_shell_script world�hello world

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functions

fib() {� N=”$1”� if [ “$N” -eq 0 ]; then� echo 0� elif [ “$N” -eq 1 ]; then� echo 1� else� echo $(($(fib $(($N-2))) + $(fib $(($N-1)))))� fi�}��fib “$1”

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functions

$ ./fibonacci 10�55

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the case for python

That last slide looked painful
Complex control structures, functions, closures, etc. are a hassle in Bash
Solution: Use Python!

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the case for python

argparse

Easy CLI

fabric

Easy deployment

salt

Generally useful for infrastructure-related tasks

Psutil

Monitor system info

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the case for python

Use Bash when:

The functionality you want is easily expressed as a composition of command line tools
Common file manipulation operations

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the case for python

Use Python when:

You need “heavy lifting” with complex control structures, messy state, recursion, OOP, etc.

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shebang!

#!/path/to/something
#!/bin/sh
#!/bin/bash
#!/bin/sed
#!/usr/bin/python

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shebang!

#!/path/to/interpreter
If you call your script as an executable, this line is what determines the program used to execute the lines below it

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She bang! (Tangent)

Make sure your script is consistent with it’s shebang
If you use bash-specific features (bashisms) in a script with #!/bin/sh, you’re gonna have a bad time
checkbashisms can help

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running your script

$ path/to/interpreter my-shell-script

$ bash my-shell-script
$ python my-shell-script

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running your script

$ path/to/my-shell-script

$ ./my-shell-script
where #! matters
must be executable by you (or whoever else you want running your script)
chmod u+x my-shell-script

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other resources

LDP (Linux Documentation Project)

Bash Guide for Beginners

http://www.tldp.org/LDP/Bash-Beginners-Guide/html/

Advanced Bash Scripting Guide

http://tldp.org/LDP/abs/html/

Scripting topic guide

Attached to the lab!

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lab assignment

Lab has more detail than this lecture
Make a Unix phonebook!
decal.ocf.berkeley.edu/labs/b3