This presentation by Chris Tankersley at Midwest PHP 2018 provides an overview of Bash scripting, including its history, basic syntax, variable declaration, flow control structures, and function usage. It emphasizes the use of Bash for system scripts, quick prototyping, and the development of portable scripts. Additionally, it references the Google Shell Style Guide for best practices in structuring larger scripts.

1. BASHing at the CLI
Chris Tankersley
@dragonmantank
Midwest PHP 2018
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2. Midwest PHP 2018
Commandline Scripting
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3. Midwest PHP 2018
What is Bash?
●
Bourne Shell
– Introduced with Unix System 7 in 1977
– Still present on most systems as `sh`
●
Bourne Again Shell
– 1989 as part of the GNU Project
– Also took parts from C Shell (`csh`) and Korn Shell (`ksh`)
– Added syntactic sugar on top of the Bourne Shell
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4. Midwest PHP 2018
Run directly or through scripts
$ for FILE in `ls`; do echo ${FILE}; done
#!/bin/bash
for FILE in `ls`; do
echo ${FILE}
done
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Why use Bash?
●
System Scripts
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Quick prototyping
●
Ultra-portable scripts*
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Anatomy of a script
#!/bin/bash
// Logic goes here
echo “Hello World”
exit 0
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Variables
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Declaring Variables
MY_VAR=”something”
MY_NUMBER=1
LOG_DIR=”/var/log”
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Using Variables
MY_VAR=”something”
MY_NUMBER=1
LOG_DIR=”/var/log”
APP_LOG=${LOG_DIR}/app.log
echo $MY_VAR
echo ${MY_VAR}
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Declaring Arrays
FILES=(
‘app.log’
‘app.err’
‘access.log’
)
echo ${FILES[0]}
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Declaring Hashes
declare -AANIMALS
ANIMALS[“moo”]=”cow”
ANIMALS[“dog”]=”woof”
echo ${ANIMALS[moo]}
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Storing Output
FILES=`ls /var/log`
FILE_DATA=$(cat /var/log/app.log)
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Built In Variables
$BASH = Location of the bash executable
$FUNCNAME = Name of the current function
$HOME = Home directory of current user
$PATH = Search path for binaries
$PWD = Current working directory
$TMOUT = Time before a script times out
$UID = Current User ID
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14. Midwest PHP 2018
Built In Variables
$0, $1, … = Positional Parameters
$# = Number of arguments
“$@” = All arguments as quote delimited words
$! = PID of the last job run in background
$? = Exit code of the last command
$$ = PID of the script itself
http://tldp.org/LDP/abs/html/internalvariables.html
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Flow Control
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Numerical Comparison Operators
-eq - Equal To
-ne - Not Equal To
-gt, > - Greater Than
-ge, >= - Greater Than or Equal
-lt, < - Less Than
-le, <= - Less Than or Equal
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String Comparison Operators
==, = - Equal To
!= - Not Equal
< - Less than ASCII order
> - Greater than ASCII order
-n - Not null
-z - Is Null, 0 length
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Logic Operators
&&, -a – And
||, -o – Or
&& and || short circuit
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If/Else Statements
if [[ ${MY_VAR} -eq 0 ]]; then
echo “MY_VAR equals 0”
elif [[ ${MY_VAR} -eq 1 ]]; then
Echo “MY_VAR equals 1”
else
echo “MY_VAR equals ${MY_VAR}”
fi
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Case Statements
case ${MY_VAR} in
1) echo “1”;;
2) echo “2”;;
*) echo “Some other number”;;
esac
Don’t put strings in quotes
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For Each Loops
FILES=(
‘app.log’
‘app.err’
)
for FILENAME in ${FILES[@]}; do
echo ${FILENAME}
done
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For Each Loops
for FILENAME in `ls /var/log`; do
echo ${FILENAME}
done
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While Loops
COUNTER=0
while [[ ${COUNTER} -lt 10 ]];
do
((COUNTER++))
done
echo ${COUNTER}
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Functions
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Basic Functions
function hello_world()
{
echo “Hello Midwest PHP!”
}
hello_world
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Function Arguments
function app_log()
{
echo “[$(date)] ${1}”
}
app_log “This is my log message”
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Function Return Values
function check_zero()
{
if [[ ${1} -eq 0 ]]; then
return 1
else
return 0
fi
}
IS_ZERO=`check_zero 1`
IS_ZERO=`check_zero 0`
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Variable Scope
MY_VAR=”Midwest PHP”
function hello_world()
{
local MY_VAR=”Hello World”
echo ${MY_VAR}
}
echo ${MY_VAR}
hello_world
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Putting it all together
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Google Shell Style Guide
https://google.github.io/styleguide/shell.xml
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Structuring Larger Scripts
●
Define functions at the top
●
Break functions out into their own files
– source filename.sh
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Function-Only Scripts
if [[ "${BASH_SOURCE[0]}" == "${0}" ]]; then
# This script is being invoked as a script
echo 'This script is not intended to be run standalone. Please use with
the system-micro.sh script'
exit 1
fi
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Know when to use a higher level language
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Thank You!
• Software Engineer for InQuest
• Author of “Docker for Developers”
• https://leanpub.com/dockerfordevs
• Co-Host of “Jerks Talk Games”
• http://jerkstalkgames.com
• http://ctankersley.com
• chris@ctankersley.com
• @dragonmantank
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