LPIC - 109 Scripting

Objective

• Manage Shell Environment
• Shell Scripting
• SQL Queries

More reference - Scripting (Tag)

Shell Environment

1 - Shell Config

Global & Local vars

• Global - Available from current shell & any child processes spawned from the shell
  • printenv - Show all global env vars 📌
  • export - make var available to all programs launched from the shell
• Local - Only available in the shell that creates it
  • set - Show all env vars for a specific process
  • env - set var for just one program
  • Recommend using lowercase letters (distinguish from system env vars)

# var is available to all
export FCEDIT=/usr/bin/vim

# var is available for one program
env FCEDIT=/usr/bin/vim mail

Common env vars

Note: For multiple X sessions, DISPLAY always starts with 0:0 for the first session, then 1:0

Variable	Explanation
SHELL	Path to current shell
HOSTNAME	Current hostname
PS1	Default bash prompt
TERM	Current terminal type (e.g. xterm)
DISPLAY	Identify X display. Usually 0:0

3 Ways to start Bash shell

• At login time - As default login shell
• Interactive shell (launched via Terminal GUI)
• Non-interactive shell - To run a script

Shell config files

• Global
  • Login - /etc/profile (main default startup file)
  • Interactive - /etc/bashrc
• User
  • Login - ~/.bash_profile
  • Interactive - ~/.bashrc
• ~/.inputrc - Customize keyboard config
  • X uses its own keyboard config. So this doesn’t affect programs running in X

Shell Scripting

1 - General

Notes

• Piping - Redirect output to another command

• Run script directly from command prompt - New subshell

• Make local shell env vars availbale in the script 📌

  exec ./xx.sh

Shebang / hashbang

• #! - Tells Linux it’s a script
• /bin/bash - Path to interpreter to run the shellscript (e.g. Bash)

Source a script

• Run in the current shell. Script has access to env vars defined in the calling shell. 📌
• source ~/.bashrc
• . ~/.bashrc

Shell Variables

• $0 - Script name

• $$ - PID of current shell

• $? - Exit status of last command (e.g. 0)

  # change exit status (from 0 to 120)
  exit 120

Command-line Arguments

# example - test.sh
echo $1 checked in $2 days ago

# run the script
./test.sh Merikanto 10

Command Substitution

• Assign the command’s output to a user var in the script
• Can also capture function output using $() 📌

# use ``
v1=`date`

# or $()
v2=$(who)

echo $v1
echo $v2

File Descriptor

• Non-negative number
• 0 - STDIN (keyboard)
• 1 - STDOUT
• 2 - STDERR

Read input

Silent reading - Text is displayed, but sets text color same as terminal background color 📌

# read & use input
read name
useradd -aG $name

# specify a prompt
read -p "Your name:" first last
echo -n "Hello $last, $first"	# -n: suppress newline char

# read password / silent reading
read -s -p "Password: " pass

# use timer
read -t 5 -p "Your name:" name	# timeout is 5s

# count input char
read -n1 -p "Enter [Y/N]" answer	# 1 char, Y or N

Running multiple commands

# use ;
date; who

List all active aliases

alias -p

2 - Synatx

Math

• Floating point arthmetic is controlled by a built-in var - scale (set to desired number of decimals) 📌
• zsh - Supports advanced math functions & features

# $[] - int only
result=$[ 25 * 5 ]

# floating point calc
scale=3
bc

# substitute as var
v1=$(echo "scale=3; 2.0/3" | bc)

Condition Tests - Numeric

Test - If [ … ]	Description
n1 -eq n2	n1 == n2
n1 -ne n2	n1 != n2
n1 -ge n2	n1 >= n2
n1 -gt n2	n1 > n2
n1 -le n2	n1 <= n2
n1 -lt n2	n1 < n2

Condition Tests - File

Test - If [ … ]	Description
-e [file]	If file exists
-f [file]	If file exists & is a file
-d [file]	If file exists & is a directory
-s [file]	If file exists & not empty
-r [file]	If file exists & readable
-w [file]	If file exists & writeable
-x [file]	If file exists & executable

Logics - Combine tests with boolean symbols

• && - And
• || - Or

If

if [ ... ]
    then
        ...
    else
        ...
fi

Case

# input is 1 char, Y or N
read -n1 -p "Enter [Y/N]" answer	

case $answer in 
Y | y)	echo "\nOK, continue...";;

N | n)	echo "\nBye"
        exit;;
        
*)		echo "\nWrong Input"
        exit;;

For

seq - Similar to range(), but is [a, b] （闭区间）

# execute 4 times, default interval = 1
for x in `seq 2 5`

# example
for file in $(ls | sort) ; do
    if [ -d $file ]
    then
        echo "$file is a directory"
    fi
    
    if [ -f $file ]
    then
        echo "$file is a file"
    fi
done

While

while [ ... ]
do
    ...
done

Function

Use return to specify single int value to define exit status

# Format 1
function hello {
    ...
}

# Format 2
hello() {
    ...
}

Example - Putting it altogether

Accepts source & target filename. Aborts when target file exits

#!/bin/bash

doit() {
    cp $1 $1
}

function check {
    if [ -s $2 ]
        then
            echo "Target exits. Exiting..."
            exit
    fi
}

check $1 $2
doit  $1 $2

3 - SQL Queries

All Syntax is used in MySQL

Update

UPDATE objects SET size=5 
    WHERE name="lizard";

Exact Matches

SELECT * FROM objects 
    WHERE color="green";

Multiple Tests

Return an ordered list

SELECT * FROM objects 
    WHERE type="soft" 
    AND value>7.50
    ORDER BY value;

Deletion

# delete all data from a table (keeps the table)
DELETE * from objects;

# delete table
DROP TABLE objects;

Other Syntax

• JOIN - combine data from multiple tables
• GROUP BY - Used with math operators, e.g. SUM()

SELECT objects.name, objects.value, SUM(value)
    FROM objects, locations
    WHERE locations.name=objects.name
    GROUP BY value;