Linux+

Basics

Output redirection

Lets you redirect the output of a command from the monitor to another device, such as a file.

Script format

The shebang (#!) signals to the OS which shell to run the shell script with.

If you don’t specify the shell, the system runs the script with the default shell defined in your user account in /etc/passwd

Advanced

Displaying messages

Use echo to display messages to the command line:

# no quotes
echo This is a message without a single quote

# output blank line
echo 

# quotes bc contains single quote (')
echo "This ain't a message without a single quote"

Variables

Variables set aside memory locations to temporarily store information so you can recall that information later in a script or program by referencing the variable name.

Command	Description
`set`	Display all global variables.
`printenv`	Display local variables set for the session.
`export`	Mark a variable as exportable, which means that any child processes spawned from your shell can use it.
`env`	Lets you run a script and modify the env vars internal to the script without affecting the system environment variables.

Environment variables

Env vars track specific system information:

set
BASH=/bin/bash
BASHOPTS=checkwinsize:cmdhist:complete_fullquote:expand_aliases:extglob:extquote:force_fignore:globasciiranges:histappend:interactive_comments:login_shell:progcomp:promptvars:sourcepath
BASH_ALIASES=()
BASH_ARGC=([0]="0")
BASH_ARGV=()
BASH_CMDS=()
BASH_COMPLETION_VERSINFO=([0]="2" [1]="11")
BASH_LINENO=()
BASH_REMATCH=()
BASH_SOURCE=()
...

# script
cat envvars.sh 
#!/bin/bash
# display user information from the system
echo User info for userid: $USER
echo UID: $UID
echo HOME: $HOME

# output
./envvars.sh 
User info for userid: linuxuser
UID: 1001
HOME: /home/linuxuser

User variables

User variables let you store your own data in your shell scripts. To assign values, use = with no spaces:

var1=10
var2=23.45
var3=testing
var4="This has spaces in it"

# script
cat user_vars.sh 
#!/bin/bash
# testing variables
days=10
guest=Katie
echo $guest checked in $days ago

# output
./user_vars.sh 
Katie checked in 10 ago

Args and positional vars

Pass command-line arguments to the shell with positional variables:

# script
cat positional_args.sh 
#!/bin/bash
# Positional vars
echo $1 checked in $2 days ago

# output
./positional_args.sh Ricky 8
Ricky checked in 8 days ago

Exit status

$?

When a script ends, it returns its exit status to the parent shell to tell us whether or not the script completed successfully.

# successful command
who
linuxuser pts/0        2024-04-24 14:51 (10.0.2.2)

# exit status
echo $?
0

# unsuccessful command
./tester.sh
-bash: ./tester.sh: No such file or directory

# exit status
echo $?
127

By default, a successful script exits with a 0 status. You can change this with the exit command. Change the status if you want to indicate specific errors by changing the exit status value. This helps with debugging:

# create new shell
/bin/bash

# reassign exit
exit 123
exit

# verify reassignment
echo $?
123

Writing scripts

Command substitution

Lets you assign the output of a command to a user variable in the shell script:

var=$(command)

# assign
today=$(date) 
me=$(who)

# use vars
echo $today echo $me
Sat Apr 27 12:13:07 PM EDT 2024 echo linuxuser pts/0 2024-04-27 12:06 (10.0.2.2)

Math

Bash only evaluates values as text strings. To perform math, you have to place the equation in brackets:

# only integers
result=$[ 25 * 5 ]

bc

Program that lets you perform floating-point calculations:

# standard usage
bc
bc 1.07.1
Copyright 1991-1994, 1997, 1998, 2000, 2004, 2006, 2008, 2012-2017 Free Software Foundation, Inc.
This is free software with ABSOLUTELY NO WARRANTY.
For details type \`warranty\'. 
12 * 5.4
64.8
3.14 * (4 + 5)
28.26
quit

# quiet mode, no copyright info
bc -q
3.44 / 5
0

# set scale to set num of digits to right of decimal
scale=4
3.44 / 5
.6880
quit

# use bc in script
# variable=$(echo "options; express" | bc)
var1=$(echo "scale=4; 3.44 / 5" | bc)
echo $var1
.6880

if statement

# syntax
if [ <condition> ]
then
    <commands>
fi

# comparing numbers
if [ $1 -eq $2 ]
then
	echo "Both values are equal"
	exit
fi

if [ $1 -gt $2 ]
then
	echo "The first val is greater than the second val"
	exit
fi


if [ $1 -lt $2 ]
then
	echo "The first val is less than the second val"
	exit	
fi

Numeric condition tests

Test	Description
n1 -eq n2	checks if n1 is equal to n2
n1 -ge n2	checks if n1 is greater than or equal to n2
n1 -gt n2	checks if n1 is greater to n2
n1 -le n2	checks if n1 is less than or equal to n2
n1 -lt n2	checks if n1 is less than n2
n1 -ne n2	checks if n1 is not equal to n2

String condition tests

Test	Description
str1 = str2	checks if str1 is the same as str2
str1 != str2	checks if str1 is not the same as str2
str1 < str2	checks if str1 is less than str2
str1 > str2	checks if str1 is greater than str2
-n str1	checks if str1 has length greater than zero
-z str1	checks if str1 has length of zero

File condition tests

Test	Description
-d file	checks if file exists and is a dir
-e file	checks if file exists
-f file	checks if file exists and is a file
-r file	checks if file exists and is readable
-s file	checks if file exists and is not empty
-w file	checks if file exists and is writable
-x file	checks if file exists and is executable
-O file	checks if file exists and is owned by current user
-G file	checks if file exists and the default groups is the same as the current user
file1 -nt file2	checks if file1 is newer than file2
file1 -ot file2	checks if file1 is older than file2

case statement

write a case statement instead of writing multiple if condition tests:

# syntax
case <variable> in
<pattern1>) commands1;;
<pattern2> | <pattern3>) commands2;; # list more than one patterns on a line
*) default <commands>;;
esac

# check current user
case $USER in
su | linuxuser)
	echo "Welcome $USER"
	echo "Please enjoy your visit";;
testing)
	echo "Special testing account";;
admin)
	echo "Don't forget to log off when you're done";;
*)
	echo "Sorry, you're not allowed here!";;
esac

Loops

for loop

Iterates through every element in a series, such as files in a directory or lines in a text document:

# syntax
for <variable> in <series>; do
    <commands>
done

# iterate through the files in the Home folder
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 loop

A while loop keeps looping as long as the condition specified evaluates true. The condition in a while loop is the same as the if statement:

# syntax
while [<condition> ] ; do
    <commands>
done

# example
number=$1
factorial=1
while [ $number -gt 0 ] ; do
	factorial=$[ $factorial * $number ]
	number=$[ $number - 1 ]
done
echo The factorial of $1 is $factorial

Text manipulation

globbing: Lets you use wildcard characters to search for multiple files and directories in your script.
parameter expansion: Placing braces around a variable name ({test1}) lets you specify a substring value from the variable based on offset and length.
Parameter expansion like a pro
read: Lets you read text files line by line, then you can process using standard text manipulation tools.
regular expressions: Find and replace specific strings in files.