croguerrero/bash-examples

Notes SHELL SCRITPS

Work with special characters

$PATH $SHELL

Implementing and/or List

&& ans list --- A string of commands where the next command is only executed if
the previous command exited with astatus of zero

|| ans list --- A string of commands where the next command is only executed if
the previous command exited with a non-zero status

Exit Statuses

Zero Exit Status:
    Implies the script or program ran to
    completion and everything is fine.
Non-Zero Exit Status:
    Results can vary based on the script
    or program that generated the exit status.

the $? variable is used to determine the exit status of the last command executed.

Redirecting I/O

Redirecting stdout and stderr.

> file_name
>> file_name  # append data to file
< file_name
2> file_name
2>> file_name
1> file_name
1>> file_name

---

echo "abc 123">file
exec 5<> file
read -n3 var <&5
echo $var
exec 5>&-

Variables

Variables are used to define a value that can be used in subsequent commands.

VARIABLE_NAME = VARIABLE_VALUE
echo $VARIABLE_NAME

---

BASH FUNCTIONS

Its a way to group code in a logical way.

function name {
    code
 exit status
}
function hello {
    echo "Hello World"
}
hello
quit 

---

ARRAYS

Multiple values are stored in an array.

Number=(1 2 3 4 5)
Number+= 9 #append 9 to the array
echo ${Number[0]}
echo ${Number[@]}
echo ${#Number[*]} #length of array

---

Sustitution

rpm -qf /usr/bin/bash
rpm -qf `wich yum`
rpm -qf $(which yum)
TIME = $(date +%H:%M:%S)

Process Sustitution

bash <(curl -s https://raw.githubusercontent.com/user/repo/master/script.sh)

Loops

for i in $(ls); do echo $i done

for i in {1..10}; do touch file$i done

while [ $i -le 10 ]; do echo $i i=$((i+1)) done

until [ $i -gt 10 ]; do echo $i i=$((i+1)) done

SIGNALS

Programs in Linux are managed pratilly by signals from the kernel.

SIGINT: Interrupt SIGKILL: Kill SIGTERM: Terminate SIGUSR1: User defined 1

Using Exit Status, Test and Built

if ()
then 
    command
else 
    command
fi 

Heredocs

A heredoc is a special type of redirection that allows you to pass multiple lines of input to a command.

cat << DELIMITER This is a heredoc. It is useful for passing multiple lines of input to a command. DELIMITER

Herestring

Is a pared-down version of heredoc.

command <<< "$WORD"

Bash Builtins for Troubleshoot Problems

set -x set -u set -e

Aditional Builtins

• break: Breaks out of a loop.
  • continue: Continues to the next iteration of a loop.
  • readonly: Makes a variable readonly cannot be modified. If we put -x intialy on /bin/bash we can view how process code

What is REGEX

Regular expressions are a way to match or search patterns.

Regex Metacharacter

grep -E "^[0-9]+$" grep -E '<is>|am' # <> is used for matching the word is or am
grep -oE '<is>|am' super.txt grep -E '(B|C)AT' super.txt

Best Practices