Learning Basic Bash Scripting

License

The guide may be used and shared for educational, non-commercial, not-for-profit purposes. Users are free to modify and distribute content.

Let's create a simple scirpt

1. We will create a file, first.sh (you can use any text editor)
2. The first line will be #! /bin/sh (it tells the interpreter that it is a bash script).
3. We will write a command ls(list the subfolders in the directory)

#! /bin/sh
ls

4. Run the script by executing command ./first.sh

Suppose, you have to push a commit to your git repository. Instead of writing the commands one by one, we can write a shell script that automates the whole process. Create a file git.sh

#! /bin/sh 
git add .
git commit -m $1
git push origin master

We will automate the process of pushing commit to the repo. $1 is a variable that will get the input that the user will provide. To run this script, execute ./git.sh ‘initial commit’. ‘initial commit’ will be placed at the $1.

Comments

Comments in shell scripts are followed by #.

# This is a comment.

Variables

Variables are declared using the =

MY_NAME='Morty' #No spaces 

NAME="John"
echo $NAME
echo "$NAME"
echo "${NAME}!"

Variables are accessed using the $ sign.

String

String are assigned to a variable

NAME="John"
echo "Hi $NAME"  #=> Hi John
echo 'Hi $NAME'  #=> Hi $NAME

Echo

Echo is used to display the statement in the shell just like printf in C language.

MY_NAME='Morty' 
echo "My name is $MY_NAME"

<Output>
# My name is Morty

Conditional Statements

Let’s see how an if else statement looks in bash scripting.

if [[condition]];
then
# write the then instructions here
else
# write the else instructions here
fi

if [[ -z "$string" ]]; then
  echo "String is empty"
elif [[ -n "$string" ]]; then
  echo "String is not empty"
fi

Conditional execution

These are some of conditions

git commit && git push
git commit || echo "Commit failed"

Functions

Functions are group of statements that together perform a task.

get_name() {
  echo "John"
}
echo "You are $(get_name)"

Loops

Loops are a sequence of instruction s that is continually repeated until a certain condition is reached

Basic for loop

for i in /etc/rc.*; do
  echo $i
done

Forever

while true; do
  ···
done

Ranges

for i in {1..5}; do
    echo "Welcome $i"
done

#With step size
for i in {5..50..5}; do
    echo "Welcome $i"
done

Functions

Defining Functions

myfunc() {
    echo "hello $1"
}
# ------------------------------------

# Same as above (alternate syntax)
function myfunc() {
    echo "hello $1"
}

myfunc "John"

Returning values

myfunc() {
    local myresult='some value'
    echo $myresult
}
# ------------------------------------
result="$(myfunc)"

Raising Errors

myfunc() {
  return 1
}
# ------------------------------------
if myfunc; then
  echo "success"
else
  echo "failure"
fi

Conditional Statements

Note that [[ is actually a command/program that returns either 0 (true) or 1 (false). Any program that obeys the same logic (like all base utils, such as grep(1) or ping(1)) can be used as condition, see examples.

Conditions

Condition	Function
[[ -z STRING ]]	Empty string
[[ -n STRING ]]	Not empty string
[[ STRING == STRING ]]	Not Equal
[[ NUM -eq NUM ]]	Equal
[[ NUM -ne NUM ]]	Not equal
[[ NUM -lt NUM ]]	Less than
[[ NUM -le NUM ]]	Less than or equal
[[ NUM -gt NUM ]]	Greater than
[[ NUM -ge NUM ]]	Greater than or equal
[[ STRING =~ STRING ]]	Regexp
(( NUM < NUM ))	Numeric conditions
[[ -o noclobber ]]	If OPTIONNAME is enabled
[[ ! EXPR ]]	Not
[[ X ]] && [[ Y ]]	And
[[ X ]] | | [[ Y ]]	Or

File Conditions

Condition	Function
[[ -e FILE ]]	Exists
[[ -r FILE ]]	Readable
[[ -h FILE ]]	Symlink
[[ -d FILE ]]	Directory
[[ -w FILE ]]	Writable
[[ -s FILE ]]	Size is > 0 bytes
[[ -f FILE ]]	File
[[ -x FILE ]]	Executable
[[ FILE1 -nt FILE2 ]]	1 is more recent than 2
[[ FILE1 -ot FILE2 ]]	2 is more recent than 1
[[ FILE1 -ef FILE2 ]]	Same files