Boost up with bash scripts

Specify shell for your script

Your shell script can be executed by different shells, to explicitly specify the shell executable, leave a comment with format !{absolute_shell_path}.

#!/bin/bash

Variable

Declare a variable

In shell scripts, we declare a variable without any keyword. And the value a variable can have no punctuation for strings, they are strings by default. If contains spaces, use double quote to delimitate it, or the John in this context will be recognized as a command.

FIRST_NAME=John
LAST_NAME=Smith
FULL_NAME="John Smith"

Referencing a variable

Since strings have no punctuations, we use double quote to prevent globbing and word splitting when referencing it. echo accepts variadic parameters so they are concatenated naturally.

echo My full name is: $FIRST_NAME $LAST_NAME
echo Hello "$FULL_NAME"

INFO

The best practice for referencing a string variable is always punctuate with double quote.

Assigning variable inline

Unlike some programming languages, shell script invokes a function or executable at the begin of a command and may assign the value to variable after the invoked.

echo "What's your first name?"
read -r FIRST_NAME
echo "What's your last name?"
read -r LAST_NAME

echo Hello "$FIRST_NAME $LAST_NAME"

INFO

-r option stands for raw string.

Positional arguments

We can specify positional arguments for values we'd like to pass when invoking our shell script.

# in test.sh
echo Hello "$1" "$2"

INFO

It's also a good practice to brace place holder symbol with double quote.

Call this script with values.

./test.sh John Smith

# Hello John Smith

Control flows

Test operator

Test operator (or test expression) is the core role of control flow statements. There are three ways to write a test operator:

TIP

• Use $ to retrieve value returned by statement or command.
• [ ... ] and [[ ... ]] should have one space for both side of [ and ].

bash

condition=$(test "$1" -gt "$2")

bash

condition=$([ "$1" -gt "$2" ])

bash

condition=$([[ $1 > $2 ]])

However, using [[ ... ]] id much preferred because it offers more natural syntax for use.

1. Pattern Matching:

   • Supports advanced pattern matching with the == operator.

2. Extended Logical Operators:

   • Allows the use of logical operators (&& and ||) for combining multiple conditions.

3. Regular Expressions:

   • Supports the =~ operator for regular expression matching.

4. Extended String Comparison:

   • Provides additional string comparison operators (<, >, <=, etc.).

5. Logical Negation:

   • Supports the ! operator for logical negation.

6. Variable Expansion:

   • Variables don't need to be quoted in most cases for improved readability.

7. Improved Quoting:

   • Less sensitive to quoting issues, making it more forgiving.

Using [[ ... ]] is recommended when advanced conditional expressions are needed in Bash scripting.

if, then, elif, else and fi

if  [[ $1 > 70  ]]; then
   echo "You might be too old to drive..."
elif [[ $1 < 16  ]]; then
   echo "You are too young to drive!"
else
   echo "Let's go get your license."
fi

INFO

• Every condition expression should be terminated by semicolon ;.
• fi is the reverse of if. 😅

case in

case-in statement is rather advanced switch like control flow statement, it offers some pattern matching feature, way better than the old stupid switch in c lang.

bash

case $1 in
    "apple" | "banana")
        echo "This is fruit."
        ;;
    "car")
        echo "This is a vehicle."
        ;;
    *)
        echo "I don't know what it is."
        ;;
esac

cs

switch (item)
{
    case "apple" or "banana":
        Console.WriteLine("This is fruit.");
        break;
    case "car":
        Console.WriteLine("This is a vehicle.");
        break;
    default:
        Console.WriteLine("I don't know what it is.");
        break;
}

INFO

esac is the reverse of case.😅

Array

Init an array

arr=(1 2 3 4 5)

Access array element

first=${arr[0]}

WARNING

Access array element using ${arr[idx]} instead of $(arr[idx]).

for in

arr=(1 2 3 4 5)

for item in "${arr[@]}"; do
    echo "$item"
done

INFO

"${arr[@]}" references an whole iterable array instead of first element.

while, do and done

while [[  ]]

Function

user="Jane"
get_user() {
   local user="$1" # scoped, Jane will not be overridden.
   local age="$2"
   echo "Current user is $user, $age years old.";
}
get_user "John" "18" # invoke the function only after the definition.

INFO

1. Use local keyword to declare a function scope only variable.
2. Set positional parameters by order instead of a parameter list.
   • Positional parameter are local by default.
3. Function only available after definition.