Running Multi-Line Shell Code at Once From Single Prompt

You can run multi-line shell code at once by using a shell script or by using a command line tool such as the bash or sh command to execute the code in a single prompt.

To create a shell script, you can use a text editor to write the code and save it with a .sh file extension. For example, you can create a file called "script.sh" and add the following code −

#!/bin/bash
echo "Hello, World!"
echo "This is a shell script."

Then you can run the script by using the command bash script.sh or sh script.sh.

Another way is to use the << symbol, also known as a here-string, which allows you to pass multiple lines of text to a command as its standard input. For example −

bash << EOF
echo "Hello, World!"
echo "This is a shell script."
EOF
You can also use << with other commands, like this

cat << EOF
Line 1
Line 2
Line 3
EOF

You can use the here-doc syntax with any command that reads from standard input.

Multi-Line Code Techniques

There are several techniques that can be used to write multi-line code in a shell script or at the command prompt. Some of the most common include −

Using a text editor − You can use a text editor such as nano, vi, or emacs to write your code and save it to a file with a .sh file extension. Then, you can execute the script by running bash script.sh or sh script.sh.

Using the \ character − You can use the \ character at the end of each line to indicate that the command continues on the next line. For example −

echo "This is a very long command" \
     "that spans multiple lines"

Using parentheses − You can use parentheses to group commands together and span multiple lines. For example −

 (echo "This is the first line"
 echo "This is the second line")

Using ; − You can use ; to separate commands and execute them on the same line

echo "This is first command"; echo "This is second command"

Using && − You can use && to run multiple commands where each command runs if the previous one succeeded

command1 && command2 && command3

Using || − You can use || to run multiple commands where if first command fails, second command runs

command1 || command2

Using the << symbol (here-string) − You can use the << symbol to pass multiple lines of text to a command as its standard input, as mentioned earlier.

bash << EOF
echo "Hello, World!"
echo "This is a shell script."
EOF

Each of these techniques has its own advantages and disadvantages, and you should choose the one that best suits your needs and is most readable for your use case.

Using a Backslash

Using a backslash (\) is a common technique for writing multi-line code in a shell script or at the command prompt.

For example, you can use the backslash to split a long echo command into multiple lines −

echo "This is a very long command that spans multiple lines" \
     "and is broken up into multiple lines for readability"

You can also use the backslash to split a long command that includes multiple arguments or options into multiple lines. For example −

command --option1 value1 \
         --option2 value2 \
         --option3 value3

Using Parentheses and Curly Braces

Using parentheses and curly braces are another common technique for writing multi-line code in a shell script or at the command prompt.

Parentheses are used to group commands together and execute them as a single unit. For example, you can use parentheses to group multiple echo commands together −

(echo "This is the first line"
 echo "This is the second line")

Curly braces are used to create a block of commands that are executed together as a single unit. It's similar to using parentheses but with curly braces, you can give the block a name and then refer to it later as a single command. For example −

# Define a block of commands
block_of_commands() {
   echo "This is the first command"
   echo "This is the second command"
}

# Execute the block of commands
block_of_commands

Both parentheses and curly braces are useful for creating reusable blocks of code that can be executed multiple times, with different arguments or in different contexts.

Using the EOF tag

Using the EOF (end-of-file) tag is a technique for writing multi-line code in a shell script or at the command prompt.

For example, you can use the EOF tag to create a simple script that prints "Hello, World!" and "This is a shell script." −

bash << EOF
echo "Hello, World!"
echo "This is a shell script."
EOF

You can also use the EOF tag with other commands, such as cat to print the contents of a file −

cat << EOF
Line 1
Line 2
Line 3
EOF

The EOF tag technique is similar to using a text editor to write your code and save it to a file with a .sh file extension, but it allows you to write and execute the code all in one step.

Using Double Ampersand

Using double ampersand (&&) is a technique for writing multi-line code in a shell script or at the command prompt. The double ampersand operator allows you to run multiple commands in succession, where each command runs only if the previous one succeeded.

For example, you can use the double ampersand operator to run a command that checks if a file exists, and then another command that runs only if the file exists −

test -e file.txt && echo "file exists"

You can also chain multiple commands together using the double ampersand operator, where each command runs only if the previous one succeeded −

command1 && command2 && command3

Using Semicolons

Using semicolons (;) is a technique for writing multi-line code in a shell script or at the command prompt. The semicolon is used to separate commands and execute them on the same line.

For example, you can use the semicolon to run multiple echo commands on the same line −

echo "This is first command"; echo "This is second command"

or you can use semicolon to run multiple commands of different types −

ls -l; echo "Hello World!"; pwd

This technique is useful when you want to run multiple commands in a specific order and you don't care about the success or failure of any particular command.

Conclusion

Writing multi-line code in a shell script or at the command prompt can be done using several techniques, including using a text editor, using the \ character, using parentheses and curly braces, using the << symbol (here-string), using ;, && and ||. Each technique has its own advantages and disadvantages, and it's important to choose the one that best suits your needs and is most readable for your use case.