How to use Here Document in bash programming

A block of code or text which can be redirected to the command script or interactive program is called here document or HereDoc. Different types of scripting languages like bash, sh, csh, ksh, etc., can take text input directly using here-document without using any text file. So when the coder needs less amount of text data, then using code and data in the same file is the better option, and it can be done easily by using here-document in the script. Without scripting language, here document can also be used in various high-level languages like php, perl, etc. How you can use here-document in the bash script is shown in this tutorial.

To use here-document in any bash script, you have to use the symbol << followed by any delimiting identifier after any bash command and close the HereDoc by using the same delimiting identifier at the end of the text. The syntax of writing HereDoc is shown below.

Example-1: Use of Simple heredoc Text

When the heredoc is used in any script, it is necessary to keep the same name for the starting and ending delimiter. Suppose the command is `cat` and the heredoc delimiter is ADDTEXT. Create a bash script file named heredoc1.bash with the following code to check the simple use of heredoc.

Run the following command to execute the script.

Output:

The following output will appear after executing the script.

Example-2: Use of heredoc with ‘-’ symbol

HereDoc uses the ‘–‘ symbol to suppress any tab space from each line of heredoc text. In the following example, tab space is added at the start of each line, and the ‘–‘ symbol is used before the starting delimiter. When the script executes, all tab spaces are omitted from the starting of each line, but it creates no effect on normal space. Create a bash file named heredoc2.bash with the following script to test the function of ‘–‘.

Run the following command to execute the script.

Output:

The following output will appear after executing the script. The tab space has been removed from the heredoc content.

Example-3: Use of Variable within heredoc Text

Create a new bash script with the following code. Here, two variables named start and end are declared. These variables are used within the heredoc text. If you use a quotation mark at the starting delimiter of heredoc, then the variable’s value will not print in the console.

Run the following command to execute the script.

Output:

The following output will appear after executing the script. The output shows the heredoc content with the value of the variables.

Example-4: Create a new bash file using HerDoc

In the previous examples, how you can use heredoc in any bash script are shown. You can also create a new bash file using heredoc, shown in the next part of this tutorial. Create a new bash file named heredoc4.sh with the following code. Here, the NewFile variable is declared to set the file name of the new bash script, which will be created after the execution of heredoc4.sh file. After the execution, a new bash file named output.sh will be created.

Run the following commands to test the above script. The first command will execute the main script file. The second command will display the content of the newly created file. The third command will run the newly created bash file.

Output:

The following output will appear after executing the script.

Example-5: Using Function with heredoc

You can pass input values to the variables of any function of the bash script from heredoc content. Create another new bash file named heredoc5.sh to test how function can be used with heredoc. Add the following code to the file. A function named BookInfo is declared in the script, which will take data from heredoc text. Six variables are declared in the function named ISBN, bookName, authorName, edition, publication, and price. To set the value of the variables properly, you have to maintain the order of the values in the heredoc section according to the variables declared in the function. After setting the data in the function variables, the price value is calculated by a 10% discount and printed the values of all variables in the console.

Run the following command to execute the script.

Output:

Here, the value of the price variable has been set to 100. After setting a 10% discount on price value, the value has been set to 90. After the execution, the following output will appear in the terminal.

Example-6: Execute commands inside the heredoc

Create a bash file named heredoc6.bash with the following script to execute `date`, `pwd`, and `whoami` command with `sudo` command.

Output:

The following output will appear after executing the above script. The output of the three commands has printed in the output.

Example-7: Use of heredoc to write multiple lines into a file

Create a bash file named heredoc6.bash with the following script to add multiple lines in the text file named temp.txt and print the file’s content. Three lines will be added to the text file after executing the code.

Output:

The following output will appear after executing the above script. The output shows that three lines are added in the temp.txt file.

Example-8: using heredoc with pipe to search and replace the content

The heredoc input can also be piped. Create a bash file with the following script to search and replace a particular character from the heredoc content. The first heredoc will print the original text, and the second heredoc will print the replaced text. The `sed` command has been used here to replace all instances of the character ‘a’ by ‘A’.

Output:

The following output will appear after executing the above script. In the output, all ‘a’ have been replaced by ‘A’.

Example-9: Use of heredoc to comment out the block of lines

One of the important use of heredoc is to comment on multiple lines of the bash script. Create a bash file with the following script to find a year is a leap year or not. The purpose of the script has been described by multi-line comments using heredoc.

Output:

The following output will appear after executing the above script. The following output shows that 2008 is a leap year and 2021 is not a leap year.

Conclusion:

The various uses of the heredoc document have shown in this tutorial by using multiple examples. It can be used with different types of shell commands and adding multi-line comments in the bash script. The uses of the heredoc will be cleared for the bash users after practicing the examples of this tutorial.