Here documents (<<EOF) in Bash Scripting - Step-by-Step Execution

Concept Flow - Here documents (<<EOF)

Start script

↓

Encounter <<EOF

↓

Read lines until EOF

↓

Pass lines as input

↓

Command executes with input

↓

Script continues or ends

The script reads lines after <<EOF until it finds EOF, then sends those lines as input to a command.

Execution Sample

Bash Scripting

cat <<EOF
Hello
World
EOF

This script sends the lines 'Hello' and 'World' as input to the cat command, which prints them.

Execution Table

Step	Action	Input Read	Command Input	Output
1	Start script
2	Encounter <<EOF
3	Read line	Hello
4	Read line	World
5	Read line	EOF
6	Pass lines to cat	Hello\nWorld	Hello\nWorld
7	cat prints input			Hello World
8	Script ends

💡 Reached EOF marker, input passed to command, script ends

Variable Tracker

Variable	Start	After Step 3	After Step 4	After Step 5	Final
input_lines	empty	Hello	Hello\nWorld	Hello\nWorld	Hello\nWorld

Key Moments - 3 Insights

Why does the script stop reading input after the EOF line?

Because the EOF marker tells the shell where the here document ends, as shown in execution_table step 5.

Are the lines inside the here document treated as commands?

No, they are treated as plain text input passed to the command before <<EOF, as seen in step 6.

What happens if the EOF marker is missing?

The shell waits indefinitely for the EOF marker, causing the script to hang, since it doesn't know where input ends.

Visual Quiz - 3 Questions

Look at the execution table, what is the command input at step 6?

A "Hello\nWorld"

B "Hello"

C "World"

D "EOF"

💡 Hint

Check the 'Command Input' column at step 6 in the execution_table.

At which step does the script stop reading lines for the here document?

A Step 3

B Step 5

C Step 4

D Step 6

💡 Hint

Look at the 'Input Read' column to find when EOF is read in the execution_table.

If the EOF marker was changed to END, what would change in the execution table?

A The output would be empty

B Step 6 would have no input

C Step 5 would read 'END' instead of 'EOF'

D The script would end earlier

💡 Hint

The EOF marker defines the end line, so changing it changes the line read at step 5.

Concept Snapshot

Here documents (<<EOF) let you send multiple lines of text as input to a command.
Syntax: command <<EOF ... EOF
The shell reads lines until it finds EOF, then passes them to the command.
Useful for feeding scripts or commands with fixed input.
EOF marker can be any word but must match exactly.
Lines inside are treated as plain text, not commands.

Full Transcript

Here documents in bash scripting allow you to send multiple lines of text as input to a command. When the shell sees <<EOF, it starts reading the following lines until it finds a line with only EOF. These lines are then passed as input to the command before <<EOF. For example, 'cat <<EOF' followed by lines 'Hello' and 'World' and then 'EOF' will send 'Hello\nWorld' to cat, which prints them. The script stops reading input when it finds the EOF marker. If the EOF marker is missing, the shell waits forever. The lines inside the here document are not commands but plain text. You can change EOF to any word, but it must match exactly to end the input.