The Problem

We often use environment variables like $SUBJECT to inject data into commands. But what if that data lives in a file?

Instead of manually copying text, you can use Command Substitution to treat a file’s content as a dynamic variable.

• The example assumes you have a Linux environment.

Technical Solutions

In Bash, there isn’t a direct sigil like $ for files, but you can achieve the same result using Command Substitution. Essentially, you tell Bash to “execute a command that reads the file” and then use the output of that command as the substitution.

There are two general approaches which have minor differences outlined in this blog post:

• Command Substitution
• Built in redirection

Pro-Tip: Dealing with Quotes

• Just like with variables, if your file contains spaces or special characters and you want to keep them intact, always wrap the substitution in double quotes:

  NODE_TYPE // bash

  [ COPY_HEX ]

  # Correct usage: 
  ls "$(< filename.txt)"

• Use Environment Variables for small, frequently accessed flags or secrets.

• Use File Substitution for larger blocks of text, templates, or data that needs to persist between reboots.

Method One: Command Substitution

1. The Standard Way (Command Substitution) The most common way to do this is using $(cat filename).

2. Create a file subject-1.txt

   NODE_TYPE // bash

   [ COPY_HEX ]

   echo "command substitution" > subject-1.txt

3. Run the cat command and places the file’s contents directly into your command line.

   NODE_TYPE // bash

   [ COPY_HEX ]

   # Command Substitution use:
   echo "Hello $(cat subject-1.txt)"

   Expected Output

   NODE_TYPE // text

   [ COPY_HEX ]

   Hello command substitution

Method Two: Built in redirection

Bash provides a shorthand that is faster because it reads the file directly without spawning an external cat process.

1. The Faster Way (Built-in Redirection)

2. Create a file subject-2.txt

   NODE_TYPE // bash

   [ COPY_HEX ]

   echo "built in redirection" > subject-2.txt

3. Bash has a shorthand for $(cat …) that is deemed more efficient because it doesn’t need to spawn a new process for the cat command. You simply use < inside the parentheses:

   NODE_TYPE // bash

   [ COPY_HEX ]

   echo "Hello $(< subject-2.txt)"

   Expected Output

   NODE_TYPE // text

   [ COPY_HEX ]

   Hello built in redirection

   Note: This specific syntax is a Bash/Zsh feature and may not work in basic /bin/sh.

Solution Differences