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Understanding the distinction between interactive and non-interactive shells is fundamental when writing reliable Bash scripts. Interactive shells are designed for on-the-fly command entry, while non-interactive shells execute prewritten scripts without user intervention.

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Two Modes of Shell Operation

  1. Interactive Mode
    You type commands at a prompt and see immediate feedback.
  2. Non-Interactive Mode
    You place commands in a file (a script) and run them all at once.
Interactive shells load initialization files like ~/.bashrc, enabling features such as command history and prompt customization. Non-interactive shells skip most of these initializations.

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Invoking Interactive vs. Non-Interactive Shells

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Interactive Example

$ ls
file1.txt  file2.txt
$ cd /tmp
$ pwd
/tmp

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Non-Interactive Example

  1. Create a script script.sh: 
    #!/bin/bash
ls
cd /tmp
pwd
  2. Make it executable and run: 
    chmod +x script.sh
./script.sh
# Output:
# file1.txt  file2.txt
# /tmp

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Comparing Shell Modes

FeatureInteractive ShellNon-Interactive Shell
InvocationDirect terminal prompt./script.sh or bash script.sh
InputKeyboardScript file or piped commands
Initialization files~/.bashrc, PROMPT_COMMAND/etc/profile, ~/.bash_profile
Common Use CasesAd hoc tasks, debuggingAutomation, cron jobs, CI/CD
Prompt Variable (PS1)Typically set (\u@\h:\w\$ )Usually unset

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Why It Matters

Some environment variables and shell behaviors differ between modes:

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Inspecting PS1

Interactive mode: 
$ echo "$PS1"
\u@\h:\w\$
Non-interactive script (show_ps1.sh): 
#!/bin/bash
echo "$PS1"
Running it produces no output because PS1 is unset in non-interactive shells.
Don’t rely on interactive-only variables (like PS1) in scripts. Always initialize or provide defaults for any environment variable your script requires.

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Best Practices for Shell Scripts

  • Start with a shebang #!/usr/bin/env bash for portability.
  • Use set -euo pipefail to catch errors early.
  • Double-quote variable expansions to prevent word splitting: "$VAR".
  • Explicitly source files when needed: 
    source ~/.bashrc
  • Test scripts in both modes if they must run interactively and non-interactively.

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Further Reading

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