Interactive vs non interactive shell

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.

Two Modes of Shell Operation

Interactive Mode
You type commands at a prompt and see immediate feedback.
Non-Interactive Mode
You place commands in a file (a script) and run them all at once.

Note

Interactive shells load initialization files like ~/.bashrc, enabling features such as command history and prompt customization. Non-interactive shells skip most of these initializations.

Invoking Interactive vs. Non-Interactive Shells

Interactive Example

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

Non-Interactive Example

Create a script script.sh:
```
#!/bin/bash
ls
cd /tmp
pwd
```

Make it executable and run:

chmod +x script.sh
./script.sh
# Output:
# file1.txt  file2.txt
# /tmp

Comparing Shell Modes

Feature	Interactive Shell	Non-Interactive Shell
Invocation	Direct terminal prompt	`./script.sh` or `bash script.sh`
Input	Keyboard	Script file or piped commands
Initialization files	`~/.bashrc`, `PROMPT_COMMAND`	`/etc/profile`, `~/.bash_profile`
Common Use Cases	Ad hoc tasks, debugging	Automation, cron jobs, CI/CD
Prompt Variable (`PS1`)	Typically set (`\u@\h:\w\$` )	Usually unset

Why It Matters

Some environment variables and shell behaviors differ between modes:

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.

Warning

Don’t rely on interactive-only variables (like PS1) in scripts. Always initialize or provide defaults for any environment variable your script requires.

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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