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LINUX_LAB / Bash scripting / 03

Input and Output

In the last chapter, I put my name into a variable myself. Here, the script asks for it. This is enough to turn a fixed message into a small check-in I can actually use.

I already know what input and output do from C++ and Java. These notes focus on the Bash parts: keeping a full line intact, choosing a format, and remembering where the newline goes.

Print · Format · Read · Try it · Revision

Open the chapter

From the repo root, in a Bash terminal:

cd learning/bash-scripting/03-input-and-output

In order	Run it	What to watch
output.sh	bash output.sh	A greeting and a blank line with echo.
formatted-output.sh	bash formatted-output.sh	Strings, integers, and a literal percent sign with printf.
input.sh	bash input.sh	Two prompts, then a summary of the values you entered.

All three print to the terminal. They don't create or change files.

echo for a simple message

name="Shaikh Mahad"
echo "Hello, $name"
echo
echo "Back to Bash."

echo adds a newline after its arguments. An echo on its own gives me a blank line. For a fixed message like this, that's all I need.

For arbitrary text or formatting, I use printf. echo can interpret values such as -n as options, and its backslash handling depends on the shell and settings.

printf when the output needs a shape

name="Shaikh Mahad"
sessions=3
score=95

printf 'Name: %s\n' "$name"
printf 'Sessions: %d\n' "$sessions"
printf 'Score: %d%%\n' "$score"

Name: Shaikh Mahad
Sessions: 3
Score: 95%

In the format	Meaning
%s	Insert a string.
%d	Format an integer. It doesn't validate arbitrary user input.
%%	Print a literal %.
\n	Print a newline. printf doesn't add one automatically.

The format stays fixed; values go in separate, quoted arguments:

message='100% ready for the next script.'
printf '%s\n' "$message"

This prints the message exactly, followed by a newline. printf "$message" would treat the message itself as a format, so % and backslashes could change the output.

read a line without changing it

This is the input line from input.sh:

IFS= read -r -p "Enter your name: " name || exit 1

Part	Why it's here
IFS=	Sets the input field separator to empty for this read only, so it doesn't split or trim whitespace.
read	Reads a line from standard input, without storing its ending newline.
-r	Keeps backslashes literal. A useful default for text.
-p "..."	Shows a prompt when reading from a terminal.
name	The variable to fill. No $ when giving its name.
|| exit 1	|| runs the command on its right only if read fails. exit 1 ends the script with a failure status.

With one variable and IFS=, this behaves more like reading a whole line with C++ getline than reading a single word with cin >>.

If the input ends before a complete line is read, the script stops instead of printing an incomplete summary. In a terminal, Ctrl+D on an empty input line signals end-of-file. Chapter 04 uses the same || rule with other commands.

Run bash input.sh and enter two lines:

Enter your name: Shaikh Mahad
Enter your course: Operating Systems

Student information
Name: Shaikh Mahad
Course: Operating Systems

Try a name with spaces or a course containing 100%. Both should come back unchanged. Pressing Enter on an empty line is valid input here; this example preserves text, but doesn't require a nonempty value.

A few read options I'll want to find again

Split a line into fields when that's intentional:

read -r -p "First and last name: " first_name last_name
printf 'First: %s\nLast: %s\n' "$first_name" "$last_name"

With the usual space/tab delimiters, Shaikh Mahad becomes Shaikh and Mahad. Extra words go into the last variable. Leaving off IFS= allows that splitting and trims surrounding delimiter whitespace.

Hide typed characters in a terminal:

IFS= read -r -s -p "Password: " password
printf '\n'
unset password

-s hides typing; it doesn't encrypt the value. Don't print it back. This snippet discards it after the demonstration.

Omit the variable name and Bash stores the line in REPLY:

read -r -p "One thing to revise: "
printf '%s\n' "$REPLY"

Your turn: a session check-in

Make a copy of input.sh for practice. Replace its course prompt with a topic prompt, then add a line asking what to revise next. Print all three answers with printf.

Try Bash quoting as the topic and paths with spaces as the reminder. The words should stay together.

The extra prompt and output

Inside your script, after reading the name:

IFS= read -r -p "Today's topic: " topic || exit 1
IFS= read -r -p "Next time, revise: " reminder || exit 1

printf '\nStudent: %s\nTopic: %s\nNext: %s\n' "$name" "$topic" "$reminder"

You can keep several %s fields in one format. The arguments fill them in order.

Quick revision

If I forget…	The reminder I need
Why two outputs run together	printf needs an explicit \n.
Why spaces or backslashes changed	Check IFS= and read -r. Quoting the output alone can't recover lost input.
Why % behaves strangely	Keep the printf format separate from the value.
Where the data goes	read uses standard input; echo and printf use standard output. The terminal is just their usual destination here.

Pipes and redirection connect those streams to commands and files. Linux command chapter 07 puts that into practice: save a result, pass it to another command, and keep errors separately.

References for the details

GNU Bash manual: Bash builtins, including echo, printf, and read. In a Bash terminal, help read and help printf are useful offline references.

← Bash 02: Variables and Expansion · Back to the learning map

Next: Bash 04, Operators and Expressions → Calculate with the values and check whether a command succeeded.