Here strings (<<<) in Bash Scripting

Introduction

Here strings let you send a small piece of text as input to a command easily, without creating a file.

You want to quickly provide input to a command that reads from standard input.

You need to test a command with a short string without typing it interactively.

You want to avoid creating temporary files for simple input data.

You want to pass a variable's content as input to a command.

You want to simplify scripts by embedding input directly.

Syntax

Bash Scripting

command <<< "string"

The string after <<< is sent as input to the command's standard input.

The string can be quoted or unquoted, but quoting preserves spaces and special characters.

Examples

Counts the number of words in the string 'hello world'.

Bash Scripting

wc -w <<< "hello world"

Searches for the word 'apple' in the given string.

Bash Scripting

grep 'apple' <<< "apple banana cherry"

Reads the string into a variable and prints it.

Bash Scripting

read line <<< "This is a line"; echo "$line"

Sample Program

This script counts words in a string and searches for the word 'cat' in another string using here strings.

Bash Scripting

#!/bin/bash

# Count words in a string using here string
word_count=$(wc -w <<< "one two three four")
echo "Word count: $word_count"

# Search for a word in a string
if grep -q 'cat' <<< "dog cat mouse"; then
  echo "Found cat"
else
  echo "Cat not found"
fi

Important Notes

Here strings add a newline at the end of the string automatically.

They are useful for short inputs; for longer inputs, consider here documents or files.

Not all shells support here strings; bash and zsh do.

Summary

Here strings (<<<) send a short string as input to a command.

They simplify passing data without files or interactive typing.

Useful for quick tests and simple scripts.

Practice

1. What does the here string operator <<< do in bash scripting?

easy

A. Redirects output to a file

B. Starts a multiline comment

C. Sends a short string as input to a command

D. Creates a new shell process

Solution

1. Step 1: Understand the purpose of here strings

   Here strings use <<< to send a short string directly as input to a command without needing a file.

2. Step 2: Compare with other redirections

   Unlike output redirection or comments, here strings provide input data inline.

3. Final Answer:

   Sends a short string as input to a command -> Option C

4. Quick Check:

   Here strings = send input string [OK]

Hint: Here strings send input text directly to commands [OK]

Common Mistakes:

• Confusing input with output redirection
• Thinking it creates files
• Mixing with multiline comments

2. Which of the following is the correct syntax to use a here string to send the text "hello world" to the cat command?

easy

A. cat <<<"hello world"

B. cat << hello

C. cat < hello

D. cat <<< hello world

Solution

1. Step 1: Identify correct here string syntax

   The here string operator is <<< followed by a quoted string to preserve spaces or special characters.

2. Step 2: Check each option

   cat <<< hello world misses quotes, which can cause word splitting. cat << hello uses here document syntax. cat < hello is input redirection from a file. cat <<<"hello world" correctly uses cat <<<"hello world".

3. Final Answer:

   cat <<<"hello world" -> Option A

4. Quick Check:

   Quotes needed with here strings = cat <<<"hello world" [OK]

Hint: Use <<< with quotes for strings containing spaces [OK]

Common Mistakes:

• Omitting quotes causing errors
• Using << instead of <<<
• Confusing input redirection with here strings

3. What is the output of the following command?

grep 'world' <<< "hello world"

medium

A. "

B. world

C. hello

D. hello world

Solution

1. Step 1: Understand grep with here string input

   The here string sends "hello world" as input to grep. Grep searches for lines containing 'world'.

2. Step 2: Determine grep output

   The entire line "hello world" contains 'world', so grep outputs the full line.

3. Final Answer:

   hello world -> Option D

4. Quick Check:

   grep outputs matching lines = hello world [OK]

Hint: grep prints full matching lines from input [OK]

Common Mistakes:

• Expecting only the matched word
• Confusing grep output with cut or awk
• Thinking here string changes grep behavior

4. Identify the error in this script snippet:

read name <<

medium

A. No error, script runs fine

B. read command cannot use here strings

C. Missing quotes around John in here string

D. Incorrect use of echo syntax

Solution

1. Step 1: Analyze the here string usage

   The here string operator works with read and does not require quotes for simple alphanumeric strings like John.

2. Step 2: Check the given snippet

   read name << correctly sets name=John (even with literal \n if present), and echo "Name is $name" outputs correctly. No syntax or runtime errors.

3. Final Answer:

   No error, script runs fine -> Option A

4. Quick Check:

   Here strings + read works without quotes for simple input [OK]

Hint: Here strings with read work without quotes for simple input [OK]

Common Mistakes:

• Assuming read can't use here strings
• Thinking quotes are always required
• Misunderstanding echo syntax

5. You want to count how many times the word "error" appears in a log line stored in a variable line. Which command correctly uses a here string to do this?

hard

A. grep 'error' < $line | wc -l

B. grep -o 'error' <<< "$line" | wc -l

C. echo $line | grep -c 'error'

D. wc -l <<< "$line" | grep 'error'

Solution

1. Step 1: Understand the goal

   We want to count occurrences of 'error' in the variable line.

2. Step 2: Analyze each option

   grep -o 'error' <<< "$line" | wc -l uses here string to send $line to grep with -o (outputs each match on a new line), then counts lines with wc -l, correctly counting occurrences.
   grep 'error' < $line | wc -l tries to redirect input from a file named by $line, which is wrong.
   echo $line | grep -c 'error' echoes line and uses grep -c, which counts matching lines, not occurrences.
   wc -l <<< "$line" | grep 'error' pipes wc -l output to grep, which is incorrect usage.

3. Final Answer:

   grep -o 'error' <<< "$line" | wc -l -> Option B

4. Quick Check:

   Use grep -o with here string to count matches [OK]

Hint: Use grep -o with <<< and wc -l to count matches [OK]

Common Mistakes:

• Using grep -c which counts lines, not matches
• Redirecting input from variable as file
• Misordering pipes causing wrong output