Input/Output¶

Reading input and producing formatted output in bash scripts.

Reading Input¶

The read Command¶

read variable

User types input, stored in variable:

echo "What's your name?"
read name
echo "Hello, $name!"

Prompt with -p¶

read -p "Enter your name: " name
echo "Hello, $name!"

Silent Input with -s¶

For passwords:

read -sp "Password: " password
echo    # Newline after hidden input
echo "Password has ${#password} characters"

Timeout with -t¶

if read -t 5 -p "Quick! Enter something: " answer; then
    echo "You entered: $answer"
else
    echo "Too slow!"
fi

Default Value¶

read -p "Name [Anonymous]: " name
name="${name:-Anonymous}"
echo "Hello, $name"

Limit Characters with -n¶

read -n 1 -p "Continue? (y/n) " answer
echo
[[ "$answer" == "y" ]] && echo "Continuing..."

Read Array with -a¶

read -p "Enter words: " -a words
echo "First word: ${words[0]}"
echo "All words: ${words[@]}"

Read with Delimiter -d¶

read -d ':' part < /etc/passwd
echo "First field: $part"

Common Options¶

Option	Purpose
`-p prompt`	Display prompt
`-s`	Silent (no echo)
`-t seconds`	Timeout
`-n count`	Read count characters
`-r`	Raw (don't interpret backslashes)
`-a array`	Read into array
`-d delim`	Use delimiter instead of newline

Reading Files¶

Line by Line¶

while IFS= read -r line; do
    echo "Line: $line"
done < file.txt

Components:

IFS= - Don't trim whitespace
-r - Don't interpret backslashes
< file.txt - Input from file

Read Specific Fields¶

# /etc/passwd: user:x:uid:gid:desc:home:shell
while IFS=: read -r user _ uid gid _ home shell; do
    echo "$user (UID: $uid) -> $shell"
done < /etc/passwd

Read from Command Output¶

while IFS= read -r file; do
    echo "Found: $file"
done < <(find . -name "*.txt")

Read from Here Document¶

while IFS= read -r line; do
    echo "Config: $line"
done << 'EOF'
setting1=value1
setting2=value2
setting3=value3
EOF

Basic Output¶

echo¶

echo "Hello, World!"
echo -n "No newline"     # No trailing newline
echo -e "Tab:\tNewline:\nDone"  # Interpret escapes

Echo escapes (with -e):

Escape	Meaning
`\n`	Newline
`\t`	Tab
`\r`	Carriage return
`\\`	Backslash
`\033[...m`	ANSI color codes

printf¶

More control over formatting:

printf "Hello, %s!\n" "World"
printf "Number: %d\n" 42
printf "Float: %.2f\n" 3.14159
printf "Hex: %x\n" 255

Format specifiers:

Specifier	Type
`%s`	String
`%d` or `%i`	Integer
`%f`	Float
`%e`	Scientific notation
`%x` / `%X`	Hexadecimal
`%o`	Octal
`%%`	Literal %

printf Width and Alignment¶

printf "%-10s %5d\n" "Alice" 25
printf "%-10s %5d\n" "Bob" 30
printf "%-10s %5d\n" "Charlie" 35

Alice         25
Bob           30
Charlie       35

Format	Meaning
`%10s`	Right-aligned, width 10
`%-10s`	Left-aligned, width 10
`%05d`	Zero-padded, width 5
`%.2f`	2 decimal places
`%10.2f`	Width 10, 2 decimals

printf to Variable¶

printf -v greeting "Hello, %s!" "World"
echo "$greeting"    # Hello, World!

Here Documents¶

Multi-line input:

cat << EOF
Line 1
Line 2
Line 3
EOF

Variable Expansion¶

name="Alice"
cat << EOF
Hello, $name!
Your home is $HOME
EOF

No Expansion (Quoted Delimiter)¶

cat << 'EOF'
Variables like $HOME won't expand
Neither will $(commands)
EOF

Indented (Tab Removal)¶

cat <<- EOF
    Indented with tabs
    Tabs are stripped
    EOF

Tabs Only

The <<- syntax only strips tabs, not spaces.

Here Document to Command¶

mysql -u root << EOF
USE mydb;
SELECT * FROM users;
EOF

Here Document to Variable¶

read -r -d '' content << 'EOF'
Line 1
Line 2
Line 3
EOF
echo "$content"

Here Strings¶

Single-line input:

grep "pattern" <<< "search in this string"

# Equivalent to:
echo "search in this string" | grep "pattern"

Colored Output¶

Basic ANSI Colors¶

# Colors
RED='\033[31m'
GREEN='\033[32m'
YELLOW='\033[33m'
BLUE='\033[34m'
RESET='\033[0m'

echo -e "${RED}Error${RESET}: Something failed"
echo -e "${GREEN}Success${RESET}: All good"
printf "${YELLOW}Warning${RESET}: Be careful\n"

tput for Portable Colors¶

RED=$(tput setaf 1)
GREEN=$(tput setaf 2)
YELLOW=$(tput setaf 3)
BOLD=$(tput bold)
RESET=$(tput sgr0)

echo "${RED}Error${RESET}"
echo "${BOLD}${GREEN}Success${RESET}"

Color Functions¶

red()    { printf '\033[31m%s\033[0m\n' "$*"; }
green()  { printf '\033[32m%s\033[0m\n' "$*"; }
yellow() { printf '\033[33m%s\033[0m\n' "$*"; }
blue()   { printf '\033[34m%s\033[0m\n' "$*"; }

error()   { red "Error: $*" >&2; }
success() { green "Success: $*"; }
warn()    { yellow "Warning: $*"; }
info()    { blue "Info: $*"; }

Check for Color Support¶

if [[ -t 1 ]] && [[ -n "$TERM" ]] && [[ "$TERM" != "dumb" ]]; then
    # Terminal supports colors
    RED='\033[31m'
    RESET='\033[0m'
else
    RED=''
    RESET=''
fi

Progress Indicators¶

Simple Counter¶

for i in {1..10}; do
    printf "\rProcessing: %d/10" "$i"
    sleep 0.5
done
echo

Percentage¶

total=100
for ((i=1; i<=total; i++)); do
    percent=$((i * 100 / total))
    printf "\r[%-50s] %d%%" $(printf '#%.0s' $(seq 1 $((i/2)))) "$percent"
    sleep 0.05
done
echo

Spinner¶

spinner() {
    local pid=$1
    local spin='-\|/'
    local i=0
    while kill -0 "$pid" 2>/dev/null; do
        printf "\r[%c] Working..." "${spin:i++%4:1}"
        sleep 0.1
    done
    printf "\rDone!          \n"
}

# Usage
sleep 3 &
spinner $!

Output Formatting¶

Tables¶

# Header
printf "%-15s %-10s %10s\n" "Name" "Status" "Count"
printf "%-15s %-10s %10s\n" "---------------" "----------" "----------"

# Data
printf "%-15s %-10s %10d\n" "Service A" "Running" 42
printf "%-15s %-10s %10d\n" "Service B" "Stopped" 0
printf "%-15s %-10s %10d\n" "Service C" "Running" 123

Name            Status          Count
--------------- ---------- ----------
Service A       Running            42
Service B       Stopped             0
Service C       Running           123

Column Command¶

echo -e "Name\tAge\tCity
Alice\t25\tNYC
Bob\t30\tLA" | column -t

Name   Age  City
Alice  25   NYC
Bob    30   LA

Logging Functions¶

#!/usr/bin/env bash

LOG_LEVEL="${LOG_LEVEL:-INFO}"

log() {
    local level="$1"
    shift
    local timestamp
    timestamp=$(date '+%Y-%m-%d %H:%M:%S')
    printf "[%s] %-5s %s\n" "$timestamp" "$level" "$*"
}

debug() { [[ "$LOG_LEVEL" == "DEBUG" ]] && log "DEBUG" "$@"; }
info()  { log "INFO" "$@"; }
warn()  { log "WARN" "$@" >&2; }
error() { log "ERROR" "$@" >&2; }
fatal() { log "FATAL" "$@" >&2; exit 1; }

# Usage
info "Starting script"
debug "Debug message (only if LOG_LEVEL=DEBUG)"
warn "This is a warning"
error "This is an error"

Try It¶

Read input:

read -p "Name: " name
echo "Hello, $name!"

Formatted output:

printf "%-10s %5d %8.2f\n" "Item" 42 3.14159

Read file:

echo -e "one\ntwo\nthree" | while read -r line; do
    echo "Got: $line"
done

Here document:

name="World"
cat << EOF
Hello, $name!
Today is $(date)
EOF

Colored output:

printf '\033[32mGreen\033[0m and \033[31mRed\033[0m\n'

Summary¶

Task	Command
Read line	`read var`
Read with prompt	`read -p "Prompt: " var`
Silent input	`read -s var`
Read file	`while read -r line; do ... done < file`
Print string	`echo "text"` or `printf "text\n"`
Formatted print	`printf "%s %d\n" str num`
Here document	`cat << EOF ... EOF`
Here string	`cmd <<< "string"`

Best practices:

Use read -r to prevent backslash interpretation
Use IFS= when reading to preserve whitespace
Prefer printf over echo for portability
Quote here document delimiter to prevent expansion
Use functions for colored/logging output