Shell by Example: Printf POSIX 

#!/bin/sh

The printf command provides formatted output, similar to printf in C. It’s more portable and powerful than echo for complex formatting.

Basic printf syntax: printf FORMAT [ARGUMENTS…]

 

printf "Hello, World!\n"

Unlike echo, printf doesn’t add a newline automatically. Use \n to add newlines.

 

printf "Line 1\n"
printf "Line 2\n"

Format specifiers:

%s - String

 
printf "String: %s\n" "hello"

%d - Decimal integer

 
printf "Integer: %d\n" 42

%f - Floating point

 
printf "Float: %f\n" 3.14159

%e - Scientific notation

 
printf "Scientific: %e\n" 1234.5

%x - Hexadecimal

 
printf "Hex: %x\n" 255

%o - Octal

 
printf "Octal: %o\n" 64

%c - Single character

 
printf "Character: %c\n" "A"

%% - Literal percent sign

 
printf "Percent: 100%%\n"

Width and precision:

 

echo "Width specifiers:"
printf "  |%10s|\n" "hello"  # Right-align, width 10
printf "  |%-10s|\n" "hello" # Left-align, width 10
printf "  |%10d|\n" 42       # Right-align number
printf "  |%-10d|\n" 42      # Left-align number

echo "Precision for floats:"
printf "  Default: %f\n" 3.14159265
printf "  2 decimals: %.2f\n" 3.14159265
printf "  Width + precision: %8.2f\n" 3.14159265

echo "Precision for strings (max length):"
printf "  %.5s\n" "Hello, World!" # First 5 characters

Zero-padding:

 

printf "Zero-padded: %05d\n" 42
printf "Zero-padded: %08.2f\n" 3.14

Multiple arguments:

 

printf "%s is %d years old\n" "Alice" 30
printf "%s is %d years old\n" "Bob" 25

Arguments cycle through format:

 

printf "%s\n" "first" "second" "third"
printf "%s %s\n" "a" "1" "b" "2" "c" "3"

Escape sequences:

 

echo "Escape sequences:"
printf "  Tab:\tafter tab\n"
printf "  Newline:\nafter newline\n"
printf "  Backslash: \\\\\n"
printf "  Single quote: \\'\n"
printf "  Bell: \a(beep)\n"

Hex and octal escape codes:

 

printf "  Hex (41 = A): \x41\n"
printf "  Octal (101 = A): \101\n"

Store printf output in variable:

 

result=$(printf "Value: %05d" 42)
echo "Stored: $result"
Bash

Bash can store directly with -v flag:

 
printf -v myvar "Value: %05d" 42
echo "Stored: $myvar"

Practical examples:

Create a table:

 

printf "\n%-15s %8s %10s\n" "Name" "Age" "Salary"
printf "%-15s %8d %10.2f\n" "Alice Smith" 30 75000
printf "%-15s %8d %10.2f\n" "Bob Johnson" 25 65000
printf "%-15s %8d %10.2f\n" "Carol White" 35 85000

Format currency:

 

format_currency() {
  printf "$%'.2f\n" "$1"
}
format_currency 1234567.89

Leading zeros for file numbering:

 

echo "File numbering:"
for i in 1 2 3 10 100; do
  printf "  file_%03d.txt\n" "$i"
done

Progress percentage:

 

show_progress() {
  printf "\rProgress: %3d%%" "$1"
}
echo "Progress simulation:"
for p in 25 50 75 100; do
  show_progress "$p"
  sleep 0.2
done
printf "\n"

Color output (ANSI escape codes):

 

printf "\033[31mRed text\033[0m\n"
printf "\033[32mGreen text\033[0m\n"
printf "\033[1mBold text\033[0m\n"
printf "\033[4mUnderlined\033[0m\n"

Format date components:

 

year=2024
month=3
day=15
printf "Date: %04d-%02d-%02d\n" "$year" "$month" "$day"

Dynamic width:

 

width=20
printf "%*s\n" "$width" "right-aligned"
printf "%-*s\n" "$width" "left-aligned"

Print to stderr:

 

printf "Error: %s\n" "Something went wrong" >&2

Repeat character N times:

 

printf '=%.0s' $(seq 1 40)
printf '\n'
Bash

Bash-specific: printf with arrays

 
nums=(1 2 3 4 5)
printf "Number: %d\n" "${nums[@]}"

Common patterns:

 

# Debug output
debug() {
  printf "[DEBUG] %s: %s\n" "$(date +%H:%M:%S)" "$*" >&2
}
debug "Starting process"

# Log with timestamp
log() {
  printf "[%s] %s\n" "$(date '+%Y-%m-%d %H:%M:%S')" "$*"
}
log "Application started"

echo "Printf examples complete"

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by Jose Diaz-Gonzalez | inspired by Go by Example | source