printf - Formatted Output

Write formatted output.

Source: src/scripting/printf_builtin.f90:builtin_printf

Synopsis

printf FORMAT [ARGUMENTS...]

Description

Outputs formatted text according to FORMAT, similar to C's printf. More portable than echo -e for escape sequences.

Format Specifiers

Format Flags

Flags appear between % and the specifier:

Width and Precision

%[flags][width][.precision]specifier

• Width: Minimum field width
• Precision:
  • For strings: maximum characters
  • For floats: decimal places
  • For integers: minimum digits

Escape Sequences

Return Value

Examples

Basic formatting

printf "Hello, %s!\n" "World"
# Output: Hello, World!

printf "Name: %s, Age: %d\n" "Alice" 30
# Output: Name: Alice, Age: 30

Number formatting

# Integers
printf "%d\n" 42        # 42
printf "%5d\n" 42       # "   42" (width 5)
printf "%05d\n" 42      # 00042 (zero-padded)
printf "%-5d|\n" 42     # "42   |" (left-aligned)

# Hexadecimal
printf "%x\n" 255       # ff
printf "%X\n" 255       # FF
printf "%#x\n" 255      # 0xff

# Octal
printf "%o\n" 8         # 10
printf "%#o\n" 8        # 010

Floating-point

printf "%f\n" 3.14159       # 3.141590
printf "%.2f\n" 3.14159     # 3.14
printf "%8.2f\n" 3.14159    # "    3.14"
printf "%e\n" 1234.5        # 1.234500e+03

String formatting

printf "%s\n" "hello"       # hello
printf "%10s\n" "hello"     # "     hello"
printf "%-10s|\n" "hello"   # "hello     |"
printf "%.3s\n" "hello"     # hel (max 3 chars)

Width from argument

printf "%*s\n" 10 "hello"   # "     hello" (width=10)
printf "%.*s\n" 3 "hello"   # hel (precision=3)

Format reuse

When there are more arguments than format specifiers, the format repeats:

printf "%s\n" apple banana cherry
# apple
# banana
# cherry

printf "%d " 1 2 3 4 5
# 1 2 3 4 5

Escape sequences

printf "Line 1\nLine 2\n"
printf "Col1\tCol2\tCol3\n"
printf "Alert: \a"

Table formatting

printf "%-15s %8s %8s\n" "Name" "Size" "Date"
printf "%-15s %8d %8s\n" "file.txt" 1234 "Mar 9"
printf "%-15s %8d %8s\n" "data.csv" 56789 "Mar 8"

# Output:
# Name                Size     Date
# file.txt            1234    Mar 9
# data.csv           56789    Mar 8

Implementation Details

From printf_builtin.f90:

! Parse format string character by character
do while (fmt_pos <= len_trim(format_str))
    ch = format_str(fmt_pos:fmt_pos)
    if (ch == '%') then
        call parse_format_spec(format_str, fmt_pos, spec)
        call apply_format(spec, args(arg_index), output)
    else if (ch == '\') then
        call handle_escape(format_str, fmt_pos, output)
    else
        output = output // ch
    end if
end do

Comparison with echo

Related Commands

• echo - Simple output
• read - Read input

Notes

• Always include \n for newlines; printf doesn't add one automatically
• For portable scripts, prefer printf over echo -e
• Arguments are converted to match format specifiers (strings to numbers, etc.)
• Non-numeric strings become 0 when used with numeric specifiers
• Quote arguments to preserve whitespace: printf "%s" "$var"