NSAS Wiki

Network System Access Solutions

User Tools

Site Tools


Sidebar

Contact

linux:shell:printf

The printf command

This is a very big topic that needs experience - please extend the descriptions and correct the details if you can!

Attention: This is about the Bash-builtin command printf - however, the description should be nearly identical for an external command that follows POSIX®.

Unlike other documentations, I don't want to redirect you to the manual page for the printf() C function family. However, if you're more experienced, that should be the most detailed description for the format strings and modifiers.

Due to mutual exclusive historical implementations of the echo command, POSIX® recommends to use printf rather than echo.

General

The printf command provides a method to print preformatted text similar to the printf() system interface (C function). It's meant as successor for echo and has far more features and possibilities.

Beside other reasons, POSIX® has a very good argument to recommend it: Both historical main flavours of the echo command are mutual exclusive, they collide. A “new” command had to be invented to solve the issue.

Syntax

printf <FORMAT> <ARGUMENTS...>

The text format is given in <FORMAT>, while all arguments the formatstring may point to are given after that, here, indicated by <ARGUMENTS…>.

Thus, a typical printf-call looks like:

printf "Surname: %s\nName: %s\n" "$SURNAME" "$LASTNAME"

where “Surname: %s\nName: %s\n” is the format specification, and the two variables are passed as arguments, the %s in the formatstring points to (for every format specifier you give, printf awaits one argument!).

Options

-v VARIf given, the output is assigned to the variable VAR instead of printed to stdout (comparable to sprintf() in some way)

The -v Option can't assign directly to array indexes in Bash versions older than Bash 4.1

Arguments

Of course in shell-meaning the arguments are just strings, however, the common C-notations plus some additions for number-constants are recognized to give a number-argument to printf:

Number-FormatDescription
NA normal decimal number
0NAn octal number
0xNA hexadecimal number
0XNA hexadecimal number
“X(a literal double-quote infront of a character): interpreted as number (underlying codeset) don't forget escaping
'X(a literal single-quote infront of a character): interpreted as number (underlying codeset) don't forget escaping

If more arguments than format specifiers are present, then the format string is re-used until the last argument is interpreted. If less format specifiers than arguments are present, then number-formats are set to zero, while string-formats are set to null (empty).

Also, to minimize surprises, when printf expects an argument, give it one, not more. I'm talking about shell word splitting, please read this article if you don't know what I mean.

<note warning> Again, attention: When a numerical format expects a number, the internal printf-command will use the common Bash arithmetic rules regarding the base. A command like the following example will throw an error, since 08 is not a valid octal number (00 to 07!):

printf "%d\n" 08

</note>

Format strings

FIXME incomplete

The format string interpretion is derived from the C printf() function family. Only format specifiers that end in one of the letters diouxXfeEgGcs are recognized.

To print a literal % (percent-sign), use %% in the format string.

Again: Every format specifier expects an associated argument provided!

These specifiers have different names, depending who you ask. But they all mean the same: A placeholder for data with a specified format:

  • format placeholder
  • conversion specification
  • formatting token
Format Description
%b Print the associated argument while interpreting backslash escapes in there
%q Print the associated argument shell-quoted, reusable as input
%d Print the associated argument as signed decimal number
%i Same as %d
%o Print the associated argument as unsigned octal number
%u Print the associated argument as unsigned decimal number
%x Print the associated argument as unsigned hexadecimal number with lower-case hex-digits (a-f)
%X Same as %x, but with upper-case hex-digits (A-F)
%f Interpret and print the associated argument as floating point number
%e Interpret the associated argument as double, and print it in <N>±e<N> format
%E Same as %e, but with an upper-case E in the printed format
%g Interprets the associated argument as double, but prints it like %f or %e
%G Same as %g, but print it like %E
%c Interprets the associated argument as character: only the first character of a given argument is printed
%s Interprets the associated argument literally as string
%b Interprets the associated argument as a string and interpreting escape sequences in it
%q Prints the associated argument in a format, that it can be re-used as shell-input (escaped spaces etc..)
%n No conversion or printing is done. Assigns the number of so far printed characters to the variable named in the corresponding argument (similat to C's printf)
%(FORMAT)T output the date-time string resulting from using FORMAT as a format string for strftime(3). The associated argument is the number of seconds since Epoch, or -1 (current time) or -2 (shell startup time)
%% No conversion is done. Produces a % (percent sign)

Some of the mentioned format specifiers can modify their behaviour by getting a format modifier:

Modifiers

To be more flexible in the output of numbers and strings, the printf command allows format modifiers. These are specified between the introducting % and the character that specifies the format:

printf "%50s\n" "This field is 50 characters wide..."

Field and printing modifiers

Field output format
<N>Any number: Specifies a minimum field width, if the text to print is smaller, it's padded with spaces, if the text is bigger, the field is expanded
.The dot: Together with a field width, the field is not expanded when the text is bigger, the text is cutted instead. ”%.s“ is an undocumented equivalent for ”%.0s“, which will force a field width of zero, effectively hiding the field from output
*The asterisk: the width is given as argument before the string or number. Usage (the ”*“ corresponds to the ”20“): printf ”%*s\n“ 20 “test string”
#“Alternative format” for numbers: see table below
-Left-bound text printing in the field (standard is right-bound)
0Pads numbers with zeros, not spaces
<space>Pad a positive number with a space, where a minus (-) is for negative numbers
+Prints all numbers signed (+ for positive, - for negative)

printf integer formatting Here is a collection of printf examples for integer printing. Several different options are shown, including a minimum width specification, left-justified, zero-filled, and also a plus sign for positive numbers.

Description Code Result
At least five wide printf(”'%5d'“, 10); ”…10“
At least five-wide, left-justified printf(”'%-5d'“, 10); “10…“
At least five-wide, zero-filled printf(”'%05d'”, 10); “00010”
At least five-wide, with a plus sign printf(”'%+5d'“, 10); ”..+10“
Five-wide, plus sign, left-justified printf(”'%-+5d'“, 10); ”+10..“

The “alternative format” modifier #:

Alternative Format
%#oThe octal number is printed with a leading zero, unless it's zero itself
%#x, %#XThe hex number is printed with a leading ”0x“/”0X“, unless it's zero
%#g, %#GThe float number is printed with trailing zeros until the number of digits for the current precision is reached (usually trailing zeros are not printed)
all number formats except %d, %o, %x, %XAlways print a decimal point in the output, even if no digits follow it

Precision

The precision for a floating- or double-number can be specified by using .<DIGITS>, where <DIGITS> is the number of digits for precision. If <DIGITS> is an asterisk (*), the precision is read from the argument that precedes the number to print, like (prints 4,3000000000):

printf "%.*f\n" 10 4,3

The format .*N to specify the N'th argument for precision does not work in Bash.

For strings, the precision specifies the maximum number of characters to print (i.e. the maximum field width). For integers, it specifies the number of digits to print (zero-padding!).

Escape codes

CodeDescription
\\Prints the character \ (backslash)
\aPrints the alert character (ASCII code 7 decimal)
\bPrints a backspace
\fPrints a form-feed
\nPrints a newline
\rPrints a carriage-return
\tPrints a horizontal tabulator
\vPrints a vertical tabulator
\”Prints a '
\?Prints a ?
\<NNN>Interprets <NNN> as octal number and prints the corresponding character from the character set
\0<NNN>same as \<NNN>
\x<NNN>Interprets <NNN> as hexadecimal number and prints the corresponding character from the character set (3 digits)
\u<NNNN>same as \x<NNN>, but 4 digits
\U<NNNNNNNN>same as \x<NNN>, but 8 digits

Examples

Snipplets

  • print the decimal representation of a hexadecimal number (preserve the sign)
    • printf “%d\n” 0x41
    • printf “%d\n” -0x41
    • printf “%+d\n” 0x41
  • print the octal representation of a decimal number
    • printf “%o\n” 65
    • printf “%05o\n” 65 (5 characters width, padded with zeros)
  • this prints a 0, since no argument is specified
    • printf “%d\n”
  • print the code number of the character A
    • printf “%d\n” \'A
    • printf “%d\n” “'A”
  • Generate a greeting banner and assign it to the variable GREETER
    • printf -v GREETER “Hello %s” “$LOGNAME”
  • Print a text at the end of the line, using tput to get the current line width
    • printf “%*s\n” $(tput cols) “Hello world!”

Small code table

This small loop prints all numbers from 0 to 127 in

  • decimal
  • octal
  • hex
for ((x=0; x <= 127; x++)); do
  printf '%3d | %04o | 0x%02x\n' "$x" "$x" "$x"
done

Ensure well-formatted MAC address

This code here will take a common MAC address and rewrite it into a well-known format (regarding leading zeros or upper/lowercase of the hex digits, …):

the_mac="0:13:ce:7:7a:ad"

# lowercase hex digits
the_mac="$(printf "%02x:%02x:%02x:%02x:%02x:%02x" 0x${the_mac//:/ 0x})"

# or the uppercase-digits variant
the_mac="$(printf "%02X:%02X:%02X:%02X:%02X:%02X" 0x${the_mac//:/ 0x})"

Replacement echo

This code was found in Solaris manpage for echo(1).

Solaris version of /usr/bin/echo is equivalent to:

printf "%b\n" "$*"

Solaris /usr/ucb/echo is equivalent to:

if [ "X$1" = "X-n" ]
then
     shift
     printf "%s" "$*"
else
     printf "%s\n" "$*"
fi

prargs Implementation

Working off the replacement echo, here is a terse implementation of prargs:

printf '"%b"\n' "$0" "$@" | nl -v0 -s": "

repeating a character (for example to print a line)

A small trick: Combining printf and parameter expansion to draw a line

length=40
printf -v line '%*s' "$length"
echo ${line// /-}

or:

length=40
eval printf -v line '%.0s-' {1..$length}

Replacement for some calls to date(1)

The %(…)T format string is a direct interface to strftime(3).

$ printf 'This is week %(%U/%Y)T.\n' -1
This is week 52/2010.

Please read the manpage of strftime(3) to get more information about the supported formats.

Generate a Random MAC Address

printf 'DE:%02X:%02X:%02X:%02X:%02X\n' $((RANDOM%256)) $((RANDOM%256)) $((RANDOM%256)) $((RANDOM%256)) $((RANDOM%256))

Center Text

printf "%*s\n" 50 "Bla bla"
printf "%`tput cols`s"|tr ' ' '#'
linux/shell/printf.txt · Last modified: 2014/05/22 09:27 by michel.pelzer