Printf in BASH

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

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

In versions newer than 4.1, one must be careful when performing expansions into the first non-option argument of printf as this opens up the possibility of an easy code injection vulnerability.

$ var='-vx[$(echo hi >&2)]'; printf "$var" hi; declare -p x
hi
declare -a x='([0]="hi")'

…where the echo can of course be replaced with any arbitrary command. If you must, either specify a hard-coded format string or use -- to signal the end of options. The exact same issue also applies to read, and a similar one to mapfile, though performing expansions into their arguments is less common.

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:

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

Take care to avoid word splitting, as accidentally passing the wrong number of arguments can produce wildly different and unexpected results. See this article.

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

Format strings

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

• …

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 introductory % and the character that specifies the format:

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

Field and printing modifiers

The "alternative format" modifier#:

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

These are interpreted if used anywhere in the format string, or in an argument corresponding to a %b format.

The following additional escape and extra rules apply only to arguments associated with a %b format:

• Backslashes in the escapes: \', \", and \? are not removed.

• Octal escapes beginning with \0 may contain up to four digits. (POSIX specifies up to three).

These are also respects in which %b differs from the escapes used by $'...' style quoting.

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

• 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

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.

Using printf inside of awk

Here's the gotcha:

$ printf "%s\n" "Foo"
Foo

$ echo "Foo" | awk '{ printf "%s\n" $1 }'
awk: (FILENAME=- FNR=1) fatal: not enough arguments to satisfy format string
	`%s
Foo'
	 ^ ran out for this one

One fix is to use commas to separate the format from the arguments:

$ echo "Foo" | awk '{ printf "%s\n", $1 }'
Foo

Or, use printf the way that awk wants you to:

$ echo "Foo" | awk '{ printf $1 "\n" }'
Foo

But then you lose the ability to pad numbers, set field widths, etc. that printf has.

Differences from C, and portability considerations

• The a, A, e, E, f, F, g, and G conversions are supported by Bash, but not required by POSIX.

• There is no wide-character support (wprintf). For instance, if you use %c, you're actually asking for the first byte of the argument. Likewise, the maximum field width modifier (dot) in combination with %s goes by bytes, not characters. This limits some of printf's functionality to working with ascii only. ksh93's printf supports the L modifier with %s and %c (but so far not %S or %C) in order to treat precision as character width, not byte count. zsh appears to adjust itself dynamically based upon LANG and LC_CTYPE. If LC_CTYPE=C, zsh will throw "character not in range" errors, and otherwise supports wide characters automatically if a variable-width encoding is set for the current locale.

• Bash recognizes and skips over any characters present in the length modifiers specified by POSIX during format string parsing.

builtins/printf.def

#define LENMODS "hjlLtz"
...
/* skip possible format modifiers */
modstart = fmt;
while (*fmt && strchr (LENMODS, *fmt))
fmt++;

• mksh has no built-in printf by default (usually). There is an unsupported compile-time option to include a very poor, basically unusable implementation. For the most part you must rely upon the system's /usr/bin/printf or equivalent. The mksh maintainer recommends using print. The development version (post- R40f) adds a new parameter expansion in the form of ${name@Q}which fills the role of printf %q – expanding in a shell-escaped format.

• ksh93 optimizes builtins run from within a command substitution and which have no redirections to run in the shell's process. Therefore the printf -v functionality can be closely matched byvar=$(printf …) without a big performance hit.

    # Illustrates Bash-like behavior. Redefining printf is usually unnecessary / not recommended.
    function printf {
        case $1 in
            -v)
                shift
                nameref x=$1
                shift
                x=$(command printf "$@")
                ;;
            *)
                command printf "$@"
        esac
    }
    builtin cut
    print $$
    printf -v 'foo[2]' '%d\n' "$(cut -d ' ' -f 1 /proc/self/stat)"
    typeset -p foo
    # 22461
    # typeset -a foo=([2]=22461)

• The optional Bash loadable print may be useful for ksh compatibility and to overcome some of echo's portability pitfalls. Bash, ksh93, and zsh's print have an -f option which takes a printfformat string and applies it to the remaining arguments. Bash lists the synopsis as: print: print [-Rnprs] [-u unit] [-f format] [arguments]. However, only -Rrnfu are actually functional. Internally, -p is a noop (it doesn't tie in with Bash coprocs at all), and -s only sets a flag but has no effect. -Cev are unimplemented.

http://pubs.opengroup.org/onlinepubs/9699919799/utilities/printf.html

http://pubs.opengroup.org/onlinepubs/9699919799/functions/printf.html

http://wiki.bash-hackers.org/snipplets/print_horizontal_line

http://mywiki.wooledge.org/BashFAQ/018