bash-hackers-wiki/docs/commands/builtin/printf.md

# The printf command

!!! warning "FIXME"
    This is a very big topic that needs experience - please fill in
    missing information, extend the descriptions, and correct the details
    if you can!

!!! info "Bash-Builtin"
    This is about the Bash-builtin command `printf` - however, the
    description should be nearly identical for an external command that
    follows POSIX(r).

    [GNU Awk](http://www.gnu.org/software/gawk/manual/gawk.html#Printf)
    expects a comma after the format string and between each of
    the arguments of a **printf** command. For examples, see: [code
    snippet](#differences-from-awk-printf).

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 conflicting historical implementations of the `echo` command,
POSIX(r) recommends that `printf` is preferred over `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(r) 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" "$SURNAME" "$FIRSTNAME"

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

| Option | Description |
|--------|-------------|
|`-v VAR`|If 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.

!!! danger "Danger"
    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](../../commands/builtin/read.md), and a similar one to
    [mapfile](../../commands/builtin/mapfile.md), 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`:

|  Number-Format |  Description |
|----------------|--------------|
|`N`|A normal decimal number|
|`0N`|An octal number|
|`0xN`|A hexadecimal number|
|`0XN`|A 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**|

<u>**If more arguments than format specifiers**</u> 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](../../syntax/expansion/wordsplit.md), as
accidentally passing the wrong number of arguments can produce wildly
different and unexpected results. See [this article](../../syntax/words.md).

!!! danger "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.

<u>**Again:**</u> 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 **char**: only the first character of a given argument is printed|
|`%s`|Interprets the associated argument literally as string|
|`%n`|Assigns the number of characters printed so far to the variable named in the corresponding argument. Can't specify an array index. If the given name is already an array, the value is assigned to the zeroth element.|
|`%a`|Interprets the associated argument as **double**, and prints it in the form of a C99 [hexadecimal floating-point literal](http://www.exploringbinary.com/hexadecimal-floating-point-constants/).|
|`%A`|Same as `%a`, but print it like `%E`|
|`%(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). If no corresponding argument is supplies, the current time is used as default|
|`%%`|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
introductory `%` 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 shorter, it's padded with spaces, if the text is longer, the field is expanded|
|`.`|**The dot**: Together with a field width, the field is **not** expanded when the text is longer, the text is truncated 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**)|
|`0`|Pads 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)|
|`'`|For decimal conversions, the thousands grouping separator is applied to the integer portion of the output according to the current LC_NUMERIC|

<u>**The "alternative format" modifier `#`:**</u>

|Alternative|Format|
|-----------|------|
|`%#o`|The octal number is printed with a leading zero, unless it's zero itself|
|`%#x`, `%#X`|The hex number is printed with a leading "`0x`"/"`0X`", unless it's zero|
|`%#g`, `%#G`|The 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`, `%X`|Always 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

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

|Code|Description|
|----|-----------|
|`\\`|Prints the character `\` (backslash)|
|`\a`|Prints the alert character (ASCII code 7 decimal)|
|`\b`|Prints a backspace|
|`\f`|Prints a form-feed|
|`\n`|Prints a newline|
|`\r`|Prints a carriage-return|
|`\t`|Prints a horizontal tabulator|
|`\v`|Prints 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**|

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

|Escape Rule|Description|
|-----------|-----------|
|`\c`|Terminate output similarly to the `\c` escape used by `echo -e`. printf produces no additional output after coming across a `\c` escape in a `%b` argument.|

-   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
[\$\'\...\'](../../syntax/quoting.md#ansi_c_like_strings) 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
    -   `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

```bash
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.

## differences from awk printf

Awk also derives its *printf()* function from C, and therefore has
similar format specifiers. However, in all versions of awk the space
character is used as a string concatenation operator, so it cannot be
used as an argument separator. **Arguments to awk printf must be
separated by commas.** Some versions of awk do not require printf
arguments to be surrounded by parentheses, but you should use them
anyway to provide portability.

In the following example, the two strings are concatenated by the
intervening space so that no argument remains to fill the format.


    $ 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

Simply replacing the space with a comma and adding parentheses yields
correct awk syntax.

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

With appropriate metacharacter escaping the bash printf can be called
from inside awk (as from perl and other languages that support shell
callout) as long as you don't care about program efficiency or
readability.

    echo "Foo" | awk '{ system( "printf \"%s\\n \" \"" $1 "\""  ) }'
    Foo

## 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.

``` c title="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 by
    `var=$(printf ...)` without a big performance hit.

```bash
# 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](../../commands/builtin/echo.md)'s portability pitfalls. Bash, ksh93,
    and zsh's `print` have an `-f` option which takes a `printf` format
    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.
-   Assigning to variables: The `printf -v` way is slightly different to
    the way using command-substitution. [Command
    substitution](../../syntax/expansion/cmdsubst.md) removes trailing newlines
    before substituting the text, `printf -v` preserves all output.

## See also

-   SUS: [printf
    utility](http://pubs.opengroup.org/onlinepubs/9699919799/utilities/printf.html)
    and [printf()
    function](http://pubs.opengroup.org/onlinepubs/9699919799/functions/printf.html)
-   [Code snip: Print a horizontal
    line](../../snipplets/print_horizontal_line.md) uses some `printf` examples
-   [Greg's BashFAQ 18: How can I use numbers with leading zeros in a
    loop, e.g., 01, 02?](http://mywiki.wooledge.org/BashFAQ/018)