184 lines
7.7 KiB
Markdown
Executable File
184 lines
7.7 KiB
Markdown
Executable File
# The `printf` command
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This command lets you print the value of a variable by formatting it using rules. It is pretty similar to the printf in C language.
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### Syntax:
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```
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$printf [-v variable_name] format [arguments]
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```
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### Options:
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| OPTION | Description |
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| --- | --- |
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| `FORMAT` | FORMAT controls the output, and defines the way that the ARGUMENTs will be expressed in the output |
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| `ARGUMENT` | An ARGUMENT will be inserted into the formatted output according to the definition of FORMAT |
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| `--help` | Display help and exit | |
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| `--version` | Output version information adn exit | |
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### Formats:
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The anatomy of the FORMAT string can be extracted into three different parts,
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- _ordinary characters_, which are copied exactly the same characters as were used originally to the output.
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- _interpreted character_ sequences, which are escaped with a backslash ("\\").
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- _conversion specifications_, this one will define the way the ARGUMENTs will be expressed as part of the output.
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You can see those parts in this example,
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```
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printf " %s is where over %d million developers shape \"the future of sofware.\" " Github 65
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```
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The output:
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```
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Github is where over 65 million developers shape "the future of sofware."
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```
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There are two conversion specifications `%s` and `%d`, and there are two escaped characters which are the opening and closing double-quotes wrapping the words of _the future of software_. Other than that are the ordinary characters.
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### Conversion Specifications:
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Each conversion specification begins with a `%` and ends with a `conversion character`. Between the `%` and the `conversion character` there may be, in order:
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| | |
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| --- | --- |
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| `-` | A minus sign. This tells printf to left-adjust the conversion of the argument |
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| _number_ | An integer that specifies field width; printf prints a conversion of ARGUMENT in a field at least number characters wide. If necessary it will be padded on the left (or right, if left-adjustment is called for) to make up the field width |
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| `.` | A period, which separates the field width from the precision |
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| _number_ | An integer, the precision, which specifies the maximum number of characters to be printed from a string, or the number of digits after the decimal point of a floating-point value, or the minimum number of digits for an integer |
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| `h` or `l` | These differentiate between a short and a long integer, respectively, and are generally only needed for computer programming |
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The conversion characters tell `printf` what kind of argument to print out, are as follows:
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| Conversion char | Argument type |
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| --- | --- |
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| `s` | A string |
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| `c` | An integer, expressed as a character corresponds ASCII code |
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| `d, i` | An integer as a decimal number |
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| `o` | An integer as an unsigned octal number |
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| `x, X` | An integer as an unsigned hexadecimal number |
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| `u` | An integer as an unsigned decimal number |
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| `f` | A floating-point number with a default precision of 6 |
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| `e, E` | A floating-point number in scientific notation |
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| `p` | A memory address pointer |
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| `%` | No conversion |
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Here is the list of some examples of the `printf` output the ARGUMENT. we can put any word but in this one we put a 'linuxcommand` word and enclosed it with quotes so we can see easier the position related to the whitespaces.
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| FORMAT string | ARGUMENT string | Output string |
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| --- | --- | --- |
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| `"%s"` | `"linuxcommand"` | "linuxcommand" |
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| `"%5s"` | `"linuxcommand"` | "linuxcommand" |
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| `"%.5s"` | `"linuxcommand"` | "linux" |
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| `"%-8s"` | `"linuxcommand"` | "linuxcommand" |
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| `"%-15s"` | `"linuxcommand"` | "linuxcommand " |
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| `"%12.5s"` | `"linuxcommand"` | " linux" |
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| `"%-12.5"` | `"linuxcommand"` | "linux " |
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| `"%-12.4"` | `"linuxcommand"` | "linu " |
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Notes:
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- `printf` requires the number of conversion strings to match the number of ARGUMENTs
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- `printf` maps the conversion strings one-to-one, and expects to find exactly one ARGUMENT for each conversion string
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- Conversion strings are always interpreted from left to right.
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Here's the example:
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The input
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```
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printf "We know %f is %s %d" 12.07 "larger than" 12
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```
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The output:
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```
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We know 12.070000 is larger than 12
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```
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The example above shows 3 arguments, _12.07_, _larger than_, and _12_. Each of them interpreted from left to right one-to-one with the given 3 conversion strings (`%f`, `%d`, `%s`).
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Character sequences which are interpreted as special characters by `printf`:
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| Escaped char | Description |
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| --- | --- |
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| `\a` | issues an alert (plays a bell). Usually ASCII BEL characters |
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| `\b` | prints a backspace |
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| `\c` | instructs `printf` to produce no further output |
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| `\e` | prints an escape character (ASCII code 27) |
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| `\f` | prints a form feed |
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| `\n` | prints a newline |
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| `\r` | prints a carriage return |
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| `\t` | prints a horizontal tab |
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| `\v` | prints a vertical tab |
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| `\"` | prints a double-quote (") |
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| `\\` | prints a backslash (\) |
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| `\NNN` | prints a byte with octal value `NNN` (1 to 3 digits)
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| `\xHH` | prints a byte with hexadecimal value `HH` (1 to 2 digits)
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| `\uHHHH`| prints the unicode character with hexadecimal value `HHHH` (4 digits) |
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| `\UHHHHHHHH` | prints the unicode character with hexadecimal value `HHHHHHHH` (8 digits) |
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| `%b` | prints ARGUMENT as a string with "\\" escapes interpreted as listed above, with the exception that octal escapes take the form `\0` or `\0NN` |
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### Examples:
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The format specifiers usually used with printf are stated in the examples below:
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- %s
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```
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$printf "%s\n" "Printf command documentation!"
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```
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This will print `Printf command documentation!` in the shell.
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### Other important attributes of printf command:
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- `%b` - Prints arguments by expanding backslash escape sequences.
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- `%q` - Prints arguments in a shell-quoted format which is reusable as input.
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- `%d` , `%i` - Prints arguments in the format of signed decimal integers.
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- `%u` - Prints arguments in the format of unsigned decimal integers.
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- `%o` - Prints arguments in the format of unsigned octal(base 8) integers.
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- `%x`, `%X` - Prints arguments in the format of unsigned hexadecimal(base 16) integers. %x prints lower-case letters and %X prints upper-case letters.
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- `%e`, `%E` - Prints arguments in the format of floating-point numbers in exponential notation. %e prints lower-case letters and %E prints upper-case.
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- `%a`, `%A` - Prints arguments in the format of floating-point numbers in hexadecimal(base 16) fractional notation. %a prints lower-case letters and %A prints upper-case.
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- `%g`, `%G` - Prints arguments in the format of floating-point numbers in normal or exponential notation, whichever is more appropriate for the given value and precision. %g prints lower-case letters and %G prints upper-case.
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- `%c` - Prints arguments as single characters.
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- `%f` - Prints arguments as floating-point numbers.
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- `%s` - Prints arguments as strings.
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- `%%` - Prints a "%" symbol.
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#### More Examples:
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The input:
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```
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printf 'Hello\nyoung\nman!'
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```
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The output:
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```
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hello
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young
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man!
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```
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The two `\n` break the sentence into 3 parts of words.
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The input:
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```
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printf "%f\n" 2.5 5.75
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```
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The output
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```
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2.500000
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5.750000
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```
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The `%f` specifier combined with the `\n` interpreted the two arguments in the form of floating point in the seperated new lines. |