commands:builtin:printf

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commands:builtin:printf [2012/06/08 09:39]
ormaaj Add the single quote modifier. And some cleanup.
commands:builtin:printf [2016/11/30 15:39] (current)
medievalist [differences from awk printf]
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 Thus, a typical ''​printf''​-call looks like: Thus, a typical ''​printf''​-call looks like:
 <​code>​ <​code>​
-printf "​Surname:​ %s\nName: %s\n" "​$SURNAME"​ "$LASTNAME"+printf "​Surname:​ %s\nName: %s\n" "​$SURNAME"​ "$FIRSTNAME"
 </​code>​ </​code>​
 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!). 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!).
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 ==== Format strings ==== ==== Format strings ====
  
-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.+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 ''<​nowiki>​%%</​nowiki>''​ in the format string. To print a literal ''​%''​ (percent-sign),​ use ''<​nowiki>​%%</​nowiki>''​ in the format string.
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 |''​%g''​|Interprets the associated argument as **double**, but prints it like ''​%f''​ or ''​%e''​| |''​%g''​|Interprets the associated argument as **double**, but prints it like ''​%f''​ or ''​%e''​|
 |''​%G''​|Same as ''​%g'',​ but print it like ''​%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|+|''​%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| |''​%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.| |''​%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.|
-|''​%(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)|+|''​%a''​|Interprets the associated argument as **double**, and prints it in the form of a C99 [[http://​www.exploringbinary.com/​hexadecimal-floating-point-constants/​ | hexadecimal floating-point literal]].| 
 +|''​%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)| |''​%%''​|No conversion is done. Produces a ''​%''​ (percent sign)|
  
 Some of the mentioned format specifiers can modify their behaviour by getting a format modifier: Some of the mentioned format specifiers can modify their behaviour by getting a format modifier:
- 
 ==== Modifiers ==== ==== Modifiers ====
  
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 ==== Escape codes ==== ==== Escape codes ====
 +
 +These are interpreted if used anywhere in the format string, or in an argument corresponding to a ''​%b''​ format.
  
 ^Code^Description^ ^Code^Description^
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 |''​\0<​NNN>''​|same as ''​\<​NNN>''​| |''​\0<​NNN>''​|same as ''​\<​NNN>''​|
 |''​\x<​NNN>''​|Interprets ''<​NNN>''​ as **hexadecimal** number and prints the corresponding character from the character set (**3 digits**)| |''​\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<​NNNN>''​|same as ''​\x<​NNN>'',​ but **4 digits**| 
-|''​\U<​NNNNNNNN>''​|same as ''​\x<​NNN>'',​ but **8 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: 
 + 
 +|''​\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: ''​\<​nowiki>'</​nowiki>'',​ ''​\"'',​ 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#​ansi_c_like_strings ​$'​...'​]] style quoting.
  
 ===== Examples ===== ===== Examples =====
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 Please read the manpage of ''​strftime(3)''​ to get more information about the supported formats. Please read the manpage of ''​strftime(3)''​ to get more information about the supported formats.
  
-==== Using printf inside of awk ====+===== differences from awk printf =====
  
-Here'​s ​the gotcha:+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.
  
 <​code>​ <​code>​
- 
-$ printf "​%s\n"​ "​Foo"​ 
-Foo 
  
 $ echo "​Foo"​ | awk '{ printf "​%s\n"​ $1 }' $ echo "​Foo"​ | awk '{ printf "​%s\n"​ $1 }'
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 </​code>​ </​code>​
  
-One fix is to use commas to separate ​the format from the arguments:+Simply replacing ​the space with a comma and adding parentheses yields correct awk syntax.
  
 <​code>​ <​code>​
-$ echo "​Foo"​ | awk '{ printf "​%s\n",​ $1 }'+$ echo "​Foo"​ | awk '{ printf"​%s\n",​ $1 }'
 Foo Foo
 </​code>​ </​code>​
  
-Or, use **printf** ​the way that awk wants you to:+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.
  
 <​code>​ <​code>​
-echo "​Foo"​ | awk '{ printf $1 "\n" }'+echo "​Foo"​ | awk '​{ ​system( "printf ​\"​%s\\n \" \"" ​$1 "​\"​" ​ ) }'
 Foo Foo
 </​code>​ </​code>​
  
-But then you lose the ability to pad numbersset field widthsetcthat **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. Likewisethe maximum ​field width modifier (dot) in combination with ''​%s''​ goes by bytesnot charactersThis 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. 
 +<code c|builtins/​printf.def>​ 
 +#define LENMODS "​hjlLtz"​ 
 +... 
 +/skip possible format modifiers */ 
 +modstart = fmt; 
 +while (*fmt && strchr (LENMODS, *fmt)) 
 +fmt++; 
 +</​code>​ 
 + 
 +  * 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. 
 +<​code>​ 
 +# 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) 
 +</​code>​ 
 + 
 +  * The optional Bash loadable ''​print''​ may be useful for ksh compatibility and to overcome some of [[commands/​builtin/​echo | echo]]'​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. [[syntax:​expansion:​cmdsubst | Command substitution]] removes trailing newlines before substituting the text, ''​printf -v''​ preserves all output.
  
 ===== See also ===== ===== See also =====
  • commands/builtin/printf.1339148390.txt
  • Last modified: 2012/06/08 09:39
  • by ormaaj