Awk

This page compares BusyBox's implementation of awk (using this config file) with gawk (versions >= 3.1.8) and FreeBSD 9's nawk, which is based on Bell Labs/Brian Kernighan's 2007 version of awk.

It's not intended as a tutorial, but rather as a technical summary and list of "gotchas" (places where different implementations may behave in different or unexpected ways).

Invoking awk

awk [-F char_or_regex] [-v var=value ...] [--] 'awk script...' [ ARGV[1] ... ARGV[ARGC-1] ]
awk [-F char_or_regex] [-v var=value ...] -f scriptfile ... [--] [ ARGV[1] ... ARGV[ARGC-1] ]

Gawk makes a third invocation pattern possible, which mixes -f scriptfile options with an 'awk script...' on the command-line:

gawk [-F char_or_regex] [-v var=value ...] -f scriptfile ... -e 'awk script...' [--] [ ARGV[1] ... ARGV[ARGC-1] ]

Awkenough is a small set of Awk utility routines and a C stub that makes it easier to write shell scripts with awk shebang lines. (I'll make an Alpine package for this soon.) It also permits mixing -f scriptfile and command-line scripts, using the same -e option gawk provides. In both cases, the long-form option --source works the same as -e.

Notes

• all implementations honor \t and so on in -v FS=expr. BusyBox doesn't honor \t in -F expr; perhaps this is a bug. You can use the $'\t' escapes of BusyBox ash to work around this.

• nawk and gawk --traditional (but not gawk --posix) interpret -F t as -F '\t'. This is a weird special-case preserved for historical compatibility.

• scriptfile can be - for stdin

• ARGVs can be of any form, but awk only knows how to automatically process arguments whose form is:
  • var=unquoted_value: assignments will be made when the main loop reaches that ARGV
  • "": will be skipped
  • filename
  • -: will use stdin

If no files are processed (nor -), stdin will be processed after all command-line assignments.

• standalone scripts should look like this:

Contents of foo.awk

#!/usr/bin/awk -f # will receive the script's ARGC, ARGV with ARGV[0]=awk BEGIN { ... } ...

or like this:

Contents of bar.awk

#!/usr/bin/runawk -f /usr/share/awkenough/library.awk -f ...

• gawk places any unrecognized short options that precede -- into ARGV; other implementations (and gawk --traditional) fail/warn

• some gawk-only options:
  • --lint: warns about nonportable constructs
  • --re-interval: enables /pat{1,3}/ regexes; some versions of gawk enable by default, others don't
  • --traditional or --compat: makes gawk behave like nawk
  • --posix: like --traditional --re-interval, with some further restrictions
  • --exec scriptfile: stops option processing and disables any further -vs; meant to be used in shebang line for scripts in untrusted environments (cgi)
  • --sandbox: disable system("foo"), "foo"|getline, getline <"file", print|"foo", print >"file", and gawk's extension(obj, func); so only local resources explicitly specified in ARGV will be accessible. This feature is only available on recent versions of gawk.

• when handling -f path options where path contains no /s, gawk will first search in directories specified in an AWKPATH environment variable

Awk grammar

• Lines can be broken after any of: \ , { && || ?

(The last two only in BusyBox and gawk, and not in gawk --posix.)

Blocks

function definitions

• Functions have global scope, and their invocations can syntactically precede their definitions.

• Scalars (that is, non-arrays) are passed by value, arrays are passed by reference.

• Function calls can be nested or recursive; upon return, values of the scalar parameters the caller supplied shall be unchanged (array parameters may have been mutated). So each invocation of the function has its own local environment.

• Consider functions defined like this:

  function foo(x,y) {
      ...
      bar()
      return "result"
  }
  
  function bar(z) {
      y += 1
      return
  }
  

  Here the variables x and y are local to foo, and the variable z is local to bar. All other variable references are global: contrast the shell, where non-local variable references are instead dynamic. That is, in the shell, when foo calls bar, bar will be incrementing foo's local variable y. In awk, on the other hand, bar always only increments a global variable y.

  Note also that the number of arguments in a function call needn't match the number of parameters in the function's definition; if more arguments are supplied, they will be ignored; if fewer are supplied (as in foo's invocation of bar), the missing variables are assigned either "" or an empty array, depending on how they're used in the function.

  Note also the the return statement needn't be given an explicit return value (it will default to ""). The return statement can also be omitted altogether.

BEGIN and END

BEGIN { ... } ...
END { ... } ...

• Multiple BEGIN blocks are merged; so too multiple END blocks.

• If there are only BEGIN blocks (no main loop or END blocks), and getline isn't used, then POSIX requires awk to terminate without reading stdin or any file operands. The implementations I checked honor this, but some historical implementations discarded input in this case. For portability, you can explicitly append < /dev/null to an invocation of awk you know shouldn't consume stdin.

• If there are END block(s), and awk's processing of stdin hasn't been suppressed by encountering files in its ARGV list, then stdin will be processed before executing the END block(s).

• If there are END block(s), but exit code is invoked outside them, then the main loop stops and control passes immediately to the first END block. If exit code is invoked within an END block, or if there are no end blocks, the script terminates with result code.

• POSIX requires that inside END, all of FILENAME NR FNR NF retain their most-recent values, so long as getline is not invoked. My implementations comply, and also retain the most-recent fields $0 ..., but earlier versions of nawk did not; and Darwin's awk may not.

main loop blocks

These are of the form:

pattern         { action ... }

or:

pattern,pattern { action ... }

where action defaults to print $0; and pattern defaults to 1 (that is, to true).

• Patterns can take any of the forms:
  • /regex/
  • relational
  • pattern && pattern
  • pattern || pattern
  • ! pattern
  • ( pattern )
  • pattern ? pattern : pattern
  The first form is interpreted the same as the relational $0 ~ /regex/.
• Relationals can take any of the forms:
  • expr ~ RE
  • expr !~ RE
  • expr eqop expr
  • expr in array name
  • (expr,...) in array name
  • expr
  The schemas RE can be regex literals like /a[bc]+/ or any expression that evaluates to a string, like "a[bc]+". Note that strings require an extra level of \s: you can write /\w+/ or "\\w+". The scemas eqop can be any of: == != <= < > >=. A bare expression, as in the last form, is interpreted as false when the expression evaluates to 0 or "", else to true.

other block forms

BEGINFILE { ... }
ENDFILE { ... }

These are only available in (recent versions of) gawk.

@include "filename"

This is only available in some versions of gawk. See also Aleksey Cheusov's Runawk, which uses a different approach.

Statements

func(expr,...) # evaluates to a value but can also occur in statement contexts # when calling user-defined functions, no space can separate `func` and `(` lvalue = expr # evaluates to the value of expr but can also occur in statement contexts # lvalue is `var`, `array[k]`, or `$numeric` also: lvalue += 1 also: lvalue ++ expr # nawk doesn't permit in statement contexts break continue # historic implementations interpret break/continue outside of a while, do, or for-loop as `next` # BusyBox supports this usage; as do some versions of gawk --traditional return [expr=""] # POSIX doesn't specify behavior outside of a function definition next # read next line of input, restart from first rule BEGIN {...} NR==1 { ...; next } # get next record and start over {...} # POSIX doesn't specify behavior in BEGIN or END blocks nextfile # not in gawk --traditional exit [status=0] # will execute END actions, or exit immediately if invoked inside END in gawk and nawk, "... exit n ... END { exit; }" will preserve status=n in BusyBox awk, will revert to 0 delete array[k] delete array # not in gawk --traditional; more portably: split("",array) # even though it no longer has any elements, you cannot reuse the array name as a scalar variable if (test) action [; else if (test2) action] [; else action] if (test) { ... } [else action] while (test) action do action while (test) # will execute action at least once for (i=1; i<=NF; i++) action for (k in array) action # iteration order is unspecified; effect of adding new elements to array while iterating is undefined # parens optional for print, printf; should be used if any of the arguments contains `>` # arg arg arg : concatenated # arg,arg,arg : separated by OFS print # prints $0 print "" # prints newline print [(] expr,... [)] # statement, though it accepts (args,...) print [(] expr,... [)] > "file" print [(] expr,... [)] >> "file" print [(] expr,... [)] | "pipe command" Some awks (not BusyBox permit `for(;;print) ...` printf [(] "format", expr... [)] [... as for print ...] # statement, though it accepts (args,...)  %[n$][#][-|0][+| ]['][minwidth][.precision][wordsize]f `n$` use argv[n] instead of next, 1-based: only gawk `#` forces octals to have initial 0, hex to have initial 0x, floats must have . `-|0` means align to left, or right-align padding with zeros instead of spaces `+| ` means force sign or leading space `'` means 1,000,000: no awks width and precision can be *, supplied by argv: only gawk and nawk precision defaults to 0, gives post-. digits or maxwidth for string wordsize = hh/char h/short l ll: only nawk honors these format specifier: signed: feg d/i unsigned: xuo s c(on BusyBox, only 0-0x7fff ~> "\xff") not in awks: b(like s with \t in argv) p(pointer) n(save number chars so far in &argv) \b and \r: backspace (to margin), 0x8 and 0xd \v \f: next line same column, 0xb and 0xc \n \t \' \\ \a: alarm 0x7 \c: ignore rest of string: no awks \nnn: up to three octal digits only gawk will printf or sprintf "\0", others terminate gawk's printf and sprintf and nawk's printf also work with "%c", 0

Expressions

/pat/ # when this is a complete expr, interpreted as `$0 ~ /pat/` Differences from other EREs: {m,n} treated as literal by nawk and gawk --traditional In some versions of gawk --traditional, [[:classes:]] not available. dot and [^x] do match newline ^ $ always anchors and only match against start-of-buffer and end-of-buffer (like \` and \') \1 is \x01; no backrefs available in match patterns In gawk and nawk, [a\]1] matches a,],1. In BusyBox, it matches a,\ followed by literal 1 then ]. In BusyBox and gawk, /ab\52c/ is interpreted as /ab*c/ not /ab\*c/. In nawk and gawk --traditional, it's interpreted as latter. Literal newlines not allowed in /pat/, but ok in strings interpreted as patterns. /pat\nmore/ and "pat\\nmore" are both ok. x=$1 $2 # string concatenation $one $(one+two) # any numeric-valued expression can follow $ x=1,2 # assigns 1 to x x=(1,2) # assigns 1 SUBSEP 2 to x == != <= < > >= ~ !~ && has higher precedence than || !(k in A) test ? expr : expr in BusyBox and gawk, 0[0-7]+ is interpreted as octal and 0x[[:xdigit:]]+ is interpreted as hex + - * / % ^ (often, as in BusyBox, ** also does exponentiation) ++ -- lvalue | lvalue ++ -- lvalue += ... %= ^= value C escapes in "string" and /pat/: "\OOO" up to 3 octal digits "\\ \" \n \t \r \f \v \b \a" "\xff" # not in gawk --posix; BusyBox restricts to two digits unspecified what is behavior of \m for other m; my awks use literal m (gawk gives warning) only gawk suppresses \<newlines> inside string literals The expression `k in array` doesn't create an array entry, but the reference `array[k]` will create an entry with an uninitialized value. (`k in array` will then be true.) References to fields > NF, on the other hand, don't create new fields. Assignments to these fields do create them (increasing NF and setting any intermediate fields to uninitialized values). Assignments to any field (even <= NF) cause $0 to be recomputed using OFS, but do not cause $0 to be reparsed. (So the modified/new field may contain embedded FS characters.) Assignments to $0 do cause NF, $1, ... to be recomputed.

String vs numeric

Uninitialized values include unset variables and array elements, invalid fields (> NF) or valid fields of length 0, and unassigned function parms (which can be used as scalars or arrays). Unititalized scalars have value "", which math operators treat as 0. Can force string interpretaton by `var ""`, or force numeric interpretation by `var + 0`. "1text" is coerced as 1. In boolean contexts: false when expr evaluates to 0 or "", else true. $(expr) is not reqired to convert uninitialized or string expr, but my implementations do so. "Numeric string" := value is of the form / *[+-]?NUMBER */, and it was supplied in a way that doesn't distinguish strings from numbers: n=$1 or split("string",n[]) or getline n n=FILENAME or ARGV[1] or ENVIRON["foo"] awk -v 'n=...' ... Comparisons are numeric iff: both operands are numeric, or one is numeric and other is a "numeric string" or is uninitialized my implementations don't require that at least one operand be numeric Otherwise (one of the operands is an explicit "string"), comparison is stringwise. awk 'BEGIN {print "00" == 0}' # false, first operand is not a "numeric string" awk -v'x=00' -v'n=000' 'BEGIN { y="0000" print(x=="0", x==0, x==u, x==n, y=="0", y==0, y==u, y==n) # only second, third and fourth will be true, all other comparisons are stringwise x+y print(x=="0", x==0, x==u, x==n, y=="0", y==0, y==u, y==n) # in nawk, the previous arithmetic reference to y converts its value to a "numeric string" too # so now the last three comparisons are also true # even numeric constants are susceptible to this side-effect # (print the constant and it's converted to a numeric string, and ignores further changes to OFMT) # however, stringwise evaluation of y still gives "0000" }' In POSIX and my other implementations, referencing doesn't have that side-effect.

Arrays

flavor[1]="cherry" ... print flavor[1] for (k in flavor) action nfields = split(string, larray, [sep=FS]) # sep can be single char or regex delete array[key] # BusyBox also honors "delete array" multiarray[k1, k2]="foo" (k1,k2) in multiarray for (kk in multiarray) { split(kk,k,SUBSEP) ... } # now k[1],k[2]

Functions

cos(x), sin(x), atan2(x,y), exp(x), log(x), sqrt(x) int(x) # just truncates towards 0, use `printf "%.0f" ...` to round 0 <= rand() < 1 # 1 + int(rand() * TOPNUM) # returns number in range [1..TOPNUM] srand(seed=time_of_day) ~~> oldseed # bit arith are only gawk and BusyBox and(x,y) or(x,y) xor(x,y) lshift(x,count) rshift(x,count) compl(x) # 32-bit base 0xffffffff on BusyBox, may be 53-bit on x86_64 gawk # time functions are only gawk and BusyBox systime() # seconds since epoch strftime("format",systime()=current) mktime("YYYY MM DD HH MM SS [dst?=-1]") ~~> seconds since epoch # HH etc values can be non-standard, e.g. -1 # uses local TZ; dst? 1 yes, 0 no, <0 auto tolower("string"), toupper("string") sprintf("format", expr...) # does require parens # gawk is multibyte-aware, so length, substr, index, and match all count by chars, not bytes length(string=$0) # nawk and gawk support length without () # nawk and gawk (but not gawk --posix) support length(array) substr("string", 1-based-start, maxlength=until_end) ~~> "substring" index("haystack","needle") ~~> 1-based, or 0 if fails match(string, /pat/) ~~> 1-based starting index, or 0 if fails # sets RSTART and RLENGTH (when match fails, they are 0 and -1) # ... match($0, /pat/) { print substr($0, RSTART, RLENGTH) } ... # prints first occurrence of /pat/ gawk has `match(string,/pat/,groups)`: fills groups[0,0start,0length,1,1start,1length,...] with text and positions of \\0, \\1, ... split("string", larray, sep=FS) ~~> nitems # splits into larray[1]...larray[nitems] # some versions of gawk permit fourth argument: lsep, to receive actual FS values (lsep[0] [1] lsep[1] ... [nitems] lsep[nitems]) # BusyBox and gawk accept "" and // as sep; with // BusyBox makes an extra empty field at end sub(/pat/, "subst", lstring=$0) ~~> 1 if successful, 0 if failed gsub(/pat/, "subst", lstring=$0) ~~> nsubs gensub(/pat/, "subst", "g" or which match, string=$0) ~~> newstring # gawk and BusyBox only # string is not mutated # "subst" honors not only & (same as \\0), but also \\1, \\2 ... system("command") ~~> exit code, output not captured # getline is a function though it doesn't use parens # all forms read one line each time they're evaluated; like "next" without restarting from first rule # returns 1 on success, 0 on EOF, -1 on error; file is not closed automatically # updates NF when assigning to $0 getline [var] # updates NR and NFR when file not supplied getline [var] < "-" # read from stdin instead of ARGV[ARGIND] # loop until we get a name while (!name) { printf("Enter a name? "); getline name < "-" } getline [var] < "file" "pipe command" | getline [var] close("file" or "pipe commnd") ~~> 0 if successful # awk doesn't automatically close files, pipes, sockets, or co-processes when they return EOF. # the return value of close() is unspecified; gawk uses value from fclose(3) or pclose(3), or -1 if the named file, piple, or co-process was not opened with a redirection. { print ... | "sort>tmpfile" } END { close("sort>tmpfile"); while ((getline < "tmpfile")>0) ... close("tmpfile") } fflush(output "file" or "pipe command") ~~> 0 if all requested buffers successfully flushed, else -1 # not in gawk --posix # if no argument is supplied, flushes stdout # if "" is supplied, flushes all open output files and pipes

Special filenames

# only in gawk, though all but /dev/fd/n (n>2) will generally still work "/dev/tty" "/dev/stdin", can also use: "-" "/dev/stdout" "/dev/stderr", can also use: | "cat 1>&2" "/dev/fd/n"

System variables

ENVIRON associative array of environment variables array is mutable, but changes don't affect environment of child processes (POSIX leaves implementation-dependent) ARGC Number of arguments on command line ARGIND 0 when processing stdin (and ARGC=1), 1 when processing ARGV[1], ... only in gawk and BusyBox; in BusyBox, mutating this affects which file is read next ARGV An array from 0...ARGC-1 containing the command-line arguments Entries of form "var=value" handled specially, entries of form "" skipped Entries that are directories: implementations and versions differ in whether they'll warn/error (gawk --traditional will error) You can: decrement ARGC or set ARGV[...]="" or delete ARGV[...] to suppress argument processing You can: append to ARGV and increment ARGC, then argument will be processed later ERRNO In gawk and BusyBox, contains string error message after getline or close fails FILENAME Current filename NR Number of current record NR is mutable but changing it has no side-effects. FNR Like NR, but relative to the current file RS Record separator (defaults to newline) If RS is "", it's interpreted as a span of empty lines, and FS will then always implicitly include "\n". nawk only honors first character of RS BusyBox and gawk permit RS to be a regex, and set RT to the actual terminating text (will be "" at EOF): awk -v 'RS=[\t\n]' 'BEGIN {ORS=""} {print $0 RT} END {print "\n"}' awk -v 'RS=oldpat' -v 'ORS=newtext' '{ if (RT=="") printf "%s", $0 # EOF else print }' # print text between matches gawk --traditional still honors regex RS, but doesn't set RT BusyBox permits FS to have 0-length matches: echo aaba|awk -F 'b*' '{print NF}' # yields 5 ORS Output record separator (defaults to newline) NF Number of fields in current record gawk and BusyBox recalculate $0 when NF is either increased or decreased (in latter case, fields are discarded); nawk does neither. FS Field separator (defaults to a space) POSIX requires that assigning a new value to FS has no effect on the current input line; it only affects the next input line. Only gawk conforms; BusyBox and FreeBSD awk will also use the new FS for current line if no fields have yet been referenced. FS=space (default): strips leading and trailing space/tabs, fields are separated by spans of space/tabs/newlines (in gawk --posix, only spans of spaces/tabs) FS=":" or "\t": each occurrence of the char separates another field FS="pat": separator is leftmost longest non-null and non-overlapping match of pattern FS="" (gawk and BusyBox only): do character-wise splitting They also accept "" and // as args to split (latter can't be assigned as value to FS) When splitting on //, BusyBox generates an additional empty field All my gawks permit sub(//,...) and gsub(//,...): matches each inter-character position, starting before first char OFS Output field separator (defaults to a space) SUBSEP Separator character for array subscripts (\034) OFMT Output format for numbers (%.6g) CONVFMT String conversion format for numbers (%.6g). used for comparisons and array indexing, defaults to whatever value OFMT has integer strings are always converted using "%d" results are unspecified if CONVFMT isn't a floating-point format spec RSTART First position in the string matched by match() function RLENGTH Length of the string matched by match() function IGNORECASE=1 in gawk and BusyBox only Affects all pattern-matching and string comparison/indexing; doesn't affect array indexing

gawk has length(A), returns count of keys rather than maxkey

Gawk-only extensions

Some versions of gawk have: switch (expression) { case value|regex : statement ... [ default: statement ] } Only gawk supports FIELDWIDTHS="2 4 2" # use FS=FS to revert to regular field-splitting Only some versions of gawk support FPAT="a pat that every field satisfies" # use FS=FS to revert to regular field-splitting also patsplit("string",larray,[pat=FPAT,lsep]) Only gawk has PROCINFO array, with fields: version:3.1.8 pid:17171 ppid:17139 pgrpid:17171 uid:0 euid:0 gid:0 egid:0 group1:0 # group1..groupn are supplementary groups FS:FS or FIELDWIDTHS or FPAT [strftime] [sorted_in] Only some versions of gawk permit calling functions indirectly: ...funcname="foo"; @funcname(1,2)... Only gawk has: asort(a, [a.backup]) ~~> nelems, string "10" sorts lexicographically but numeric 10 sorts arithmetically asorti(a, [a.backup]) ~~> nelems, this discards original values and sorts the original keys lexicographically strtonum("string") # more portably, use "string"+0 isarray(a) in gawk4 command-or-socket |& getline # co-process print ... |& command-or-socket The following special filenames may be used with the |& co-process operator for creating TCP/IP network connections: /inet{,4,6}/tcp/lport/rhost/rport Files for a TCP/IP connection on local port lport to remote host rhost on remote port rport. Use a port of 0 to have the system pick a port. Use /inet4 to force an IPv4 connection, and /inet6 to force an IPv6 connection. Plain /inet uses the system default (most likely IPv4). /inet{,4,6}/udp/lport/rhost/rport Similar, but use UDP/IP instead of TCP/IP. optional second argument to the close() function. ability to use positional specifiers with printf and sprintf(). Localizable strings. The bindtextdomain(), dcgettext(), dcngettext() functions. BINMODE, LINT, TEXTDOMAIN adding new built-in functions dynamically with the extension() function.

Useful links

These links may also be of interest:

• http://www.pement.org/awk/awk1line.txt
• http://www.catonmat.net/series/awk-one-liners-explained
• http://awk.freeshell.org/HomePage
• http://awk.freeshell.org/AwkFeatureComparison

Awk and Lua

See summary of Lua regex.

Commands in brown are from awkenough.

in lua	in awk
string.find(str, pattern, [startpos]) --> (start,stop) or nil	match(str, pat) --> returns and sets RSTART, also sets RLENGTH index(str, needle) --> start or 0 nthindex(str, needle, [nth=1, permits -1]) --> start or 0
string.match(str, pattern, [start=1]) --> (%0) or nil or (%1,%2,...)	matchstr(str, pat, [nth=1]) --> \\0, sets RSTART and RLENGTH gawk's match only matches nth=1; it returns RSTART and fills a third array argument with 0,0start,0length,1,1start...
string.gmatch(str, pattern) --> an iterator over all matches or sets of matching groups The iteration sequence will look like: (%0), (%0), (%0) ...; or like: (%1,%2...), (%1,%2...),... With this function, one can't specify the position to start matching.	gmatch(str, pat, MATCHES, STARTS) --> nmatches
string.gsub(str, pattern, replacement, [max#repls]) --> (newstr,nrepls)	gensub(pat, repl, nth/"g", str): is closest to Lua's gsub sub(pat, repl, str): mutate first match in str gsub(pat, repl, str): mutate all matches in str
string:len	length(str)
string:lower string:upper	tolower toupper
string.rep(str, count,[5.2 adds sep])	rep(str,count,[sep])
string.sub(str, start,[stop])	substr(str,start,[len]) has_prefix has_suffix
	split(str,ITEMS,[seppat],[gawk's SEPS]) --> nitems gsplit(str,ITEMS,[seppat],[SEPS]) --> nitems asplit(str, PAIRS, ["="], [" "]) --> nitems
table.concat(tbl,[sep],[start],[stop]) --> string	concat([start=1], [len=to_end], [fs=OFS], [A]) --> string if you want to preserve existing FS, need to use gsplit to concat an array without specifying len: concat(start, uninitialized, OFS, A)
string:reverse	reverse([A])
table.remove(tbl, [pos=from end]) --> value formerly at tbl[pos]	pop([start=from_end], [len=to_end], [A]) --> values sep by SUBSEP
table.insert(tbl,[valpos=insert at end],value) --> nil	insert(value, [start=after_end], [A]) --> new length of array extend(VALS, [start=after_end], [A]) --> new length of array
table.sort(tbl, [lessthan])	sort(A) hsort(A) qsort(A, 1, length(A))
	isempty(A) has_value(A, value)
	includes(A, B, [onlykeys?]): is B <= A? union(A, B, [conflicts]) --> mutates A intersect(A, B, [conflicts]) --> mutates A subtract(A, B, [conflicts]) --> mutates A