.TH EXPR 1 2026-10-19 "w64devkit" "w64devkit Manual"
.SH NAME
expr \- evaluate an expression
.SH SYNOPSIS
.B expr
.IR arg ...
.SH DESCRIPTION
.B expr
evaluates an expression and writes its value and a newline to standard
output.
It reads no input.
The expression is the command-line arguments, and each operator,
operand, keyword, and parenthesis must be a separate argument:
.B expr 1+2
is the string
.BR 1+2 ,
but
.B expr 1 + 2
is 3.
Many operator characters are special to shells, so quote them; see
.BR NOTES .
.SS Values
A value is a string or a 64-bit signed integer.
Every argument is a string.
Arithmetic and comparisons use a string as an integer when the whole
string is one: optional leading white space, an optional
.B +
or
.BR \- ,
and decimal digits, with nothing after them.
A leading zero does not make a number octal, so
.B 010
counts as 10.
Hexadecimal and floating-point numbers are not integers.
A number outside the 64-bit range is replaced by the nearest limit,
and arithmetic that overflows wraps around without an error.
.PP
An integer is printed in decimal, and a string as it is, so
.B expr 007
prints
.BR 007 ,
and
.B expr 007 + 0
prints
.BR 7 .
.PP
A value is null if it is the empty string, the string
.BR 0 ,
or the integer 0.
Other strings, such as
.B 00
and
.BR \-0 ,
are not null.
.SS Operators
Operators are listed from the lowest precedence to the highest, and
operators of equal precedence group from left to right.
The comparison operators share a precedence, as do
.B +
and
.BR \- ,
and as do
.BR * ,
.BR / ,
and
.BR % .
No operator skips an operand: in
.BR "expr 1 \e| 1 / 0" ,
the division by zero is reported.
.TP
.IB arg1 " | " arg2
.I arg1
if it is not null, otherwise
.IR arg2 .
.TP
.IB arg1 " & " arg2
.I arg1
if neither argument is null, otherwise the integer 0.
.TP
.IB arg1 " < " arg2
1 if the relation holds, otherwise 0.
The comparison operators are
.BR < ,
.BR <= ,
.BR = ,
.BR == ,
.BR != ,
.BR >= ,
and
.BR > ,
and
.B =
is the same as
.BR == .
When both arguments are integers, they are compared as integers.
Otherwise both are compared as strings, byte by byte as unsigned values.
An integer is then converted back to decimal text first, so
.B 007
compares as
.BR 7 .
.TP
.IB arg1 " + " arg2
The sum.
Like the other arithmetic operators, it fails with the error
.B non\-numeric argument
unless both arguments are integers.
.TP
.IB arg1 " \- " arg2
The difference.
.TP
.IB arg1 " * " arg2
The product.
.TP
.IB arg1 " / " arg2
The quotient, rounded toward zero.
A zero divisor is the error
.BR "division by zero" .
.TP
.IB arg1 " % " arg2
The remainder, which has the sign of
.IR arg1 .
A zero divisor is the error
.BR "division by zero" .
.TP
.IB string " : " regexp
The result of matching
.I regexp
against the start of
.IR string ;
see
.BR "Pattern matching" .
.PP
The keywords below bind more tightly than any operator.
A keyword is recognized wherever an operand is expected.
Each takes the arguments after it as its operands, and an operand is a
word, a keyword with its own operands, or a parenthesized expression.
So
.B expr length abc + 1
is 4.
.TP
.BI "match " string " " regexp
The same as the
.B :
operator with these arguments.
.TP
.BI "substr " string " " pos " " length
The substring of
.I string
that starts at byte
.IR pos ,
counting from 1, and has at most
.I length
bytes.
It is empty, with no error, when
.I pos
or
.I length
is less than 1, when
.I pos
is beyond the end of
.IR string ,
or when either is not an integer.
.TP
.BI "index " string " " chars
The position, counting from 1, of the first byte of
.I string
that is any of the bytes of
.IR chars ,
or 0 if there is none.
.TP
.BI "length " string
The number of bytes in
.IR string .
.TP
.BI "quote " token
.I token
as a string, even if it is a keyword or a parenthesis.
Any other word is already a string where an operand is expected, even
one that looks like an operator, so
.B expr \(aq*\(aq
prints
.BR * .
.TP
.BI "( " expression " )"
The value of
.IR expression ,
which overrides precedence.
Each parenthesis must be an argument by itself, so the argument
.B (1)
is only a string.
A
.B )
where an operand is expected is a syntax error.
.SS Pattern matching
The
.B :
operator and
.B match
treat
.I regexp
as a POSIX basic regular expression, as implemented by the GNU regex
library that busybox carries.
It has intervals,
\fB\e{\fIm\fB,\fIn\fB\e}\fR,
back-references, and bracket expressions with character classes such as
.BR [[:digit:]] .
It also has the GNU extensions
.BR \e+ ,
.BR \e? ,
.BR \e| ,
.BR \ew ,
.BR \eW ,
.BR \es ,
.BR \eS ,
.BR \eb ,
.BR \eB ,
.BR \e< ,
.BR \e> ,
.BR \e\(ga ,
and
.BR \e\(aq ,
the last two anchoring the start and end of
.IR string .
The characters
.BR + ,
.BR ? ,
.BR | ,
.BR { ,
and
.B }
are ordinary unless escaped.
Matching is case sensitive, and
.B .\&
matches any byte, including a newline.
.PP
The match must begin at the first byte of
.IR string ,
but it need not reach the end, so end
.I regexp
with
.B $
to require that.
The value is:
.IP \(bu 2
With no match, the empty string if
.I regexp
has a group,
.BR \e( ...\e) ,
and otherwise the integer 0.
.IP \(bu 2
With a match and a group that took part in it, the text matched by the
first group, which may be empty.
.IP \(bu 2
Otherwise, the number of bytes matched, as an integer.
.PP
A
.I regexp
that begins with
.B ^
draws a warning on standard error:
.PP
.RS 4
.EX
expr: warning: \(aq^a\(aq: using \(aq^\(aq as the first character
of a basic regular expression is not portable; it is ignored
.EE
.RE
.PP
Despite the message, the
.B ^
is the usual start anchor and the evaluation continues.
Write a literal
.B ^
there as
.BR \e^ .
A
.I regexp
that does not compile is an error; see
.BR "EXIT STATUS" .
.SH OPTIONS
.B expr
has no options.
Every argument, even one that starts with a dash, is part of the
expression, and
.B \-\-
is an ordinary string, so
.B expr \-\- 1
is a syntax error.
Only one argument is special.
.TP
.B \-\-help
As the first argument, print a usage message on standard output and exit
with status 0, ignoring the rest of the command line.
To write the string
.B \-\-help
first in an expression, use
.BR "quote \-\-help" .
.SH ENVIRONMENT
.TP
.B BB_GLOBBING
Unless it is
.BR 0 ,
wildcards in the arguments are expanded when
.B expr
starts, which can break expressions that contain
.B *
or
.BR ? ;
see
.BR NOTES .
A busybox process started with it unset sets it to
.B 0
for its children, so commands run from the kit's shell receive their
arguments as the shell expanded them.
.TP
.BR BB_TERMINAL_MODE ", " BB_SKIP_ANSI_EMULATION
When standard output is a console, decide whether the console or busybox
interprets the escape sequences in the output, such as for color.
See
.BR busybox (1).
.TP
.B CONEMUPID
When it is set and neither of the above is, busybox uses terminal mode
0: it turns virtual terminal output off and emulates escape sequences
itself.
.TP
.BR LC_ALL ", " LC_CTYPE ", " LANG
If the first of these that is set has the value
.BR C ,
output to a console is not converted, as described in
.BR NOTES ,
and the console interprets the bytes in its own code page.
.PP
Variables that affect every busybox applet are described in
.BR busybox (1).
.SH EXIT STATUS
.TP
.B 0
The value is not null, or the first argument was
.BR \-\-help .
.TP
.B 1
The value is null.
This is not an error.
.TP
.B 2
An error.
It is reported on standard error as
.BI "expr: " message
with one of these messages:
.RS
.TP
.B syntax error
An argument is missing, misplaced, or left over, or the parentheses do
not pair.
.TP
.B too few arguments
There are no arguments.
.TP
.B non\-numeric argument
An arithmetic operator was given an argument that is not an integer.
.TP
.B division by zero
The divisor of
.B /
or
.B %
is 0.
.TP
.BI "bad regex \(aq" regexp "\(aq: " reason
The regular expression does not compile, for a reason such as an
unmatched
.BR \e( .
.TP
.BI "standard output: " reason
Standard output could not be written, such as when it is closed or is
a pipe whose reader has exited.
.RE
.SH EXAMPLES
Add two numbers:
.PP
.RS 4
.EX
$ expr 2 + 3
5
.EE
.RE
.PP
Quote an operator that the shell treats specially:
.PP
.RS 4
.EX
$ expr 6 \e* 7
42
.EE
.RE
.PP
Increment a shell variable:
.PP
.RS 4
.EX
$ i=$(expr $i + 1)
.EE
.RE
.PP
Count the bytes in a string:
.PP
.RS 4
.EX
$ expr length "hello world"
11
.EE
.RE
.PP
Take the first three bytes of a string:
.PP
.RS 4
.EX
$ expr substr abcdef 1 3
abc
.EE
.RE
.PP
Print the part of a name after the last dot:
.PP
.RS 4
.EX
$ expr file.tar.gz : \(aq.*\e.\e(.*\e)\(aq
gz
.EE
.RE
.PP
Test a name against a pattern with the exit status:
.PP
.RS 4
.EX
$ expr "$f" : \(aq.*\e.c$\(aq >/dev/null && echo C source
.EE
.RE
.PP
Use a default for an empty value:
.PP
.RS 4
.EX
$ name=
$ expr "$name" \e| anonymous
anonymous
.EE
.RE
.SH NOTES
Differences from GNU
.BR expr :
.IP \(bu 2
Integers are 64-bit, with no arbitrary-precision arithmetic.
.IP \(bu 2
Strings are bytes, not multibyte characters.
.IP \(bu 2
Every error exits with status 2, where GNU uses 3 for some of them.
.IP \(bu 2
.B |
and
.B &
always evaluate both operands, so
.B expr 1 \e| 1 / 0
fails instead of printing 1.
.IP \(bu 2
When both operands of
.B |
are null, the value is the second operand, such as the empty string,
where POSIX and GNU give 0.
.IP \(bu 2
There is no
.B +
prefix to force a string, only
.BR quote ,
no
.BR \-\-version ,
and no
.B \-\-
to end options.
.PP
In the kit's shell, quote or backslash
.BR * ,
.BR ( ,
.BR ) ,
.BR < ,
.BR > ,
.BR & ,
and
.BR | ,
and write a regular expression in single quotes so that the shell keeps
its backslashes.
In
.BR cmd.exe ,
put
.BR < ,
.BR > ,
.BR & ,
and
.B |
in double quotes or precede each with
.BR ^ ,
and write
.B %
as
.B %%
in a batch file.
.PP
When busybox is started from
.B cmd.exe
or PowerShell, the C runtime expands wildcards in its arguments, quoted
or not, before
.B expr
runs.
An argument containing
.B *
or
.BR ? ,
such as the operator in
.BR "expr 6 * 7" ,
is replaced by the names of the files it matches, and
.B expr
reports a syntax error.
An argument that matches no file is left as it is, so whether it fails
depends on the directory.
Set
.B BB_GLOBBING
to
.B 0
first.
The kit's shell does its own expansion and is not affected.
.PP
Arguments may contain any Unicode character on Windows 10 version 1903
and later, and are limited to the system code page on earlier versions.
.B expr
works on bytes, so
.BR length ,
.BR substr ,
.BR index ,
and regular expressions count a UTF-8 character outside ASCII as 2 to 4
bytes, and
.B .\&
matches one byte of it.
Strings are ordered by byte value.
.PP
Dividing
.B \-9223372036854775808
by
.BR \-1 ,
with
.B /
or
.BR % ,
raises a processor exception.
Windows ends
.B expr
with status
.BR 0xC0000095 ,
and
.B expr
prints no message.
The kit's shell then prints the Windows error message and
.B Error 0xc0000095
on standard error, and sets
.B $?\&
to 149, the low byte.
.PP
Standard output is binary, so the newline is a bare LF.
When standard output is a console, the output is converted for display
from the ANSI code page, which is UTF-8 on Windows 10 version 1903 and
later.
Escape sequences, such as those for color, take effect, and busybox
emulates them on consoles without virtual terminal support.
.PP
The shell computes integer expressions itself with
.BR $((...)) ,
without starting a process; see
.BR sh (1).
.SH SEE ALSO
.BR awk (1),
.BR bc (1),
.BR busybox (1),
.BR grep (1),
.BR sed (1),
.BR sh (1),
.BR test (1)