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| First, you have to define what you mean by "number". The most common case when people ask this seems to be "a non-negative integer, with no leading + sign". Or in other words, a string of all digits. Other times, people want to validate a floating-point input, with optional sign and optional decimal point. | |
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| === Hand Parsing === First, you have to define what you mean by "number". The most common case when people ask this seems to be "a non-negative integer, with no leading + sign". Or in other words, a string of all digits. This can be checked using standard [[glob|globs]]: |
=== Hand parsing === If you're validating a simple "string of digits", you can do it with a [[glob]]: |
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| {{{ # Bash if [[ $foo = *[^0-9]* ]]; then echo "'$foo' has a non-digit somewhere in it" |
{{{#!highlight bash # Bash / Ksh if [[ -n $foo && $foo != *[!0123456789]* ]]; then printf '"%s" is strictly numeric\n' "$foo" |
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| echo "'$foo' is strictly numeric" | printf '"%s" has a non-digit somewhere in it or is empty\n' "$foo" fi >&2 }}} Avoid `[0-9]` or `[[:digit:]]` which in some locales and some systems can match characters other than 0123456789. The same thing can be done in POSIX shells as well, using {{{case}}}: {{{#!highlight bash # POSIX case $var in '') printf 'var is empty\n';; *[!0123456789]*) printf '%s has a non-digit somewhere in it\n' "$var";; *) printf '%s is strictly numeric\n' "$var";; esac >&2 }}} Of course, if all you care about is valid vs. invalid, you can combine cases: {{{#!highlight bash # POSIX case $var in '' | *[!0123456789]*) printf '%s\n' "$0: $var: invalid digit" >&2; exit 1;; esac }}} If you need to allow a leading negative sign, or if want a valid floating-point number or something else more complex, then there are a few possible ways. Standard globs aren't expressive enough to do this, but you can trim off any sign and then compare: {{{#!highlight bash # POSIX case ${var#[-+]} in # notice ${var#prefix} substitution to trim sign '') printf 'var is empty\n';; .) printf 'var is just a dot\n';; *.*.*) printf '"%s" has more than one decimal point in it\n' "$var";; *[!0123456789.]*) printf '"%s" has a non-digit somewhere in it\n' "$var";; *) printf '"%s" looks like a valid float\n' "$var";; esac >&2 }}} Or in Bash, we can use [[glob|extended globs]]: {{{#!highlight bash # Bash -- extended globs must be enabled explicitly in versions prior to 4.1. # Check whether the variable is all digits. shopt -s extglob [[ $var = +([0123456789]) ]] }}} A more complex case: {{{#!highlight bash # Bash / ksh shopt -s extglob # not necessary in ksh and bash 4.1 or newer if [[ $foo = @(*[0123456789]*|!([+-]|)) && $foo = ?([+-])*([0123456789])?(.*([0123456789])) ]]; then echo 'foo is a floating-point number' |
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| Optionally, `case..esac` may have been used in shells with extended pattern matching. The leading test of {{{$foo}}} is to ensure that it contains at least one digit, isn't empty, and contains more than just + or - by itself. | |
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| The same thing can be done in Korn and POSIX shells as well, using {{{case}}}: | If your definition of "a valid number" is even more complex, or if you need a solution that works in legacy Bourne shells, you might prefer to use an external tool's [[RegularExpression|regular expression]] syntax. Here is a portable version (explained in detail [[http://www.wplug.org/wiki/Meeting-20100612#EXERCISE_TWO|here]]), using {{{awk}}} (not `egrep` which is line-based so would be tricked by variables that contain newline characters): |
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| {{{ # ksh, POSIX case "$foo" in *[!0-9]*) echo "'$foo' has a non-digit somewhere in it" ;; *) echo "'$foo' is strictly numeric" ;; esac }}} |
{{{#!highlight bash # Bourne |
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| If what you actually mean is "a valid floating-point number" or something else more complex, then there are a few possible ways. Standard globs aren't expressive enough to do this, but we can use [[glob|extended globs]]: {{{ # Bash -- extended globs must be enabled. # Check whether the variable is all digits. shopt -s extglob [[ $var == +([0-9]) ]] }}} A more complex case: {{{ # Bash use case instead of [[ ]] for old ksh shopt -s extglob [[ $foo = *[0-9]* && $foo = ?([+-])*([0-9])?(.*([0-9])) ]] && echo "foo is a floating-point number" }}} The leading test of {{{$foo}}} is to ensure that it contains at least one digit. The extended glob, by itself, would match the empty string, or a lone {{{+}}} or {{{-}}}, which may not be desirable behavior. If your definition of "a valid number" is even more complex, or if you need a solution that works in legacy Bourne shells, you might prefer to use an external tool's [[RegularExpression|regular expression]] syntax. Here is a portable version, using {{{egrep}}}: {{{ # Bourne if test "$foo" && echo "$foo" | egrep '^[-+]?[0-9]*(\.[0-9]*)?$' >/dev/null then echo "'$foo' might be a number" |
if awk -- 'BEGIN {exit !(ARGV[1] ~ /^[-+]?([0123456789]+\.?|[0123456789]*\.[0123456789]+)$/)}' "$foo"; then printf '"%s" is a number\n' "$foo" |
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| echo "'$foo' might not be a number" | printf '"%s" is not a number\n' "$foo" |
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| Bash version 3 and above have regular expression support in the `[[...]]` construct. | |
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| (Like the extended globs, this [[RegularExpression|extended regular expression]] will match a lone {{{+}}} or {{{-}}}. The initial {{{test}}} command only requires a non-empty string. Closing the last "bug" is left as an exercise for the reader, mostly because GreyCat is too damned lazy to learn {{{expr(1)}}}.) | {{{#!highlight bash # Bash # The regexp must be stored in a var and expanded for backward compatibility with versions < 3.2 |
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| Bash version 3 and above have regular expression support in the [[ command. {{{ # put it in a var for backward compatibility with versions <3.2 regexp='^[-+]?[0-9]*(\.[0-9]*)?$' if [[ $foo = *[0-9]* && $foo =~ $var ]]; then echo "'$foo' looks rather like a number" |
regexp='^[-+]?[0123456789]*(\.[0123456789]*)?$' if [[ $foo = *[0123456789]* && $foo =~ $regexp ]]; then printf '"%s" looks rather like a number\n' "$foo" |
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| echo "'$foo' doesn't look particularly numeric to me" | printf '"%s" doesn't look particularly numeric to me\n' "$foo" |
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=== Using the parsing done by [ and printf === {{{ |
=== Using the parsing done by [ and printf (or "using eq") === {{{#!highlight bash |
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| if [ "$foo" -eq "$foo" ] 2>/dev/null;then echo "$foo is an integer" fi }}} [ parses the variable and interprets it as in integer because of the -eq. If the parsing succeds the test is trivially true, if it fails [ prints an error message that 2>/dev/null hides and set a status different from 0. However this method fails if the shell is ksh, because ksh evaluates the variable as an arithmetic expression, see below to learn while. You can use the same trick with printf: {{{ # posix if printf "%f" "$foo" >/dev/null 2>&1;then echo "$foo is a float" |
if [ "$foo" -eq "$foo" ] 2>/dev/null; then printf '"%s" is an integer\n' "$foo" |
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| `[` parses the variable and interprets it a decimal integer because of the `-eq`. If the parsing succeeds the test is trivially true; if it fails `[` prints an error message that `2>/dev/null` hides and sets a status different from 0. However this method fails if the shell is ksh, because ksh evaluates the variable as an arithmetic expression (and that would constitute an arbitrary command injection vulnerability). | |
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| You can use %d to parse an integer. Take care that the parsing might (is supposed to be?) be locale dependant. |
Be careful: the following trick with `printf` (not supported by all shells, and the list of supported float representations varies with the shell as well; not to mention the command injection vulnerability in ksh or zsh) |
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| === Using the integer type === | {{{#!highlight bash if printf %f "$foo" >/dev/null 2>&1; then printf '"%s" is a float\n' "$foo" fi }}} is broken: about the arguments of the {{{a}}}, {{{A}}}, {{{e}}}, {{{E}}}, {{{f}}}, {{{F}}}, {{{g}}}, or {{{G}}} format modifiers, POSIX specifies that ''if the leading character is a single-quote or double-quote, the value shall be the numeric value in the underlying codeset of the character following the single-quote or double-quote.'' Hence this fails when {{{foo}}} expands to a string with a leading single-quote or double-quote: the previous command will happily validate the string as a float. It also returns 0 when {{{foo}}} expands to a number with a leading {{{0x}}}, which is a valid number in a shell script but may not work elsewhere. |
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| If you just want to guarantee ahead of time that a variable contains an integer, without actually checking, you can give the variable the "integer" attribute. {{{ declare -i foo foo=-10+1; echo "$foo" # prints -9 foo="hello"; echo "$foo" # the value of the variable "hello" is evaluated; if unset, foo is 0 foo="Some random string" # results in an error. }}} Any value assigned to a variable with the integer attribute set is evaluated as an [[ArithmeticExpression|arithmetic expression]] just like inside `$(( ))`. Bash will raise an error if you try to assign an invalid arithmetic expression. In Bash and ksh93, if a variable which has been declared integer is used in a `read` command, the user's input is treated as an [[ArithmeticExpression|arithmetic expression]], as with assignment. In particular, if the user types an identifier, the variable will be set to the value of the variable with that name, and `read` will give no other indication of a problem. {{{ # Bash (and ksh93, if you replace declare with typeset) $ declare -i foo $ read foo hello $ echo $foo # prints 0; 'hello' is unset, so is treated as 0 for arithmetic purposes $ hello=5 $ read foo # user types hello again hello $ echo $foo # prints 5, the value of 'hello' as an arithmetic expression }}} Pretty useless if you want to read only integers. In the older Korn shell (ksh88), if a variable is declared integer and used in a `read` command, and the user types an invalid integer, the shell complains, the read command returns an error status, and the value of the variable is unchanged. {{{ # ksh88 $ typeset -i foo $ foo=42 $ read foo hello ksh: hello: bad number $ echo $? 1 $ echo $foo 42 }}} |
You can use `%d` to parse an integer. Take care that the parsing might be (is supposed to be?) [[locale]]-dependent. |
How can I tell whether a variable contains a valid number?
First, you have to define what you mean by "number". The most common case when people ask this seems to be "a non-negative integer, with no leading + sign". Or in other words, a string of all digits. Other times, people want to validate a floating-point input, with optional sign and optional decimal point.
Hand parsing
If you're validating a simple "string of digits", you can do it with a glob:
Avoid [0-9] or [[:digit:]] which in some locales and some systems can match characters other than 0123456789.
The same thing can be done in POSIX shells as well, using case:
Of course, if all you care about is valid vs. invalid, you can combine cases:
If you need to allow a leading negative sign, or if want a valid floating-point number or something else more complex, then there are a few possible ways. Standard globs aren't expressive enough to do this, but you can trim off any sign and then compare:
1 # POSIX
2 case ${var#[-+]} in # notice ${var#prefix} substitution to trim sign
3 '')
4 printf 'var is empty\n';;
5 .)
6 printf 'var is just a dot\n';;
7 *.*.*)
8 printf '"%s" has more than one decimal point in it\n' "$var";;
9 *[!0123456789.]*)
10 printf '"%s" has a non-digit somewhere in it\n' "$var";;
11 *)
12 printf '"%s" looks like a valid float\n' "$var";;
13 esac >&2
Or in Bash, we can use extended globs:
A more complex case:
Optionally, case..esac may have been used in shells with extended pattern matching. The leading test of $foo is to ensure that it contains at least one digit, isn't empty, and contains more than just + or - by itself.
If your definition of "a valid number" is even more complex, or if you need a solution that works in legacy Bourne shells, you might prefer to use an external tool's regular expression syntax. Here is a portable version (explained in detail here), using awk (not egrep which is line-based so would be tricked by variables that contain newline characters):
Bash version 3 and above have regular expression support in the [[...]] construct.
1 # Bash
2 # The regexp must be stored in a var and expanded for backward compatibility with versions < 3.2
3
4 regexp='^[-+]?[0123456789]*(\.[0123456789]*)?$'
5 if [[ $foo = *[0123456789]* && $foo =~ $regexp ]]; then
6 printf '"%s" looks rather like a number\n' "$foo"
7 else
8 printf '"%s" doesn't look particularly numeric to me\n' "$foo"
9 fi
Using the parsing done by [ and printf (or "using eq")
[ parses the variable and interprets it a decimal integer because of the -eq. If the parsing succeeds the test is trivially true; if it fails [ prints an error message that 2>/dev/null hides and sets a status different from 0. However this method fails if the shell is ksh, because ksh evaluates the variable as an arithmetic expression (and that would constitute an arbitrary command injection vulnerability).
Be careful: the following trick with printf (not supported by all shells, and the list of supported float representations varies with the shell as well; not to mention the command injection vulnerability in ksh or zsh)
is broken: about the arguments of the a, A, e, E, f, F, g, or G format modifiers, POSIX specifies that if the leading character is a single-quote or double-quote, the value shall be the numeric value in the underlying codeset of the character following the single-quote or double-quote. Hence this fails when foo expands to a string with a leading single-quote or double-quote: the previous command will happily validate the string as a float. It also returns 0 when foo expands to a number with a leading 0x, which is a valid number in a shell script but may not work elsewhere.
You can use %d to parse an integer. Take care that the parsing might be (is supposed to be?) locale-dependent.