Differences between revisions 30 and 53 (spanning 23 versions)
Revision 30 as of 2012-05-11 20:51:25
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Editor: e36freak
Comment: typo correction, incorrect variable name
Revision 53 as of 2022-04-19 05:23:33
Size: 5741
Editor: emanuele6
Comment: clarify that that enabling the extglob shopt is not necessary to use extglob patterns inside of [[ in bash4.1
Deletions are marked like this. Additions are marked like this.
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{{{
# Bash
if [[ $foo != *[!0-9]* ]]; then
    echo "'$foo' is strictly numeric"
{{{#!highlight bash
# Bash / Ksh
if [[ -n $foo && $foo != *[!0123456789]* ]]; then
    printf '"%s" is strictly numeric\n' "$foo"
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    echo "'$foo' has a non-digit somewhere in it"     printf '"%s" has a non-digit somewhere in it or is empty\n' "$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 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 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
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.

Korn shell has extended globs enabled by default, but lacks `[[`, so we must use `case` to do the glob-matching:

{{{
# Korn
case $foo in
  *[0-9]*)
    case $foo in
        ?([+-])*([0-9])?(.*([0-9]))) echo "foo is a number";;
    esac;;
esac
}}}

Note that this uses the same extended glob as the Bash example before it; the third closing parenthesis at the end of it is actually part of the case syntax.

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 {{{egrep}}}:

{{{
# Bourne
if echo "$foo" | egrep '^[-+]?([0-9]+\.?|[0-9]*\.[0-9]+)$' >/dev/null
then
    echo "'$foo' is a number"
if awk -- 'BEGIN {exit !(ARGV[1] ~ /^[-+]?([0123456789]+\.?|[0123456789]*\.[0123456789]+)$/)}' "$foo"; then
    printf '"%s" is a number\n' "$foo"
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    echo "'$foo' is not a number"     printf '"%s" is not a number\n' "$foo"
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Bash version 3 and above have regular expression support in the `[[...]]` construct.
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Bash version 3 and above have regular expression support in the [[ command. Due to bugs and changes in the implementation of the `=~` feature throughout bash 3.x, we '''do not recommend''' using it, but people do it anyway, so we have to maintain this example (''and keep restoring this warning, too, when people delete it''): {{{#!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
# Put the RE in a var for backward compatibility with versions <3.2
regexp='^[-+]?[0-9]*(\.[0-9]*)?$' 
if [[ $foo = *[0-9]* && $foo =~ $regexp ]]; then
    echo "'$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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Line 87: Line 103:

{{{
{{{#!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 sets a status different from 0. However this method fails if the shell is ksh, because ksh evaluates the variable as an arithmetic expression.

You can use a similar 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"
Line 104: Line 109:
`[` 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)

{{{#!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.
Line 106: Line 122:

=== Using the integer type ===

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.

{{{
# Bash
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
}}}

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:

   1 # Bash / Ksh
   2 if [[ -n $foo && $foo != *[!0123456789]* ]]; then
   3     printf '"%s" is strictly numeric\n' "$foo"
   4 else
   5     printf '"%s" has a non-digit somewhere in it or is empty\n' "$foo"
   6 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:

   1 # POSIX
   2 case $var in
   3     '')
   4         printf 'var is empty\n';;
   5     *[!0123456789]*)
   6         printf '%s has a non-digit somewhere in it\n' "$var";;
   7     *)
   8         printf '%s is strictly numeric\n' "$var";;
   9 esac >&2

Of course, if all you care about is valid vs. invalid, you can combine cases:

   1 # POSIX
   2 case $var in
   3     '' | *[!0123456789]*)
   4         printf '%s\n' "$0: $var: invalid digit" >&2; exit 1;;
   5 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:

   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:

   1 # Bash -- extended globs must be enabled explicitly in versions prior to 4.1.
   2 # Check whether the variable is all digits.
   3 shopt -s extglob
   4 [[ $var = +([0123456789]) ]]

A more complex case:

   1 # Bash / ksh
   2 shopt -s extglob # not necessary in ksh and bash 4.1 or newer
   3 
   4 if [[ $foo = @(*[0123456789]*|!([+-]|)) && $foo = ?([+-])*([0123456789])?(.*([0123456789])) ]]; then
   5   echo 'foo is a floating-point number'
   6 fi

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):

   1 # Bourne
   2 
   3 if awk -- 'BEGIN {exit !(ARGV[1] ~ /^[-+]?([0123456789]+\.?|[0123456789]*\.[0123456789]+)$/)}' "$foo"; then
   4     printf '"%s" is a number\n' "$foo"
   5 else
   6     printf '"%s" is not a number\n' "$foo"
   7 fi

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")

   1 # fails with ksh
   2 if [ "$foo" -eq "$foo" ] 2>/dev/null; then
   3     printf '"%s" is an integer\n' "$foo"
   4 fi

[ 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)

   1 if printf %f "$foo" >/dev/null 2>&1; then
   2     printf '"%s" is a float\n' "$foo"
   3 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.

You can use %d to parse an integer. Take care that the parsing might be (is supposed to be?) locale-dependent.

BashFAQ/054 (last edited 2022-04-19 05:23:33 by emanuele6)