---

3.2.5.2 Conditional Constructs

if ¶

The syntax of the if command is:

if test-commands; then
  consequent-commands;
[elif more-test-commands; then
  more-consequents;]
[else alternate-consequents;]
fi

The test-commands list is executed, and if its return status is zero, the consequent-commands list is executed. If test-commands returns a non-zero status, each elif list is executed in turn, and if its exit status is zero, the corresponding more-consequents is executed and the command completes. If ‘else alternate-consequents’ is present, and the final command in the final if or elif clause has a non-zero exit status, then alternate-consequents is executed. The return status is the exit status of the last command executed, or zero if no condition tested true.

case ¶

The syntax of the case command is:

case word in
    [ [(] pattern [| pattern]…) command-list ;;]…
esac

case will selectively execute the command-list corresponding to the first pattern that matches word. The match is performed according to the rules described below in Pattern Matching. If the nocasematch shell option (see the description of shopt in The Shopt Builtin) is enabled, the match is performed without regard to the case of alphabetic characters. The ‘|’ is used to separate multiple patterns, and the ‘)’ operator terminates a pattern list. A list of patterns and an associated command-list is known as a clause.

Each clause must be terminated with ‘;;’, ‘;&’, or ‘;;&’. The word undergoes tilde expansion, parameter expansion, command substitution, arithmetic expansion, and quote removal (see Shell Parameter Expansion) before matching is attempted. Each pattern undergoes tilde expansion, parameter expansion, command substitution, arithmetic expansion, process substitution, and quote removal.

There may be an arbitrary number of case clauses, each terminated by a ‘;;’, ‘;&’, or ‘;;&’. The first pattern that matches determines the command-list that is executed. It’s a common idiom to use ‘*’ as the final pattern to define the default case, since that pattern will always match.

Here is an example using case in a script that could be used to describe one interesting feature of an animal:

echo -n "Enter the name of an animal: "
read ANIMAL
echo -n "The $ANIMAL has "
case $ANIMAL in
  horse | dog | cat) echo -n "four";;
  man | kangaroo ) echo -n "two";;
  *) echo -n "an unknown number of";;
esac
echo " legs."

If the ‘;;’ operator is used, no subsequent matches are attempted after the first pattern match. Using ‘;&’ in place of ‘;;’ causes execution to continue with the command-list associated with the next clause, if any. Using ‘;;&’ in place of ‘;;’ causes the shell to test the patterns in the next clause, if any, and execute any associated command-list on a successful match, continuing the case statement execution as if the pattern list had not matched.

The return status is zero if no pattern is matched. Otherwise, the return status is the exit status of the command-list executed.

select ¶

The select construct allows the easy generation of menus. It has almost the same syntax as the for command:

select name [in words …]; do commands; done

The list of words following in is expanded, generating a list of items, and the set of expanded words is printed on the standard error output stream, each preceded by a number. If the ‘in words’ is omitted, the positional parameters are printed, as if ‘in "$@"’ had been specified. select then displays the PS3 prompt and reads a line from the standard input. If the line consists of a number corresponding to one of the displayed words, then the value of name is set to that word. If the line is empty, the words and prompt are displayed again. If EOF is read, the select command completes and returns 1. Any other value read causes name to be set to null. The line read is saved in the variable REPLY.

The commands are executed after each selection until a break command is executed, at which point the select command completes.

Here is an example that allows the user to pick a filename from the current directory, and displays the name and index of the file selected.

select fname in *;
do
	echo you picked $fname \($REPLY\)
	break;
done

((…))

(( expression ))

The arithmetic expression is evaluated according to the rules described below (see Shell Arithmetic). The expression undergoes the same expansions as if it were within double quotes, but double quote characters in expression are not treated specially are removed. If the value of the expression is non-zero, the return status is 0; otherwise the return status is 1.

[[…]] ¶

[[ expression ]]

Return a status of 0 or 1 depending on the evaluation of the conditional expression expression. Expressions are composed of the primaries described below in Bash Conditional Expressions. The words between the [[ and ]] do not undergo word splitting and filename expansion. The shell performs tilde expansion, parameter and variable expansion, arithmetic expansion, command substitution, process substitution, and quote removal on those words (the expansions that would occur if the words were enclosed in double quotes). Conditional operators such as ‘-f’ must be unquoted to be recognized as primaries.

When used with [[, the ‘<’ and ‘>’ operators sort lexicographically using the current locale.

When the ‘==’ and ‘!=’ operators are used, the string to the right of the operator is considered a pattern and matched according to the rules described below in Pattern Matching, as if the extglob shell option were enabled. The ‘=’ operator is identical to ‘==’. If the nocasematch shell option (see the description of shopt in The Shopt Builtin) is enabled, the match is performed without regard to the case of alphabetic characters. The return value is 0 if the string matches (‘==’) or does not match (‘!=’) the pattern, and 1 otherwise.

If you quote any part of the pattern, using any of the shell’s quoting mechanisms, the quoted portion is matched literally. This means every character in the quoted portion matches itself, instead of having any special pattern matching meaning.

An additional binary operator, ‘=~’, is available, with the same precedence as ‘==’ and ‘!=’. When you use ‘=~’, the string to the right of the operator is considered a POSIX extended regular expression pattern and matched accordingly (using the POSIX regcomp and regexec interfaces usually described in regex(3)). The return value is 0 if the string matches the pattern, and 1 if it does not. If the regular expression is syntactically incorrect, the conditional expression returns 2. If the nocasematch shell option (see the description of shopt in The Shopt Builtin) is enabled, the match is performed without regard to the case of alphabetic characters.

You can quote any part of the pattern to force the quoted portion to be matched literally instead of as a regular expression (see above). If the pattern is stored in a shell variable, quoting the variable expansion forces the entire pattern to be matched literally.

The pattern will match if it matches any part of the string. If you want to force the pattern to match the entire string, anchor the pattern using the ‘^’ and ‘$’ regular expression operators.

For example, the following will match a line (stored in the shell variable line) if there is a sequence of characters anywhere in the value consisting of any number, including zero, of characters in the space character class, immediately followed by zero or one instances of ‘a’, then a ‘b’:

[[ $line =~ [[:space:]]*(a)?b ]]

That means values for line like ‘aab’, ‘ aaaaaab’, ‘xaby’, and ‘ ab’ will all match, as will a line containing a ‘b’ anywhere in its value.

If you want to match a character that’s special to the regular expression grammar (‘^$|[]()\.*+?’), it has to be quoted to remove its special meaning. This means that in the pattern ‘xxx.txt’, the ‘.’ matches any character in the string (its usual regular expression meaning), but in the pattern ‘"xxx.txt"’, it can only match a literal ‘.’.

Likewise, if you want to include a character in your pattern that has a special meaning to the regular expression grammar, you must make sure it’s not quoted. If you want to anchor a pattern at the beginning or end of the string, for instance, you cannot quote the ‘^’ or ‘$’ characters using any form of shell quoting.

If you want to match ‘initial string’ at the start of a line, the following will work:

[[ $line =~ ^"initial string" ]]

but this will not:

[[ $line =~ "^initial string" ]]

because in the second example the ‘^’ is quoted and doesn’t have its usual special meaning.

It is sometimes difficult to specify a regular expression properly without using quotes, or to keep track of the quoting used by regular expressions while paying attention to shell quoting and the shell’s quote removal. Storing the regular expression in a shell variable is often a useful way to avoid problems with quoting characters that are special to the shell. For example, the following is equivalent to the pattern used above:

pattern='[[:space:]]*(a)?b'
[[ $line =~ $pattern ]]

Shell programmers should take special care with backslashes, since backslashes are used by both the shell and regular expressions to remove the special meaning from the following character. This means that after the shell’s word expansions complete (see Shell Expansions), any backslashes remaining in parts of the pattern that were originally not quoted can remove the special meaning of pattern characters. If any part of the pattern is quoted, the shell does its best to ensure that the regular expression treats those remaining backslashes as literal, if they appeared in a quoted portion.

The following two sets of commands are not equivalent:

pattern='\.'

[[ . =~ $pattern ]]
[[ . =~ \. ]]

[[ . =~ "$pattern" ]]
[[ . =~ '\.' ]]

The first two matches will succeed, but the second two will not, because in the second two the backslash will be part of the pattern to be matched. In the first two examples, the pattern passed to the regular expression parser is ‘\.’. The backslash removes the special meaning from ‘.’, so the literal ‘.’ matches. In the second two examples, the pattern passed to the regular expression parser has the backslash quoted (e.g., ‘\\\.’), which will not match the string, since it does not contain a backslash. If the string in the first examples were anything other than ‘.’, say ‘a’, the pattern would not match, because the quoted ‘.’ in the pattern loses its special meaning of matching any single character.

Bracket expressions in regular expressions can be sources of errors as well, since characters that are normally special in regular expressions lose their special meanings between brackets. However, you can use bracket expressions to match special pattern characters without quoting them, so they are sometimes useful for this purpose.

Though it might seem like a strange way to write it, the following pattern will match a ‘.’ in the string:

[[ . =~ [.] ]]

The shell performs any word expansions before passing the pattern to the regular expression functions, so you can assume that the shell’s quoting takes precedence. As noted above, the regular expression parser will interpret any unquoted backslashes remaining in the pattern after shell expansion according to its own rules. The intention is to avoid making shell programmers quote things twice as much as possible, so shell quoting should be sufficient to quote special pattern characters where that’s necessary.

The array variable BASH_REMATCH records which parts of the string matched the pattern. The element of BASH_REMATCH with index 0 contains the portion of the string matching the entire regular expression. Substrings matched by parenthesized subexpressions within the regular expression are saved in the remaining BASH_REMATCH indices. The element of BASH_REMATCH with index n is the portion of the string matching the nth parenthesized subexpression.

Bash sets BASH_REMATCH in the global scope; declaring it as a local variable will lead to unexpected results.

Expressions may be combined using the following operators, listed in decreasing order of precedence:

( expression )

Returns the value of expression. This may be used to override the normal precedence of operators.

! expression

True if expression is false.

expression1 && expression2

True if both expression1 and expression2 are true.

expression1 || expression2

True if either expression1 or expression2 is true.

The && and || operators do not evaluate expression2 if the value of expression1 is sufficient to determine the return value of the entire conditional expression.