ifif 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.
casecase command is:
casewordin [ [(]pattern[|pattern]...)command-list;;]... esac
case will selectively execute the command-list corresponding to
the first pattern that matches word.
If the shell option nocasematch
(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 before matching is attempted. Each pattern undergoes tilde expansion, parameter expansion, command substitution, and arithmetic expansion.
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.
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 $ANIMAL in
horse | dog | cat) echo -n "four";;
man | kangaroo ) echo -n "two";;
*) echo -n "an unknown number of";;
esac
echo " legs."
If the ‘;;’ operator is used, no subsequent matches are attempted after the first pattern match. Using ‘;&’ in place of ‘;;’ causes execution to continue with the command-list associated with the next clause, if any. Using ‘;;&’ in place of ‘;;’ causes the shell to test the patterns in the next clause, if any, and execute any associated command-list on a successful match.
The return status is zero if no pattern is matched. Otherwise, the
return status is the exit status of the command-list executed.
selectselect construct allows the easy generation of menus.
It has almost the same syntax as the for command:
select name [in words ...]; do commands; done
The list of words following in is expanded, generating a list
of items. The set of expanded words is printed on the standard
error output stream, each preceded by a number. If the
‘in words’ is omitted, the positional parameters are printed,
as if ‘in "$@"’ had been specified.
The PS3 prompt is then displayed and a line is read 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.
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). If the value of the expression is non-zero, the return status is 0; otherwise the return status is 1. This is exactly equivalent to
let "expression"
See Bash Builtins, for a full description of the let builtin.
[[...]][[ 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. Word splitting and filename expansion are not performed on the words between the ‘[[’ and ‘]]’; tilde expansion, parameter and variable expansion, arithmetic expansion, command substitution, process substitution, and quote removal are performed. 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.
If the shell option nocasematch
(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.
Any part of the pattern may be quoted to force it to be matched as a
string.
An additional binary operator, ‘=~’, is available, with the same
precedence as ‘==’ and ‘!=’.
When it is used, the string to the right of the operator is considered
an extended regular expression and matched accordingly (as in regex3)).
The return value is 0 if the string matches
the pattern, and 1 otherwise.
If the regular expression is syntactically incorrect, the conditional
expression's return value is 2.
If the shell option nocasematch
(see the description of shopt in The Shopt Builtin)
is enabled, the match is performed without regard to the case
of alphabetic characters.
Any part of the pattern may be quoted to force it to be matched as a
string.
Substrings matched by parenthesized subexpressions within the regular
expression are saved in the array variable BASH_REMATCH.
The element of BASH_REMATCH with index 0 is the portion of the string
matching the entire regular expression.
The element of BASH_REMATCH with index n is the portion of the
string matching the nth parenthesized subexpression.
Expressions may be combined using the following operators, listed in decreasing order of precedence:
( expression )! expression && expression2 || expression2&& and || operators do not evaluate expression2 if the
value of expression1 is sufficient to determine the return
value of the entire conditional expression.