POSIX-compliant shells

http://www.opengroup.org/onlinepubs/009695399/utilities/xcu_chap02.html#tag_02_06

${parameter:-word}

Use Default Values. If parameter is unset or null, the expansion of word shall be substituted; otherwise, the value of parameter shall be substituted.

${parameter:=word}

Assign Default Values. If parameter is unset or null, the expansion of word shall be assigned to parameter. In all cases, the final value of parameter shall be substituted. Only variables, not positional parameters or special parameters, can be assigned in this way.

${parameter:?[word]}

Indicate Error if Null or Unset. If parameter is unset or null, the expansion of word (or a message indicating it is unset if word is omitted) shall be written to standard error and the shell exits with a non-zero exit status. Otherwise, the value of parameter shall be substituted. An interactive shell need not exit.

${parameter:+word}

Use Alternative Value. If parameter is unset or null, null shall be substituted; otherwise, the expansion of word shall be substituted.

In the parameter expansions shown previously, use of the colon in the format shall result in a test for a parameter that is unset or null; omission of the colon shall result in a test for a parameter that is only unset. The following table summarizes the effect of the colon:

${#parameter}

String Length. The length in characters of the value of parameter shall be substituted. If parameter is '*' or '@', the result of the expansion is unspecified.

The following four varieties of parameter expansion provide for substring processing. In each case, pattern matching notation (see Pattern Matching Notation), rather than regular expression notation, shall be used to evaluate the patterns.

If parameter is '*' or '@', the result of the expansion is unspecified. Enclosing the full parameter expansion string in double-quotes shall not cause the following four varieties of pattern characters to be quoted, whereas quoting characters within the braces shall have this effect.

${parameter%word}

Remove Smallest Suffix Pattern. The word shall be expanded to produce a pattern. The parameter expansion shall then result in parameter, with the smallest portion of the suffix matched by the pattern deleted.

${parameter%%word}

Remove Largest Suffix Pattern. The word shall be expanded to produce a pattern. The parameter expansion shall then result in parameter, with the largest portion of the suffix matched by the pattern deleted.

${parameter#word}

Remove Smallest Prefix Pattern. The word shall be expanded to produce a pattern. The parameter expansion shall then result in parameter, with the smallest portion of the prefix matched by the pattern deleted.

${parameter##word}

Remove Largest Prefix Pattern. The word shall be expanded to produce a pattern. The parameter expansion shall then result in parameter, with the largest portion of the prefix matched by the pattern deleted.

${parameter:-word}

${x:-$(ls)}

${parameter:=word}

unset X
echo ${X:=abc}
abc

echo ${X}
abc

${parameter:?word}

unset posix
echo ${posix:?}
sh: posix: parameter null or not set

${parameter:+word}

set a b c
echo ${3:+posix}
posix

${#parameter}

HOME=/usr/posix
echo ${#HOME}
10

${parameter%word}

x=file.c
echo ${x%.c}.o
file.o

${parameter%%word}

x=posix/src/std
echo ${x%%/*}
posix

${parameter#word}

x=$HOME/src/cmd
echo ${x#$HOME}
/src/cmd

${parameter##word}

x=/one/two/three
echo ${x##*/}
three

"${x#*}"
   The asterisk is a pattern character.
${x#"*"}
   The literal asterisk is quoted and not special.

Command substitution allows the output of a command to be substituted in place of the command name itself. Command substitution shall occur when the command is enclosed as follows:

$(command)

Arithmetic expansion provides a mechanism for evaluating an arithmetic expression and substituting its value. The format for arithmetic expansion shall be as follows:

$((expression))

# repeat a command 100 times
x=100
while [ $x -gt 0 ]
do
  command
  x=$(($x-1))
done

 x=100

 $ echo $(($x-1))
 99

 $ echo $(( ($x-1) % 10 /3 << 2 ))
 12

 $ echo $(( ($x-1) % 10 /3 << 2 > 11 ))
 1

 $ echo $((x-=11))
 89

 $ echo $((x))
 89

 $ echo $(($x-=11))
 bash: 100-=11: attempted assignment to non-variable (error token is "-=11")