已经在 第 3.4.6 节 “算术扩展” 讨论过。
使用 ${#VAR} 语法将计算一个变量当中字符的数量。如果 VAR is “*” or “@”, this value is substituted with the number of positional parameters or number of elements in an array in general. This
is demonstrated in the example below:
[bob in ~]echo$SHELL/bin/bash[bob in ~]echo${#SHELL}9[bob in ~]ARRAY=(one two three)[bob in ~]echo${#ARRAY}3
${
VAR:-WORD}
如果 VAR 没有定义或者是空值，WORD 的扩展就被替换掉；否则 VAR 的值被替换掉：
[bob in ~]echo${TEST:-test}test[bob in ~]echo$TEST[bob in ~]exportTEST=a_string[bob in ~]echo${TEST:-test}a_string[bob in ~]echo${TEST2:-$TEST}a_string
这种形式常常使用在条件测试中，比如在这个里面：
[ -z "${COLUMNS:-}" ] && COLUMNS=80
这是以下形式的简化写法：
if [ -z "${COLUMNS:-}" ]; then
COLUMNS=80
fi
参见 第 7.1.2.3 节 “字符串比较” 得到更多关于这种类型条件测试的信息。
如果 (-) 使用等号(=)替换， the value is assigned to the parameter if it does not exist:
[bob in ~]echo$TEST2[bob in ~]echo${TEST2:=$TEST}a_string[bob in ~]echo$TEST2a_string
以下语法测试一个变量是否存在。如果没有设置，WORD 的扩展打印到标准输出且非交互shell退出。证明：
[bob in ~]catvartest.sh#!/bin/bash # This script tests whether a variable is set. If not, # it exits printing a message. echo ${TESTVAR:?"There's so much I still wanted to do..."} echo "TESTVAR is set, we can proceed."[bob in testdir]./vartest.sh ./vartest.sh: line 6: TESTVAR: There's so much I still wanted to do...[bob in testdir]exportTESTVAR=present[bob in testdir]./vartest.sh present TESTVAR is set, we can proceed.
使用 “+” 来替代感叹号instead of the exclamation mark sets the variable to the expansion of WORD; 如果没有设置，什么都不会发生。
要从一个变量之中，删除等于 OFFSET 数量的字符，使用这样的语法：
${
VAR:OFFSET:LENGTH}
LENGTH 参数定义了在偏移点offset之后第一个字符开始需要保留多少字符。如果 LENGTH 省略，就是用剩余的变量的内容。
[bob in ~]exportSTRING="thisisaverylongname"[bob in ~]echo${STRING:4}isaverylongname[bob in ~]echo${STRING:6:5}avery
${
VAR#WORD}
and
${
VAR##WORD}
这个结构用来匹配在 VAR 包含 WORD。 WORD is expanded to produce a pattern just as in file name expansion. If the pattern matches the beginning of the expanded value
of VAR, then the result of the expansion is the expanded value of VAR with the shortest matching pattern (“#”) or the longest matching pattern (indicated with “##”).
If VAR is * or @, the pattern removal operation is applied to each positional parameter in turn, and the expansion is the resultant list.
If VAR is an array variable subscribed with “*” or “@”, the pattern removal operation is applied to each member of the array in turn, and the expansion is the resultant list.
This is shown in the examples below:
[bob in ~]echo${ARRAY[*]}one two one three one four[bob in ~]echo${ARRAY[*]#one}two three four[bob in ~]echo${ARRAY[*]#t}one wo one hree one four[bob in ~]echo${ARRAY[*]#t*}one wo one hree one four[bob in ~]echo${ARRAY[*]##t*}one one one four
The opposite effect is obtained using “%” and “%%”, as in this example below. WORD should match a trailing portion of string:
[bob in ~]echo$STRINGthisisaverylongname[bob in ~]echo${STRING%name}thisisaverylong