2、if语句 传统if 从句子——以条件表达式作为 if条件 if [ 条件表达式 ] then command else command fi 条件表达式 文件表达式 if [ -f file ] 如果文件存在 if [ -d ... ] 如果目录存在 if [ -s file ] 如果文件存在且非空 if [ -r file ] 如果文件存在且可读 if [ -w file ] 如果文件存在且可写 if [ -x file ] 如果文件存在且可执行 整数变量表达式 if [ int1 -eq int2 ] 如果int1等于int2 if [ int1 -ne int2 ] 如果不等于 if [ int1 -ge int2 ] 如果>= if [ int1 -gt int2 ] 如果> if [ int1 -le int2 ] 如果<= if [ int1 -lt int2 ] 如果< 字符串变量表达式 if [ $a = $b ] 如果string1等于string2 字符串允许使用赋值号做等号 if [ $string1 != $string2 ] 如果string1不等于string2 if [ -n $string ] 如果string 非空(非0），返回0(true) if [ -z $string ] 如果string 为空 if [ $sting ] 如果string 非空，返回0 (和-n类似)
* -eq -ne -lt -nt只能用于整数，不适用于字符串，字符串等于用赋值号= * 整数条件表达式，大于，小于，shell里没有> 和< ,会被当作尖括号，只有-ge,-gt,-le,lt * 整数操作符号-ge,-gt,-le,-lt, 别忘了加- * =放在别的地方是赋值,放在if [ ] 里就是字符串等于，与C语言里面的==功能一样 * 无空格的字符串，可以加" ",也可以不加 * if语句里面的=，其两边都必须加空格，否则失效 3、变量扩展表达式 ${parameter#word} ${parameter##word} The word is expanded to produce a pattern just as in filename expansion (see section 3.5.8 Filename Expansion). If the pattern matches the beginning of the expanded value of parameter, then the result of the expansion is the expanded value of parameter with the shortest matching pattern (the `#' case) or the longest matching pattern (the `##' case) deleted. If parameter is `@' or `*', the pattern removal operation is applied to each positional parameter in turn, and the expansion is the resultant list. If parameter is an array variable subscripted with `@' or `*', the pattern removal operation is applied to each member of the array in turn, and the expansion is the resultant list.
${parameter%word} ${parameter%%word} The word is expanded to produce a pattern just as in filename expansion. If the pattern matches a trailing portion of the expanded value of parameter, then the result of the expansion is the value of parameter with the shortest matching pattern (the `%' case) or the longest matching pattern (the `%%' case) deleted. If parameter is `@' or `*', the pattern removal operation is applied to each positional parameter in turn, and the expansion is the resultant list. If parameter is an array variable subscripted with `@' or `*', the pattern removal operation is applied to each member of the array in turn, and the expansion is the resultant list.
${parameter/pattern/string} ${parameter//pattern/string} The pattern is expanded to produce a pattern just as in filename expansion. Parameter is expanded and the longest match of pattern against its value is replaced with string. In the first form, only the first match is replaced. The second form causes all matches of pattern to be replaced with string. If patternbegins with `#', it must match at the beginning of the expanded value of parameter. If pattern begins with `%', it must match at the end of the expanded value of parameter. If string is null, matches of pattern are deleted and the / following pattern may be omitted. If parameter is `@' or `*', the substitution operation is applied to each positional parameter in turn, and the expansion is the resultant list. If parameter is an array variable subscripted with `@' or `*', the substitution operation is applied to each member of the array in turn, and the expansion is the resultant list.
${parameter:-word} If parameter is unset or null, the expansion of word is substituted. Otherwise, the value of parameter is substituted. ${parameter:=word} If parameter is unset or null, the expansion of word is assigned to parameter. The value of parameter is then substituted. Positional parameters and special parameters may not be assigned to in this way. ${parameter:?word} If parameter is null or unset, the expansion of word (or a message to that effect if word is not present) is written to the standard error and the shell, if it is not interactive, exits. Otherwise, the value of parameter is substituted. ${parameter:+word} If parameter is null or unset, nothing is substituted, otherwise the expansion of word is substituted. ${parameter:offset} ${parameter:offset:length} Expands to up to length characters of parameter starting at the character specified by offset. If length is omitted, expands to the substring of parameter starting at the character specified by offset. length and offset are arithmetic expressions (see section 6.5 Shell Arithmetic). This is referred to as Substring Expansion.
length must evaluate to a number greater than or equal to zero. If offset evaluates to a number less than zero, the value is used as an offset from the end of the value of parameter. If parameter is `@', the result is length positional parameters beginning at offset. If parameter is an array name indexed by `@' or`*', the result is the length members of the array beginning with ${parameter[offset]}. Substring indexing is zero-based unless the positional parameters are used, in which case the indexing starts at 1. ${!prefix*} Expands to the names of variables whose names begin with prefix, separated by the first character of the IFS special variable. ${#parameter} The length in characters of the expanded value of parameter is substituted. If parameter is `*' or `@', the value substituted is the number of positional parameters. If parameter is an array name subscripted by `*' or `@', the value substituted is the number of elements in the array. example: 假设我们定义了一个变量为： file=/dir1/dir2/dir3/my.file.txt 可以用${ }分别替换得到不同的值： ${file#*/}：删掉第一个 / 及其左边的字符串：dir1/dir2/dir3/my.file.txt ${file##*/}：删掉最后一个 / 及其左边的字符串：my.file.txt ${file#*.}：删掉第一个 . 及其左边的字符串：file.txt ${file##*.}：删掉最后一个 . 及其左边的字符串：txt ${file%/*}：删掉最后一个 / 及其右边的字符串：/dir1/dir2/dir3 ${file%%/*}：删掉第一个 / 及其右边的字符串：(空值) ${file%.*}：删掉最后一个 . 及其右边的字符串：/dir1/dir2/dir3/my.file ${file%%.*}：删掉第一个 . 及其右边的字符串：/dir1/dir2/dir3/my
