第 26 章. 列表结构

链接命令

与列表

command-1 && command-2 && command-3 && ... command-n

每个命令依次执行，前提是前一个命令的返回值为真（零）。在第一个假（非零）返回时，命令链终止（第一个返回假的是最后一个执行的命令）。

YongYe 的早期版本 俄罗斯方块游戏脚本 中一个有趣的双条件 与列表 的用法

equation() { # core algorithm used for doubling and halving the coordinates [[ ${cdx} ]] && ((y=cy+(ccy-cdy)${2}2)) eval ${1}+=\"${x} ${y} \" }

示例 26-1. 使用 与列表 测试命令行参数

#!/bin/bash # and list if [ ! -z "$1" ] && echo "Argument #1 = $1" && [ ! -z "$2" ] && \ # ^^ ^^ ^^ echo "Argument #2 = $2" then echo "At least 2 arguments passed to script." # All the chained commands return true. else echo "Fewer than 2 arguments passed to script." # At least one of the chained commands returns false. fi # Note that "if [ ! -z $1 ]" works, but its alleged equivalent, # "if [ -n $1 ]" does not. # However, quoting fixes this. # if "[ -n "$1" ]" works. # ^ ^ Careful! # It is always best to QUOTE the variables being tested. # This accomplishes the same thing, using "pure" if/then statements. if [ ! -z "$1" ] then echo "Argument #1 = $1" fi if [ ! -z "$2" ] then echo "Argument #2 = $2" echo "At least 2 arguments passed to script." else echo "Fewer than 2 arguments passed to script." fi # It's longer and more ponderous than using an "and list". exit $?

示例 26-2. 另一个使用 与列表 的命令行参数测试

#!/bin/bash ARGS=1 # Number of arguments expected. E_BADARGS=85 # Exit value if incorrect number of args passed. test $# -ne $ARGS && \ # ^^^^^^^^^^^^ condition #1 echo "Usage: `basename $0` $ARGS argument(s)" && exit $E_BADARGS # ^^ # If condition #1 tests true (wrong number of args passed to script), #+ then the rest of the line executes, and script terminates. # Line below executes only if the above test fails. echo "Correct number of arguments passed to this script." exit 0 # To check exit value, do a "echo $?" after script termination.

当然，与列表 也可以 设置 变量为默认值。

arg1=$@ && [ -z "$arg1" ] && arg1=DEFAULT # Set $arg1 to command-line arguments, if any. # But . . . set to DEFAULT if not specified on command-line.

或列表

command-1 || command-2 || command-3 || ... command-n

只要前一个命令返回 假，每个命令就会依次执行。在第一个 真 返回时，命令链终止（第一个返回 真 的是最后一个执行的命令）。这显然是 “与列表” 的逆运算。

示例 26-3. 结合 或列表 和 与列表 使用

#!/bin/bash # delete.sh, a not-so-cunning file deletion utility. # Usage: delete filename E_BADARGS=85 if [ -z "$1" ] then echo "Usage: `basename $0` filename" exit $E_BADARGS # No arg? Bail out. else file=$1 # Set filename. fi [ ! -f "$file" ] && echo "File \"$file\" not found. \ Cowardly refusing to delete a nonexistent file." # AND LIST, to give error message if file not present. # Note echo message continuing on to a second line after an escape. [ ! -f "$file" ] || (rm -f $file; echo "File \"$file\" deleted.") # OR LIST, to delete file if present. # Note logic inversion above. # AND LIST executes on true, OR LIST on false. exit $?

如果 或列表 中的第一个命令返回 真，它将执行。