24.1. 复杂函数与函数复杂度

退出与返回

exit status（退出状态）

函数返回一个值，称为 exit status（退出状态）。这类似于命令返回的 exit status。退出状态可以通过 return 语句显式指定，否则它就是函数中最后一个命令的退出状态（成功时为 0，失败时为非零错误代码）。这个 exit status 可以在脚本中通过引用 $? 来使用。这种机制有效地允许脚本函数拥有类似于 C 函数的 “返回值”。

return

终止一个函数。return 命令可以选择接受一个 integer（整数）参数 [1]，该参数将作为函数的 “退出状态” 返回给调用脚本，并且此退出状态被赋值给变量 $?。

示例 24-7. 两个数字的最大值

#!/bin/bash # max.sh: Maximum of two integers. E_PARAM_ERR=250 # If less than 2 params passed to function. EQUAL=251 # Return value if both params equal. # Error values out of range of any #+ params that might be fed to the function. max2 () # Returns larger of two numbers. { # Note: numbers compared must be less than 250. if [ -z "$2" ] then return $E_PARAM_ERR fi if [ "$1" -eq "$2" ] then return $EQUAL else if [ "$1" -gt "$2" ] then return $1 else return $2 fi fi } max2 33 34 return_val=$? if [ "$return_val" -eq $E_PARAM_ERR ] then echo "Need to pass two parameters to the function." elif [ "$return_val" -eq $EQUAL ] then echo "The two numbers are equal." else echo "The larger of the two numbers is $return_val." fi exit 0 # Exercise (easy): # --------------- # Convert this to an interactive script, #+ that is, have the script ask for input (two numbers).

要使函数返回字符串或数组，请使用专用变量。 count_lines_in_etc_passwd() { [[ -r /etc/passwd ]] && REPLY=$(echo $(wc -l < /etc/passwd)) # If /etc/passwd is readable, set REPLY to line count. # Returns both a parameter value and status information. # The 'echo' seems unnecessary, but . . . #+ it removes excess whitespace from the output. } if count_lines_in_etc_passwd then echo "There are $REPLY lines in /etc/passwd." else echo "Cannot count lines in /etc/passwd." fi # Thanks, S.C.

示例 24-8. 将数字转换为罗马数字

#!/bin/bash # Arabic number to Roman numeral conversion # Range: 0 - 200 # It's crude, but it works. # Extending the range and otherwise improving the script is left as an exercise. # Usage: roman number-to-convert LIMIT=200 E_ARG_ERR=65 E_OUT_OF_RANGE=66 if [ -z "$1" ] then echo "Usage: `basename $0` number-to-convert" exit $E_ARG_ERR fi num=$1 if [ "$num" -gt $LIMIT ] then echo "Out of range!" exit $E_OUT_OF_RANGE fi to_roman () # Must declare function before first call to it. { number=$1 factor=$2 rchar=$3 let "remainder = number - factor" while [ "$remainder" -ge 0 ] do echo -n $rchar let "number -= factor" let "remainder = number - factor" done return $number # Exercises: # --------- # 1) Explain how this function works. # Hint: division by successive subtraction. # 2) Extend to range of the function. # Hint: use "echo" and command-substitution capture. } to_roman $num 100 C num=$? to_roman $num 90 LXXXX num=$? to_roman $num 50 L num=$? to_roman $num 40 XL num=$? to_roman $num 10 X num=$? to_roman $num 9 IX num=$? to_roman $num 5 V num=$? to_roman $num 4 IV num=$? to_roman $num 1 I # Successive calls to conversion function! # Is this really necessary??? Can it be simplified? echo exit

另请参见 示例 11-29。

函数可以返回的最大正整数是 255。return 命令与 exit status 的概念紧密相关，这解释了这种特定的限制。幸运的是，对于那些需要从函数返回较大整数值的情况，有各种 解决方法。 示例 24-9. 测试函数中的大返回值 #!/bin/bash # return-test.sh # The largest positive value a function can return is 255. return_test () # Returns whatever passed to it. { return $1 } return_test 27 # o.k. echo $? # Returns 27. return_test 255 # Still o.k. echo $? # Returns 255. return_test 257 # Error! echo $? # Returns 1 (return code for miscellaneous error). # ========================================================= return_test -151896 # Do large negative numbers work? echo $? # Will this return -151896? # No! It returns 168. # Version of Bash before 2.05b permitted #+ large negative integer return values. # It happened to be a useful feature. # Newer versions of Bash unfortunately plug this loophole. # This may break older scripts. # Caution! # ========================================================= exit 0 获取较大整数 “返回值” 的一种解决方法是简单地将 “返回值” 赋值给全局变量。 Return_Val= # Global variable to hold oversize return value of function. alt_return_test () { fvar=$1 Return_Val=$fvar return # Returns 0 (success). } alt_return_test 1 echo $? # 0 echo "return value = $Return_Val" # 1 alt_return_test 256 echo "return value = $Return_Val" # 256 alt_return_test 257 echo "return value = $Return_Val" # 257 alt_return_test 25701 echo "return value = $Return_Val" #25701 一种更优雅的方法是让函数 echo 其 “返回值到 stdout”，然后通过 命令替换 捕获它。请参阅 第 36.7 节中对此的 讨论。 示例 24-10. 比较两个大整数 #!/bin/bash # max2.sh: Maximum of two LARGE integers. # This is the previous "max.sh" example, #+ modified to permit comparing large integers. EQUAL=0 # Return value if both params equal. E_PARAM_ERR=-99999 # Not enough params passed to function. # ^^^^^^ Out of range of any params that might be passed. max2 () # "Returns" larger of two numbers. { if [ -z "$2" ] then echo $E_PARAM_ERR return fi if [ "$1" -eq "$2" ] then echo $EQUAL return else if [ "$1" -gt "$2" ] then retval=$1 else retval=$2 fi fi echo $retval # Echoes (to stdout), rather than returning value. # Why? } return_val=$(max2 33001 33997) # ^^^^ Function name # ^^^^^ ^^^^^ Params passed # This is actually a form of command substitution: #+ treating a function as if it were a command, #+ and assigning the stdout of the function to the variable "return_val." # ========================= OUTPUT ======================== if [ "$return_val" -eq "$E_PARAM_ERR" ] then echo "Error in parameters passed to comparison function!" elif [ "$return_val" -eq "$EQUAL" ] then echo "The two numbers are equal." else echo "The larger of the two numbers is $return_val." fi # ========================================================= exit 0 # Exercises: # --------- # 1) Find a more elegant way of testing #+ the parameters passed to the function. # 2) Simplify the if/then structure at "OUTPUT." # 3) Rewrite the script to take input from command-line parameters. 这是另一个捕获函数 “返回值” 的示例。理解它需要一些 awk 知识。 month_length () # Takes month number as an argument. { # Returns number of days in month. monthD="31 28 31 30 31 30 31 31 30 31 30 31" # Declare as local? echo "$monthD" | awk '{ print $'"${1}"' }' # Tricky. # ^^^^^^^^^ # Parameter passed to function ($1 -- month number), then to awk. # Awk sees this as "print $1 . . . print $12" (depending on month number) # Template for passing a parameter to embedded awk script: # $'"${script_parameter}"' # Here's a slightly simpler awk construct: # echo $monthD | awk -v month=$1 '{print $(month)}' # Uses the -v awk option, which assigns a variable value #+ prior to execution of the awk program block. # Thank you, Rich. # Needs error checking for correct parameter range (1-12) #+ and for February in leap year. } # ---------------------------------------------- # Usage example: month=4 # April, for example (4th month). days_in=$(month_length $month) echo $days_in # 30 # ---------------------------------------------- 另请参见 示例 A-7 和 示例 A-37。 练习使用我们刚刚学到的知识，扩展之前的 罗马数字示例 以接受任意大的输入。

重定向

重定向函数的 stdin

函数本质上是一个 代码块，这意味着它的stdin可以被重定向（如 示例 3-1 中所示）。

示例 24-11. 从用户名获取真实姓名

#!/bin/bash # realname.sh # # From username, gets "real name" from /etc/passwd. ARGCOUNT=1 # Expect one arg. E_WRONGARGS=85 file=/etc/passwd pattern=$1 if [ $# -ne "$ARGCOUNT" ] then echo "Usage: `basename $0` USERNAME" exit $E_WRONGARGS fi file_excerpt () # Scan file for pattern, { #+ then print relevant portion of line. while read line # "while" does not necessarily need [ condition ] do echo "$line" | grep $1 | awk -F":" '{ print $5 }' # Have awk use ":" delimiter. done } <$file # Redirect into function's stdin. file_excerpt $pattern # Yes, this entire script could be reduced to # grep PATTERN /etc/passwd | awk -F":" '{ print $5 }' # or # awk -F: '/PATTERN/ {print $5}' # or # awk -F: '($1 == "username") { print $5 }' # real name from username # However, it might not be as instructive. exit 0

还有一种替代方法，可能不那么令人困惑，可以重定向函数的stdin。这涉及到将stdin重定向到函数内嵌的带括号的代码块。

# Instead of: Function () { ... } < file # Try this: Function () { { ... } < file } # Similarly, Function () # This works. { { echo $* } | tr a b } Function () # This doesn't work. { echo $* } | tr a b # A nested code block is mandatory here. # Thanks, S.C.

Emmanuel Rouat 的 示例 bashrc 文件 包含一些具有启发性的函数示例。