高级 Bash 脚本编程指南
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36.7. 各种技巧

36.7.1. 更强大的脚本的思路

您有一个想通过编写 Bash 脚本解决的问题。不幸的是，您不太清楚从哪里开始。一种方法是直接投入并编写脚本中容易的部分，并将困难的部分写成 伪代码。

#!/bin/bash

ARGCOUNT=1                     # Need name as argument.
E_WRONGARGS=65

if [ number-of-arguments is-not-equal-to "$ARGCOUNT" ]
#    ^^^^^^^^^^^^^^^^^^^ ^^^^^^^^^^^^^^^
#  Can't figure out how to code this . . .
#+ . . . so write it in pseudo-code.

then
  echo "Usage: name-of-script name"
  #            ^^^^^^^^^^^^^^     More pseudo-code.
  exit $E_WRONGARGS
fi 

. . .

exit 0


# Later on, substitute working code for the pseudo-code.

# Line 6 becomes:
if [ $# -ne "$ARGCOUNT" ]

# Line 12 becomes:
  echo "Usage: `basename $0` name"

有关使用伪代码的示例，请参见平方根练习。

要记录在特定会话期间或多个会话中运行了哪些用户脚本，请将以下行添加到您要跟踪的每个脚本中。这将持续记录脚本名称和调用时间的文件记录。

# Append (>>) following to end of each script tracked.

whoami>> $SAVE_FILE    # User invoking the script.
echo $0>> $SAVE_FILE   # Script name.
date>> $SAVE_FILE      # Date and time.
echo>> $SAVE_FILE      # Blank line as separator.

#  Of course, SAVE_FILE defined and exported as environmental variable in ~/.bashrc
#+ (something like ~/.scripts-run)

>> 运算符将行追加到文件。如果您希望将行前置到现有文件，即将其粘贴在开头，该怎么办？

file=data.txt
title="***This is the title line of data text file***"

echo $title | cat - $file >$file.new
# "cat -" concatenates stdout to $file.
#  End result is
#+ to write a new file with $title appended at *beginning*.

这是前面给出的示例 19-13 脚本的简化变体。当然，sed 也可以做到这一点。

shell 脚本可以充当另一个 shell 脚本、Tcl 或 wish 脚本，甚至 Makefile 中的嵌入式命令。可以使用 C 程序中的外部 shell 命令调用它，使用system()调用，即，system("script_name");.
将变量设置为嵌入式 sed 或 awk 脚本的内容可以提高周围 shell 包装器的可读性。请参见示例 A-1 和示例 15-20。

将包含您最喜欢和最有用的定义和函数的文件放在一起。必要时，使用点 (.) 或 source 命令将一个或多个这些“库文件”“包含”在脚本中。

# SCRIPT LIBRARY
# ------ -------

# Note:
# No "#!" here.
# No "live code" either.


# Useful variable definitions

ROOT_UID=0             # Root has $UID 0.
E_NOTROOT=101          # Not root user error. 
MAXRETVAL=255          # Maximum (positive) return value of a function.
SUCCESS=0
FAILURE=-1



# Functions

Usage ()               # "Usage:" message.
{
  if [ -z "$1" ]       # No arg passed.
  then
    msg=filename
  else
    msg=$@
  fi

  echo "Usage: `basename $0` "$msg""
}  


Check_if_root ()       # Check if root running script.
{                      # From "ex39.sh" example.
  if [ "$UID" -ne "$ROOT_UID" ]
  then
    echo "Must be root to run this script."
    exit $E_NOTROOT
  fi
}  


CreateTempfileName ()  # Creates a "unique" temp filename.
{                      # From "ex51.sh" example.
  prefix=temp
  suffix=`eval date +%s`
  Tempfilename=$prefix.$suffix
}


isalpha2 ()            # Tests whether *entire string* is alphabetic.
{                      # From "isalpha.sh" example.
  [ $# -eq 1 ] || return $FAILURE

  case $1 in
  *[!a-zA-Z]*|"") return $FAILURE;;
  *) return $SUCCESS;;
  esac                 # Thanks, S.C.
}


abs ()                           # Absolute value.
{                                # Caution: Max return value = 255.
  E_ARGERR=-999999

  if [ -z "$1" ]                 # Need arg passed.
  then
    return $E_ARGERR             # Obvious error value returned.
  fi

  if [ "$1" -ge 0 ]              # If non-negative,
  then                           #
    absval=$1                    # stays as-is.
  else                           # Otherwise,
    let "absval = (( 0 - $1 ))"  # change sign.
  fi  

  return $absval
}


tolower ()             #  Converts string(s) passed as argument(s)
{                      #+ to lowercase.

  if [ -z "$1" ]       #  If no argument(s) passed,
  then                 #+ send error message
    echo "(null)"      #+ (C-style void-pointer error message)
    return             #+ and return from function.
  fi  

  echo "$@" | tr A-Z a-z
  # Translate all passed arguments ($@).

  return

# Use command substitution to set a variable to function output.
# For example:
#    oldvar="A seT of miXed-caSe LEtTerS"
#    newvar=`tolower "$oldvar"`
#    echo "$newvar"    # a set of mixed-case letters
#
# Exercise: Rewrite this function to change lowercase passed argument(s)
#           to uppercase ... toupper()  [easy].
}

使用专门用途的注释标头来提高脚本的清晰度和可读性。

## Caution.
rm -rf *.zzy   ##  The "-rf" options to "rm" are very dangerous,
               ##+ especially with wild cards.

#+ Line continuation.
#  This is line 1
#+ of a multi-line comment,
#+ and this is the final line.

#* Note.

#o List item.

#> Another point of view.
while [ "$var1" != "end" ]    #> while test "$var1" != "end"

Dotan Barak 贡献了脚本中 进度条的模板代码。

示例 36-17. 进度条

#!/bin/bash
# progress-bar.sh

# Author: Dotan Barak (very minor revisions by ABS Guide author).
# Used in ABS Guide with permission (thanks!).


BAR_WIDTH=50
BAR_CHAR_START="["
BAR_CHAR_END="]"
BAR_CHAR_EMPTY="."
BAR_CHAR_FULL="="
BRACKET_CHARS=2
LIMIT=100

print_progress_bar()
{
        # Calculate how many characters will be full.
        let "full_limit = ((($1 - $BRACKET_CHARS) * $2) / $LIMIT)"

        # Calculate how many characters will be empty.
        let "empty_limit = ($1 - $BRACKET_CHARS) - ${full_limit}"

        # Prepare the bar.
        bar_line="${BAR_CHAR_START}"
        for ((j=0; j<full_limit; j++)); do
                bar_line="${bar_line}${BAR_CHAR_FULL}"
        done

        for ((j=0; j<empty_limit; j++)); do
                bar_line="${bar_line}${BAR_CHAR_EMPTY}"
        done

        bar_line="${bar_line}${BAR_CHAR_END}"

        printf "%3d%% %s" $2 ${bar_line}
}

# Here is a sample of code that uses it.
MAX_PERCENT=100
for ((i=0; i<=MAX_PERCENT; i++)); do
        #
        usleep 10000
        # ... Or run some other commands ...
        #
        print_progress_bar ${BAR_WIDTH} ${i}
        echo -en "\r"
done

echo ""

exit

if-test 结构的特别巧妙的用法是用于注释块。

#!/bin/bash

COMMENT_BLOCK=
#  Try setting the above variable to some value
#+ for an unpleasant surprise.

if [ $COMMENT_BLOCK ]; then

Comment block --
=================================
This is a comment line.
This is another comment line.
This is yet another comment line.
=================================

echo "This will not echo."

Comment blocks are error-free! Whee!

fi

echo "No more comments, please."

exit 0

将其与使用 here 文档注释掉代码块进行比较。

使用 $? 退出状态变量，脚本可以测试参数是否仅包含数字，以便可以将其视为整数。

#!/bin/bash

SUCCESS=0
E_BADINPUT=85

test "$1" -ne 0 -o "$1" -eq 0 2>/dev/null
# An integer is either equal to 0 or not equal to 0.
# 2>/dev/null suppresses error message.

if [ $? -ne "$SUCCESS" ]
then
  echo "Usage: `basename $0` integer-input"
  exit $E_BADINPUT
fi

let "sum = $1 + 25"             # Would give error if $1 not integer.
echo "Sum = $sum"

# Any variable, not just a command-line parameter, can be tested this way.

exit 0

函数返回值的 0 - 255 范围是一个严重的限制。全局变量和其他解决方法通常存在问题。函数将值传回脚本主体的另一种方法是让函数写入stdout（通常使用 echo）“返回值”，并将此值分配给变量。这实际上是命令替换的一种变体。

示例 36-18. 返回值技巧

#!/bin/bash
# multiplication.sh

multiply ()                     # Multiplies params passed.
{                               # Will accept a variable number of args.

  local product=1

  until [ -z "$1" ]             # Until uses up arguments passed...
  do
    let "product *= $1"
    shift
  done

  echo $product                 #  Will not echo to stdout,
}                               #+ since this will be assigned to a variable.

mult1=15383; mult2=25211
val1=`multiply $mult1 $mult2`
# Assigns stdout (echo) of function to the variable val1.
echo "$mult1 X $mult2 = $val1"                   # 387820813

mult1=25; mult2=5; mult3=20
val2=`multiply $mult1 $mult2 $mult3`
echo "$mult1 X $mult2 X $mult3 = $val2"          # 2500

mult1=188; mult2=37; mult3=25; mult4=47
val3=`multiply $mult1 $mult2 $mult3 $mult4`
echo "$mult1 X $mult2 X $mult3 X $mult4 = $val3" # 8173300

exit 0

相同的技术也适用于字母数字字符串。这意味着函数可以“返回”非数值。

capitalize_ichar ()          #  Capitalizes initial character
{                            #+ of argument string(s) passed.

  string0="$@"               # Accepts multiple arguments.

  firstchar=${string0:0:1}   # First character.
  string1=${string0:1}       # Rest of string(s).

  FirstChar=`echo "$firstchar" | tr a-z A-Z`
                             # Capitalize first character.

  echo "$FirstChar$string1"  # Output to stdout.

}  

newstring=`capitalize_ichar "every sentence should start with a capital letter."`
echo "$newstring"          # Every sentence should start with a capital letter.

甚至函数可以使用此方法“返回”多个值。

示例 36-19. 更多返回值技巧

#!/bin/bash
# sum-product.sh
# A function may "return" more than one value.

sum_and_product ()   # Calculates both sum and product of passed args.
{
  echo $(( $1 + $2 )) $(( $1 * $2 ))
# Echoes to stdout each calculated value, separated by space.
}

echo
echo "Enter first number "
read first

echo
echo "Enter second number "
read second
echo

retval=`sum_and_product $first $second`      # Assigns output of function.
sum=`echo "$retval" | awk '{print $1}'`      # Assigns first field.
product=`echo "$retval" | awk '{print $2}'`  # Assigns second field.

echo "$first + $second = $sum"
echo "$first * $second = $product"
echo

exit 0

要使其工作，函数中只能有一个 echo 语句。如果您更改前面的示例

sum_and_product ()
{
  echo "This is the sum_and_product function." # This messes things up!
  echo $(( $1 + $2 )) $(( $1 * $2 ))
}
...
retval=`sum_and_product $first $second`      # Assigns output of function.
# Now, this will not work correctly.

我们技巧包中的下一个技巧是将数组传递给函数，然后“返回”数组回到脚本主体。

传递数组涉及使用命令替换将数组的空格分隔的元素加载到变量中。从函数中获取作为“返回值”的数组使用先前提到过的策略，即在函数中 echoing 数组，然后调用命令替换和 ( ... ) 运算符将其分配给数组。

示例 36-20. 传递和返回数组

#!/bin/bash
# array-function.sh: Passing an array to a function and ...
#                   "returning" an array from a function


Pass_Array ()
{
  local passed_array   # Local variable!
  passed_array=( `echo "$1"` )
  echo "${passed_array[@]}"
  #  List all the elements of the new array
  #+ declared and set within the function.
}


original_array=( element1 element2 element3 element4 element5 )

echo
echo "original_array = ${original_array[@]}"
#                      List all elements of original array.


# This is the trick that permits passing an array to a function.
# **********************************
argument=`echo ${original_array[@]}`
# **********************************
#  Pack a variable
#+ with all the space-separated elements of the original array.
#
# Attempting to just pass the array itself will not work.


# This is the trick that allows grabbing an array as a "return value".
# *****************************************
returned_array=( `Pass_Array "$argument"` )
# *****************************************
# Assign 'echoed' output of function to array variable.

echo "returned_array = ${returned_array[@]}"

echo "============================================================="

#  Now, try it again,
#+ attempting to access (list) the array from outside the function.
Pass_Array "$argument"

# The function itself lists the array, but ...
#+ accessing the array from outside the function is forbidden.
echo "Passed array (within function) = ${passed_array[@]}"
# NULL VALUE since the array is a variable local to the function.

echo

############################################

# And here is an even more explicit example:

ret_array ()
{
  for element in {11..20}
  do
    echo "$element "   #  Echo individual elements
  done                 #+ of what will be assembled into an array.
}

arr=( $(ret_array) )   #  Assemble into array.

echo "Capturing array \"arr\" from function ret_array () ..."
echo "Third element of array \"arr\" is ${arr[2]}."   # 13  (zero-indexed)
echo -n "Entire array is: "
echo ${arr[@]}                # 11 12 13 14 15 16 17 18 19 20

echo

exit 0

#  Nathan Coulter points out that passing arrays with elements containing
#+ whitespace breaks this example.

有关将数组传递给函数的更详细示例，请参见示例 A-10。

使用双括号结构，可以使用 C 风格的语法来设置和递增/递减变量，以及在 for 和 while 循环中。请参见示例 11-13 和示例 11-18。

在脚本开头设置 path 和 umask 使其更具可移植性——更可能在“外国”机器上运行，该机器的用户可能搞砸了$PATH和 umask。

#!/bin/bash
PATH=/bin:/usr/bin:/usr/local/bin ; export PATH
umask 022   # Files that the script creates will have 755 permission.

# Thanks to Ian D. Allen, for this tip.

一个有用的脚本编写技术是重复地将过滤器的输出（通过管道）反馈到同一个过滤器，但使用不同的参数和/或选项。特别适合这样做的是 tr 和 grep。

# From "wstrings.sh" example.

wlist=`strings "$1" | tr A-Z a-z | tr '[:space:]' Z | \
tr -cs '[:alpha:]' Z | tr -s '\173-\377' Z | tr Z ' '`

示例 36-21. 字谜的乐趣

#!/bin/bash
# agram.sh: Playing games with anagrams.

# Find anagrams of...
LETTERSET=etaoinshrdlu
FILTER='.......'       # How many letters minimum?
#       1234567

anagram "$LETTERSET" | # Find all anagrams of the letterset...
grep "$FILTER" |       # With at least 7 letters,
grep '^is' |           # starting with 'is'
grep -v 's$' |         # no plurals
grep -v 'ed$'          # no past tense verbs
# Possible to add many combinations of conditions and filters.

#  Uses "anagram" utility
#+ that is part of the author's "yawl" word list package.
#  http://ibiblio.org/pub/Linux/libs/yawl-0.3.2.tar.gz
#  http://bash.deta.in/yawl-0.3.2.tar.gz

exit 0                 # End of code.


bash$ sh agram.sh
islander
isolate
isolead
isotheral



#  Exercises:
#  ---------
#  Modify this script to take the LETTERSET as a command-line parameter.
#  Parameterize the filters in lines 11 - 13 (as with $FILTER),
#+ so that they can be specified by passing arguments to a function.

#  For a slightly different approach to anagramming,
#+ see the agram2.sh script.

另请参见示例 29-4、示例 16-25 和示例 A-9。

使用 “匿名 here 文档”来注释掉代码块，以避免不得不使用 # 单独注释掉每一行。请参见示例 19-11。

在依赖于可能未安装的命令的机器上运行脚本是危险的。使用 whatis 以避免潜在的问题。

CMD=command1                 # First choice.
PlanB=command2               # Fallback option.

command_test=$(whatis "$CMD" | grep 'nothing appropriate')
#  If 'command1' not found on system , 'whatis' will return
#+ "command1: nothing appropriate."
#
#  A safer alternative is:
#     command_test=$(whereis "$CMD" | grep \/)
#  But then the sense of the following test would have to be reversed,
#+ since the $command_test variable holds content only if
#+ the $CMD exists on the system.
#     (Thanks, bojster.)


if [[ -z "$command_test" ]]  # Check whether command present.
then
  $CMD option1 option2       #  Run command1 with options.
else                         #  Otherwise,
  $PlanB                     #+ run command2. 
fi

if-grep 测试在错误情况下可能不会返回预期的结果，此时文本输出到stderr，而不是stdout.

if ls -l nonexistent_filename | grep -q 'No such file or directory'
  then echo "File \"nonexistent_filename\" does not exist."
fi

重定向 stderr到stdout修复了这个问题。

if ls -l nonexistent_filename 2>&1 | grep -q 'No such file or directory'
#                             ^^^^
  then echo "File \"nonexistent_filename\" does not exist."
fi

# Thanks, Chris Martin, for pointing this out.

如果您绝对必须访问子 shell 外部的子 shell 变量，这是一种方法。

TMPFILE=tmpfile                  # Create a temp file to store the variable.

(   # Inside the subshell ...
inner_variable=Inner
echo $inner_variable
echo $inner_variable >>$TMPFILE  # Append to temp file.
)

    # Outside the subshell ...

echo; echo "-----"; echo
echo $inner_variable             # Null, as expected.
echo "-----"; echo

# Now ...
read inner_variable <$TMPFILE    # Read back shell variable.
rm -f "$TMPFILE"                 # Get rid of temp file.
echo "$inner_variable"           # It's an ugly kludge, but it works.

run-parts 命令对于按特定顺序运行一组命令脚本非常方便，尤其是在与 cron 或 at 结合使用时。
为了对复杂脚本进行多次修订，请使用 rcs 修订控制系统软件包。
这样做的好处之一是自动更新的 ID 标头标签。rcs 中的 co 命令执行某些保留关键字的参数替换，例如，替换# $Id$在脚本中，类似于
# $Id: hello-world.sh,v 1.1 2004/10/16 02:43:05 bozo Exp $

36.7.2. 组件

如果能够从 shell 脚本调用 X-Windows 组件就好了。碰巧存在几个声称可以做到这一点的软件包，即 Xscript、Xmenu 和 widtools。前两个似乎不再维护。幸运的是，仍然可以在这里获取 widtools。

widtools（组件工具）软件包需要安装 XForms 库。此外，在典型的 Linux 系统上构建软件包之前，需要对 Makefile 进行一些明智的编辑。最后，提供的六个组件中有三个不起作用（实际上，会段错误）。

dialog 工具系列提供了一种从 shell 脚本调用“dialog”组件的方法。原始的 dialog 实用程序在文本控制台中工作，但其后继者 gdialog、Xdialog 和 kdialog 使用基于 X-Windows 的组件集。

示例 36-22. 从 shell 脚本调用的组件

#!/bin/bash
# dialog.sh: Using 'gdialog' widgets.

# Must have 'gdialog' installed on your system to run this script.
# Or, you can replace all instance of 'gdialog' below with 'kdialog' ...
# Version 1.1 (corrected 04/05/05)

# This script was inspired by the following article.
#     "Scripting for X Productivity," by Marco Fioretti,
#      LINUX JOURNAL, Issue 113, September 2003, pp. 86-9.
# Thank you, all you good people at LJ.


# Input error in dialog box.
E_INPUT=85
# Dimensions of display, input widgets.
HEIGHT=50
WIDTH=60

# Output file name (constructed out of script name).
OUTFILE=$0.output

# Display this script in a text widget.
gdialog --title "Displaying: $0" --textbox $0 $HEIGHT $WIDTH



# Now, we'll try saving input in a file.
echo -n "VARIABLE=" > $OUTFILE
gdialog --title "User Input" --inputbox "Enter variable, please:" \
$HEIGHT $WIDTH 2>> $OUTFILE


if [ "$?" -eq 0 ]
# It's good practice to check exit status.
then
  echo "Executed \"dialog box\" without errors."
else
  echo "Error(s) in \"dialog box\" execution."
        # Or, clicked on "Cancel", instead of "OK" button.
  rm $OUTFILE
  exit $E_INPUT
fi



# Now, we'll retrieve and display the saved variable.
. $OUTFILE   # 'Source' the saved file.
echo "The variable input in the \"input box\" was: "$VARIABLE""


rm $OUTFILE  # Clean up by removing the temp file.
             # Some applications may need to retain this file.

exit $?

# Exercise: Rewrite this script using the 'zenity' widget set.

xmessage 命令是弹出一个消息/查询窗口的简单方法。例如

xmessage Fatal error in script! -button exit

组件竞赛中的最新条目是 zenity。此实用程序弹出 GTK+ 对话框组件和窗口，并且在脚本中运行良好。

get_info ()
{
  zenity --entry       #  Pops up query window . . .
                       #+ and prints user entry to stdout.

                       #  Also try the --calendar and --scale options.
}

answer=$( get_info )   #  Capture stdout in $answer variable.

echo "User entered: "$answer""

有关使用组件进行脚本编写的其他方法，请尝试 Tk 或 wish（Tcl 衍生品）、PerlTk（带有 Tk 扩展的 Perl）、tksh（带有 Tk 扩展的 ksh）、XForms4Perl（带有 XForms 扩展的 Perl）、Gtk-Perl（带有 Gtk 扩展的 Perl）或 PyQt（带有 Qt 扩展的 Python）。

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