11.1. 循环

for 循环

for参数in[列表]

这是基本的循环结构。它与 C 语言中的对应结构有显著不同。

for 参数in [列表]
do
�command(s)...
done

在每次循环过程中，参数会取 [列表] 中每个连续变量的值。列表.

for arg in "$var1" "$var2" "$var3" ... "$varN" # In pass 1 of the loop, arg = $var1 # In pass 2 of the loop, arg = $var2 # In pass 3 of the loop, arg = $var3 # ... # In pass N of the loop, arg = $varN # Arguments in [list] quoted to prevent possible word splitting.

参数列表可能包含通配符。

如果 do 和 for 在同一行，则需要在列表后加分号。

for 参数in [列表] ; do

例 11-1. 简单的 for 循环

#!/bin/bash # Listing the planets. for planet in Mercury Venus Earth Mars Jupiter Saturn Uranus Neptune Pluto do echo $planet # Each planet on a separate line. done echo; echo for planet in "Mercury Venus Earth Mars Jupiter Saturn Uranus Neptune Pluto" # All planets on same line. # Entire 'list' enclosed in quotes creates a single variable. # Why? Whitespace incorporated into the variable. do echo $planet done echo; echo "Whoops! Pluto is no longer a planet!" exit 0

每个[列表]元素可能包含多个参数。这在分组处理参数时很有用。 在这种情况下，使用 set 命令（参见 例 15-16）强制解析每个[列表]元素并将每个组件分配给位置参数。

例 11-2. for 循环，每个 [列表] 元素包含两个参数

#!/bin/bash # Planets revisited. # Associate the name of each planet with its distance from the sun. for planet in "Mercury 36" "Venus 67" "Earth 93" "Mars 142" "Jupiter 483" do set -- $planet # Parses variable "planet" #+ and sets positional parameters. # The "--" prevents nasty surprises if $planet is null or #+ begins with a dash. # May need to save original positional parameters, #+ since they get overwritten. # One way of doing this is to use an array, # original_params=("$@") echo "$1 $2,000,000 miles from the sun" #-------two tabs---concatenate zeroes onto parameter $2 done # (Thanks, S.C., for additional clarification.) exit 0

变量可以提供[列表]在 for 循环 中的 [列表]。

例 11-3. Fileinfo: 操作包含在变量中的文件列表

#!/bin/bash # fileinfo.sh FILES="/usr/sbin/accept /usr/sbin/pwck /usr/sbin/chroot /usr/bin/fakefile /sbin/badblocks /sbin/ypbind" # List of files you are curious about. # Threw in a dummy file, /usr/bin/fakefile. echo for file in $FILES do if [ ! -e "$file" ] # Check if file exists. then echo "$file does not exist."; echo continue # On to next. fi ls -l $file | awk '{ print $8 " file size: " $5 }' # Print 2 fields. whatis `basename $file` # File info. # Note that the whatis database needs to have been set up for this to work. # To do this, as root run /usr/bin/makewhatis. echo done exit 0

在 for 循环 中的 [列表][列表]可以是参数化的。

例 11-4. 操作参数化的文件列表

#!/bin/bash filename="*txt" for file in $filename do echo "Contents of $file" echo "---" cat "$file" echo done

如果在 for 循环 中的 [列表][列表]包含通配符 (* 和 ?) 用于文件名扩展，则会发生 globbing。

例 11-5. 使用 for 循环操作文件

#!/bin/bash # list-glob.sh: Generating [list] in a for-loop, using "globbing" ... # Globbing = filename expansion. echo for file in * # ^ Bash performs filename expansion #+ on expressions that globbing recognizes. do ls -l "$file" # Lists all files in $PWD (current directory). # Recall that the wild card character "*" matches every filename, #+ however, in "globbing," it doesn't match dot-files. # If the pattern matches no file, it is expanded to itself. # To prevent this, set the nullglob option #+ (shopt -s nullglob). # Thanks, S.C. done echo; echo for file in [jx]* do rm -f $file # Removes only files beginning with "j" or "x" in $PWD. echo "Removed file \"$file\"". done echo exit 0

省略 for 循环 的中的 [列表]部分会导致循环操作 $@ -- 位置参数。 例 A-15 是对此的一个特别巧妙的说明。另请参见 例 15-17。

例 11-6. 缺失 for 循环 中的中的 [列表][列表]

#!/bin/bash # Invoke this script both with and without arguments, #+ and see what happens. for a do echo -n "$a " done # The 'in list' missing, therefore the loop operates on '$@' #+ (command-line argument list, including whitespace). echo exit 0

可以使用命令替换来生成 for 循环 中的[列表][列表]。 另请参见 例 16-54、例 11-11 和 例 16-48。

例 11-7. 生成 for 循环 的[列表][列表]，使用命令替换

#!/bin/bash # for-loopcmd.sh: for-loop with [list] #+ generated by command substitution. NUMBERS="9 7 3 8 37.53" for number in `echo $NUMBERS` # for number in 9 7 3 8 37.53 do echo -n "$number " done echo exit 0

这是一个使用命令替换创建 for 循环 的[列表].

例 11-8. 二进制文件的 grep 替代方案

#!/bin/bash # bin-grep.sh: Locates matching strings in a binary file. # A "grep" replacement for binary files. # Similar effect to "grep -a" E_BADARGS=65 E_NOFILE=66 if [ $# -ne 2 ] then echo "Usage: `basename $0` search_string filename" exit $E_BADARGS fi if [ ! -f "$2" ] then echo "File \"$2\" does not exist." exit $E_NOFILE fi IFS=$'\012' # Per suggestion of Anton Filippov. # was: IFS="\n" for word in $( strings "$2" | grep "$1" ) # The "strings" command lists strings in binary files. # Output then piped to "grep", which tests for desired string. do echo $word done # As S.C. points out, lines 23 - 30 could be replaced with the simpler # strings "$2" | grep "$1" | tr -s "$IFS" '[\n*]' # Try something like "./bin-grep.sh mem /bin/ls" #+ to exercise this script. exit 0

更多类似示例。

例 11-9. 列出系统上的所有用户

#!/bin/bash # userlist.sh PASSWORD_FILE=/etc/passwd n=1 # User number for name in $(awk 'BEGIN{FS=":"}{print $1}' < "$PASSWORD_FILE" ) # Field separator = : ^^^^^^ # Print first field ^^^^^^^^ # Get input from password file /etc/passwd ^^^^^^^^^^^^^^^^^ do echo "USER #$n = $name" let "n += 1" done # USER #1 = root # USER #2 = bin # USER #3 = daemon # ... # USER #33 = bozo exit $? # Discussion: # ---------- # How is it that an ordinary user, or a script run by same, #+ can read /etc/passwd? (Hint: Check the /etc/passwd file permissions.) # Is this a security hole? Why or why not?

又一个[列表]由命令替换产生的 [列表] 的示例。

例 11-10. 检查目录中所有二进制文件的作者身份

#!/bin/bash # findstring.sh: # Find a particular string in the binaries in a specified directory. directory=/usr/bin/ fstring="Free Software Foundation" # See which files come from the FSF. for file in $( find $directory -type f -name '*' | sort ) do strings -f $file | grep "$fstring" | sed -e "s%$directory%%" # In the "sed" expression, #+ it is necessary to substitute for the normal "/" delimiter #+ because "/" happens to be one of the characters filtered out. # Failure to do so gives an error message. (Try it.) done exit $? # Exercise (easy): # --------------- # Convert this script to take command-line parameters #+ for $directory and $fstring.

最后一个关于[列表]命令替换的示例，但这次 "命令" 是一个 函数。

generate_list () { echo "one two three" } for word in $(generate_list) # Let "word" grab output of function. do echo "$word" done # one # two # three

for 循环 的输出可以通过管道传递给一个或多个命令。

例 11-11. 列出目录中的 符号链接

#!/bin/bash # symlinks.sh: Lists symbolic links in a directory. directory=${1-`pwd`} # Defaults to current working directory, #+ if not otherwise specified. # Equivalent to code block below. # ---------------------------------------------------------- # ARGS=1 # Expect one command-line argument. # # if [ $# -ne "$ARGS" ] # If not 1 arg... # then # directory=`pwd` # current working directory # else # directory=$1 # fi # ---------------------------------------------------------- echo "symbolic links in directory \"$directory\"" for file in "$( find $directory -type l )" # -type l = symbolic links do echo "$file" done | sort # Otherwise file list is unsorted. # Strictly speaking, a loop isn't really necessary here, #+ since the output of the "find" command is expanded into a single word. # However, it's easy to understand and illustrative this way. # As Dominik 'Aeneas' Schnitzer points out, #+ failing to quote $( find $directory -type l ) #+ will choke on filenames with embedded whitespace. # containing whitespace. exit 0 # -------------------------------------------------------- # Jean Helou proposes the following alternative: echo "symbolic links in directory \"$directory\"" # Backup of the current IFS. One can never be too cautious. OLDIFS=$IFS IFS=: for file in $(find $directory -type l -printf "%p$IFS") do # ^^^^^^^^^^^^^^^^ echo "$file" done|sort # And, James "Mike" Conley suggests modifying Helou's code thusly: OLDIFS=$IFS IFS='' # Null IFS means no word breaks for file in $( find $directory -type l ) do echo $file done | sort # This works in the "pathological" case of a directory name having #+ an embedded colon. # "This also fixes the pathological case of the directory name having #+ a colon (or space in earlier example) as well."

在 for 循环 中的 [列表]stdout循环的 stdout 可以重定向到一个文件，如前一个示例的稍作修改的版本所示。

例 11-12. 目录中的符号链接，保存到文件

#!/bin/bash # symlinks.sh: Lists symbolic links in a directory. OUTFILE=symlinks.list # save-file directory=${1-`pwd`} # Defaults to current working directory, #+ if not otherwise specified. echo "symbolic links in directory \"$directory\"" > "$OUTFILE" echo "---------------------------" >> "$OUTFILE" for file in "$( find $directory -type l )" # -type l = symbolic links do echo "$file" done | sort >> "$OUTFILE" # stdout of loop # ^^^^^^^^^^^^^ redirected to save file. # echo "Output file = $OUTFILE" exit $?

for 循环 还有一种替代语法，C 程序员会非常熟悉。 这需要双括号。

例 11-13. C 风格的 for 循环

#!/bin/bash # Multiple ways to count up to 10. echo # Standard syntax. for a in 1 2 3 4 5 6 7 8 9 10 do echo -n "$a " done echo; echo # +==========================================+ # Using "seq" ... for a in `seq 10` do echo -n "$a " done echo; echo # +==========================================+ # Using brace expansion ... # Bash, version 3+. for a in {1..10} do echo -n "$a " done echo; echo # +==========================================+ # Now, let's do the same, using C-like syntax. LIMIT=10 for ((a=1; a <= LIMIT ; a++)) # Double parentheses, and naked "LIMIT" do echo -n "$a " done # A construct borrowed from ksh93. echo; echo # +=========================================================================+ # Let's use the C "comma operator" to increment two variables simultaneously. for ((a=1, b=1; a <= LIMIT ; a++, b++)) do # The comma concatenates operations. echo -n "$a-$b " done echo; echo exit 0

另请参见 例 27-16、例 27-17 和 例 A-6。

---

现在，一个在 "真实" 环境中使用的 for 循环。

例 11-14. 在批处理模式下使用 efax

#!/bin/bash # Faxing (must have 'efax' package installed). EXPECTED_ARGS=2 E_BADARGS=85 MODEM_PORT="/dev/ttyS2" # May be different on your machine. # ^^^^^ PCMCIA modem card default port. if [ $# -ne $EXPECTED_ARGS ] # Check for proper number of command-line args. then echo "Usage: `basename $0` phone# text-file" exit $E_BADARGS fi if [ ! -f "$2" ] then echo "File $2 is not a text file." # File is not a regular file, or does not exist. exit $E_BADARGS fi fax make $2 # Create fax-formatted files from text files. for file in $(ls $2.0*) # Concatenate the converted files. # Uses wild card (filename "globbing") #+ in variable list. do fil="$fil $file" done efax -d "$MODEM_PORT" -t "T$1" $fil # Finally, do the work. # Trying adding -o1 if above line fails. # As S.C. points out, the for-loop can be eliminated with # efax -d /dev/ttyS2 -o1 -t "T$1" $2.0* #+ but it's not quite as instructive [grin]. exit $? # Also, efax sends diagnostic messages to stdout.

关键字 do 和 done 划定了 for 循环 命令块的范围。 但是，在某些情况下，可以通过将命令块放在花括号中来省略它们 for((n=1; n<=10; n++)) # No do! { echo -n "* $n *" } # No done! # Outputs: # * 1 ** 2 ** 3 ** 4 ** 5 ** 6 ** 7 ** 8 ** 9 ** 10 * # And, echo $? returns 0, so Bash does not register an error. echo # But, note that in a classic for-loop: for n in [list] ... #+ a terminal semicolon is required. for n in 1 2 3 { echo -n "$n "; } # ^ # Thank you, YongYe, for pointing this out.

while

此结构在循环顶部测试条件，并只要该条件为真（返回 0 退出状态）就保持循环。 与 for 循环 相比，while 循环 适用于循环重复次数事先未知的情况。

while [条件]
do
�command(s)...
done

while 循环 中的括号结构只不过是我们的老朋友，if/then 测试中使用的测试括号。 实际上，while 循环 可以合法地使用更通用的 双括号结构 (while [[ condition ]])。

与 for 循环 的情况一样，将 do 与条件测试放在同一行需要分号。

while [条件] ; do

请注意，测试括号 在 while 循环中不是强制性的。 例如，参见 getopts 结构。

例 11-15. 简单的 while 循环

#!/bin/bash var0=0 LIMIT=10 while [ "$var0" -lt "$LIMIT" ] # ^ ^ # Spaces, because these are "test-brackets" . . . do echo -n "$var0 " # -n suppresses newline. # ^ Space, to separate printed out numbers. var0=`expr $var0 + 1` # var0=$(($var0+1)) also works. # var0=$((var0 + 1)) also works. # let "var0 += 1" also works. done # Various other methods also work. echo exit 0

例 11-16. 另一个 while 循环

#!/bin/bash echo # Equivalent to: while [ "$var1" != "end" ] # while test "$var1" != "end" do echo "Input variable #1 (end to exit) " read var1 # Not 'read $var1' (why?). echo "variable #1 = $var1" # Need quotes because of "#" . . . # If input is 'end', echoes it here. # Does not test for termination condition until top of loop. echo done exit 0

while 循环 可能有多个条件。 只有最后一个条件决定循环何时终止。 然而，这需要稍微不同的循环语法。

例 11-17. 具有多个条件的 while 循环

#!/bin/bash var1=unset previous=$var1 while echo "previous-variable = $previous" echo previous=$var1 [ "$var1" != end ] # Keeps track of what $var1 was previously. # Four conditions on *while*, but only the final one controls loop. # The *last* exit status is the one that counts. do echo "Input variable #1 (end to exit) " read var1 echo "variable #1 = $var1" done # Try to figure out how this all works. # It's a wee bit tricky. exit 0

与 for 循环 一样，while 循环 也可以通过使用双括号结构来采用 C 风格的语法（另请参见 例 8-5）。

例 11-18. while 循环中的 C 风格语法

#!/bin/bash # wh-loopc.sh: Count to 10 in a "while" loop. LIMIT=10 # 10 iterations. a=1 while [ "$a" -le $LIMIT ] do echo -n "$a " let "a+=1" done # No surprises, so far. echo; echo # +=================================================================+ # Now, we'll repeat with C-like syntax. ((a = 1)) # a=1 # Double parentheses permit space when setting a variable, as in C. while (( a <= LIMIT )) # Double parentheses, do #+ and no "$" preceding variables. echo -n "$a " ((a += 1)) # let "a+=1" # Yes, indeed. # Double parentheses permit incrementing a variable with C-like syntax. done echo # C and Java programmers can feel right at home in Bash. exit 0

在其测试括号内，while 循环 可以调用一个 函数。

t=0 condition () { ((t++)) if [ $t -lt 5 ] then return 0 # true else return 1 # false fi } while condition # ^^^^^^^^^ # Function call -- four loop iterations. do echo "Still going: t = $t" done # Still going: t = 1 # Still going: t = 2 # Still going: t = 3 # Still going: t = 4

与 if-test 结构类似，while 循环 可以省略测试括号。 while condition do command(s) ... done

通过将 read 命令的功能与 while 循环 结合使用，我们得到了方便的 while read 结构，它对于读取和解析文件很有用。

cat $filename | # Supply input from a file. while read line # As long as there is another line to read ... do ... done # =========== Snippet from "sd.sh" example script ========== # while read value # Read one data point at a time. do rt=$(echo "scale=$SC; $rt + $value" | bc) (( ct++ )) done am=$(echo "scale=$SC; $rt / $ct" | bc) echo $am; return $ct # This function "returns" TWO values! # Caution: This little trick will not work if $ct > 255! # To handle a larger number of data points, #+ simply comment out the "return $ct" above. } <"$datafile" # Feed in data file.

while 循环 可以将其stdin 通过在其末尾添加 < 重定向到文件。 while 循环 可以将其stdin 通过管道提供.

until

此结构在循环顶部测试条件，并只要该条件为假（与 while 循环 相反）就保持循环。

until [条件为真]
do
�command(s)...
done

请注意，until 循环 在循环的顶部测试终止条件，这与某些编程语言中的类似结构不同。

与 for 循环 的情况一样，将 do 与条件测试放在同一行需要分号。

until [条件为真] ; do

例 11-19. until 循环

#!/bin/bash END_CONDITION=end until [ "$var1" = "$END_CONDITION" ] # Tests condition here, at top of loop. do echo "Input variable #1 " echo "($END_CONDITION to exit)" read var1 echo "variable #1 = $var1" echo done # --- # # As with "for" and "while" loops, #+ an "until" loop permits C-like test constructs. LIMIT=10 var=0 until (( var > LIMIT )) do # ^^ ^ ^ ^^ No brackets, no $ prefixing variables. echo -n "$var " (( var++ )) done # 0 1 2 3 4 5 6 7 8 9 10 exit 0