Shell 本身的計算能力非常有限，字串的處理都是靠外部指令來進行的，尤其是
最常遇見的檔案名稱處理，例如： cc 這個編譯器在編譯一個 C 檔案，如
example.c時，會產生中間檔案'example.o'，所以編譯器的執行過程中，
須將'example.c'去掉副檔名將字根'example'從檔案名稱中抽取出來，
再貼上'.o'字串，最終產生一個'example.o'的字串作為中間檔名。
$ x=supercalifragilisticexpialidocious
$ echo There are ${#x} characters in $x
#-----------------------------------------------------------
結果:
There are 34 characters in supercalifragilisticexpialidocious
Wed Apr 16 07:34:09 CST 2025
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Arithmetic Expansion
原始 Bourne Shell 是沒有數值運算功能的，都是要依靠外部指令來完成數值運算的，
非常不方便，效率也差，有鑑於此，POSIX 特地在其標準中加入有限的數值運算能力。
由於不方便更動其整個語言的架構，因此數值運算的功能也是藏在變數展開的過程中。
而運算式的語法與 C 語言的運算式相同。而數學運算式的符號如下：
%getname "Yao Nan" Lien
You have entered: Yao Nan Lien
=== Passing $* ===
First Name: Yao
Last Name: Nan
=== Passing $@ ===
First Name: Yao
Last Name: Nan
=== Passing "$*" ===
First Name: Yao Nan Lien
Last Name:
=== Passing "$@" ===
First Name: Yao Nan
Last Name: Lien
# ---------------------
# example single'\'
# ----------------------
full=Joe\ Smith
在一行的末尾接一個'\'
可將次行連接起來，
合併成同一行。
# ---------------------
# example '\' before NL
# ----------------------
echo this is a \
test
兩個連續的 Backslash '\\'
# ---------------------
# example '\\'
# ----------------------
echo \\ is one backslash
2. 單引號(' ')
所有特殊符號均失去特殊意義。
3. 雙引號(" ")
除了下列四個符號外，其他的特殊符號均失去特殊意義。
$
`
"
\
例
結果
x=this is a string
x="this is a string"
echo $x
echo "$x"
echo '$x'
echo "*"
echo '*'
is: not found
this is a string
this is a string
$x
*
*
Wed Apr 16 07:34:12 CST 2025
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Using Quote
雙引號(" ")之使用
當字串中間含有須展開之變數或指令代換時，用雙引號比
單引號方便許多，例如：
# -------------------------------------
# Script: singlequote
# -------------------------------------
name=Clinton
echo 'My name is '$name' and today is '`date`
# -------------------------------------
# Script: doublequote
# -------------------------------------
name=Clinton
echo "My name is $name and today is `date`"
# -------------------------------------
# Script: doublequote2
# -------------------------------------
name=Clinton
output="My name is $name and today is `date`"
#----------------------
# 印出目錄下所有檔案
#----------------------
for i in `ls`
do
echo "==== File Name: $i ===="
cat $i
done
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printf
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唯讀的使用者變數
唯讀的使用者變數
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Examples of Using Read
利用 'read' 從 /etc/passwd 檔案中抽取資訊
while IFS=: read user password uid realname homedir shell
do
echo user $user is $realname who prefers $shell
done < /etc/passwd
cat /etc/passwd |
while IFS=: read user password uid realname homedir shell
do
echo user $user is $realname who prefers $shell
done
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Examples of Using Read
複製整個目錄
1 利用 copy -r directory 指令
2. 利用下面兩個 script
# -----------------------------------------------
# Script cpdir v1
# -----------------------------------------------
find /a/b -type d -print | #Find all directories
sed 's;/a/b/;/home/b/;' | #Change name
sed 's/^/mkdir /' | #Insert mkdir command
sh -x #Execute
# -----------------------------------------------
# Script cpdir v2
# -----------------------------------------------
find /a/b -type d -print | #Find all directories
sed 's;/a/b/;/home/b/;' | #Change name
while read newdir #Read new directory name
do
mkdir $newdir #Make new directory
done
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流程控制
Wed Apr 16 07:34:14 CST 2025
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Target of Condition Test
Wed Apr 16 07:34:14 CST 2025
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POSIX Defined Exit Status
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Test
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流程控制 - if
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流程控制 - If 之例
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流程控制 - If 之例
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Nested if Structure
if
write stu1 < message
then
:
else
if mail stu1 < message
then
echo "mail sent instead"
else
echo "Could not sent instead"
fi
fi
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流程控制 - for 迴圈
流程控制 - for 迴圈
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流程控制 - for
流程控制 - for
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More Codes
for person in stu1 stu2 stu3
do
if
who | grep $person > /dev/null
then
write $person < message
else
mail $person < message
fi
done
&& 或 || 也可以用在
測試條件式 (test) 中連同作為 AND 及 OR ，連同 NOT 作為
邏輯運算的運算子 (logical operator)。
NOT
if ! grep QQ fileX > dev/null
then
echo QQ not found in fileX
fi
=
if grep QQ fileX > /dev/null
then
: # do nothing
else
echo QQ not found in fileX
fi
AND
if grep pattern1 myfile && grep pattern2 myfile
then
echo myfile contains both patterns
fi
OR
if grep pattern1 myfile || grep pattern2 myfile
then
echo Neither pattern1 nor pattern2 found in myfile
fi
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File Testing Statement
測試檔案性質 (Test File Status)
test -r filename
Exists and readable
test -w filename
Exists and writable
test -x filename
Exists and executable
test -f filename
Exists and regular
test -d filename
Exists and directory
test -s filename
Exists and nonzero size
例
for file
do
if test -f $file
then
if test -r $file
then
if test -x $file
then
echo "File $file is
readable and executable"
fi
fi
fi
done
for file
do
if [ -f $file ]
then
if [ -r $file ]
then
if [ -x $file ]
then
echo "File $file is
readable and executable"
fi
fi
fi
done
for file
do
if [ -f $file -a -r $file -a -x $file ]
then
echo File $file is readable and executable
fi
done
for file
do
[ -f $file -a -r $file -a -x $file ] && echo File $file is readable and executable
done
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String Comparison
test "s1" = "s2"
Strings equal
test "s1" != "s2"
Strings not equal
test -z "s1"
Length of string zero
test -n "s1"
Length of string nonzero
test "s1"
Length of string nonzero
"test" 所需的參數之間以空白隔開。
注意用雙引號(" ")保護參數，防止空字串破壞程式結構。
例 Script 1
例 Script 2
noset=""
test -z $noset
test "$noset" = a
q=`who -T | grep "^s8500" | cut -c10`
if test "$q" = "+"
then
write s8500 < note
else
mail s8500 < note
fi
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Numerical Comparison
test "$n1" -eq "$n2"
Equal
test "$n1" -ne "$n2"
Not equal
test "$n1" -gt "$n2"
Greater than
test "$n1" -ge "$n2"
Greater than or equal to
test "$n1" -lt "$n2"
Less than
test "$n1" -le "$n2"
Less than or equal to
例
# -----------------------------------------------
# countuser: count number of users on the system
# -----------------------------------------------
count=`who | wc -i`
if [ "$count" -le "20" ]
then
echo "$count" users - light load
else
echo "$count" users - heavy load
fi
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流程控制 - for 迴圈
流程控制 - for 迴圈
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流程控制 - for
流程控制 - for
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While and Until
Syntax
while
command_lines
do
command_lines
done
until
command_lines
do
command_lines
done
'while' will test the condition to do the first iteration
'until' will do the first iteration before test the condition
Begin
while marks loop start
Control
Evaluate return code from last command before do
Middle
execute all commands within body
End
done marks loop end, shell reads until end is found
例 Script: Appends stdin to file outfile
echo "
Enter lines of input (end with ^D):
"
while read x
do
echo $x
done >> outfile
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算數運算 (Arithmetic Expression)
Wed Apr 16 07:34:18 CST 2025
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流程控制 - break and continue
流程控制 - break and continue
Wed Apr 16 07:34:18 CST 2025
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流程控制 - Case
流程控制 - Case
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Shift
彈性的位置參數數目
如果使用者給的位置參數的數目並非固定，希望有彈性，可用下面
的方式做到：
#-------------------------------------
# Script: concatefile v2
#-------------------------------------
for i #若沒有敘明位置參數，相當於 for i in $*
do
echo "====== FILE NAME: $i ========"
cat -n $i # option -n 會將讀入的資料加上 line number
done > outfile
file=
verbose=
quiet=
long=
while [ $# -gt 0 ] #Loop until no args left
do
case $1 in #Check first arg
-f) file=$2
shift #Shift off "-f" so that shift at end gets value in $2
;;
-v) verbose=true; quiet= ;;
-q) quiet=true; verbose= ;;
-l) long=true ;;
--) shift #By convention, - - ends options
break
;;
-*) echo $0: $1: unrecognized option >&2 ;;
*) break ;; # Nonoption argument, break while loop
esac
shift #Set up for next iteration
done
# -----------------------------------------------
# Send emails to top 10 disk space usage users
# -----------------------------------------------
cd /home #Move to top of home directories
du -s * | #Generate raw disk usage
sort -nr | #Sort numerically, highest numbers first
sed 10q | #Stop after first 10 lines
while read amount name
do
mail -s "disk usage warning" $name << EOF
Greetings. You are one of the top 10 consumers of disk space
on the system. Your home directory uses $amount disk blocks.
Please clean up unneeded files, as soon as possible.
Thanks,
Your friendly neighborhood system administrator.
EOF
done
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Open a file for both input and output
program <> file
To open file for both reading and writing.
It is up to program to be aware of this and take advantage of it
not practically useful
An undocumented feature in V7 Bourne shell
Standardized in the 1992 POSIX standard,
on many systems /bin/sh doesn't support it.
Wed Apr 16 07:34:22 CST 2025
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File Descriptor Manipulation
更複雜的I/O轉向
#------ ------------------------------------------------
# sends standard output (file descriptor 1) to 'outfile'
# and standard error (file descriptor 2) to 'errfile'.
# -----------------------------------------------------
myscript 1> ourfile 2> errfile
# ---------------------------------------
# sends standard output to 'outfile'
# and throw away standard error.
# ---------------------------------------
myscript 1> ourfile 2> /dev/null
# ----------------------------------------------------------
# sends both standard output and standard error to 'outfile'
# ----------------------------------------------------------
myscript > ourfile 2>&1
# -------------------------------------------------------
# sends both standard output and standard error to pipe
# -------------------------------------------------------
myscript 2>&1 | nextscript
A pseudosignal used by the shell, indicating
that the standard output
has hung up; sending this signal logs you out.
2
INT
Sent by a <Del> keystroke; sends an interrupt to
the current program.
3
QUIT
Sent by a <Ctrl>\ keystroke; causes the current
program to abort,
leaving behind a core dump (for use in program debugging).
9
KILL
Cannot be trapped or ignored; forces the
receiving program to die.
11
SEGV
Indicates that a segmentation violation (a
memory fault within the
UNIX system) has occurred. This signal cannot be trapped or
ignored by a
shell; it invariably terminates the receiving process and causes
a core dump.
15
TERM
Terminates the receiving program. (This signal
should be used in
preference to Signal 9, because the receiving program can catch
it and carry
out some shutdown operation, for example closing open files;
Signal 9 forces
the process to die immediately.)