一个程序演示所有的shell编程知识

演示程序

#!/bin/sh -x
#由反引号括起来的也是一条命令，Shell先执行该命令，然后将输出结果立刻代换到当前命令行中。例如定义一个变量存放date命令的输出：
echo `date`
#命令代换也可以用$()表示：
echo $(date)

#如果一个变量叫做VARNAME，用${VARNAME}可以表示它的值，在不引起歧义的情况下也可以用$VARNAME表示它的值。通过以下例子比较这两种表示法的不同：
echo $SHELL
echo $SHELLabc
echo ${SHELL}abc

#单引号用于保持引号内所有字符的字面值，即使引号内的\和回车也不例外，但是字符串中不能出现单引号
echo '$SHELL'
echo "$SHELL"
echo '$SHELL
hello world'
#双引号用于保持引号内所有字符的字面值（回车也不例外），除以下情况外：
#$加变量名可以取变量的值
#反引号仍表示命令替换
#\$表示$的字面值
#\`表示`的字面值
#\"表示"的字面值
#\\表示\的字面值
echo "$SHELL
hello world"
echo "\$SHELL
hello world"

#命令test或[可以测试一个条件是否成立，如果测试结果为真，则该命令的Exit Status为0，如果测试结果为假，则命令的Exit Status为1
VAR=2
test $VAR -gt 1
echo $?

test $VAR -gt 3
echo $?

[ $VAR -gt 3 ]
echo $?
#存在Desktop目录且VAR等于abc
VAR=abc
[ -d Desktop -a $VAR = 'abc' ]
echo $?

#在Shell中用if、then、elif、else、fi这几条命令实现分支控制
echo "Is it morning? Please answer yes or no."
read YES_OR_NO
if [ "$YES_OR_NO" = "yes" ]; then
echo "Good morning!"
elif [ "$YES_OR_NO" = "no" ]; then
echo "Good afternoon!"
else
echo "Sorry, $YES_OR_NO not recognized. Enter yes or no."
exit 1
fi

#C语言的case只能匹配整型或字符型常量表达式，而Shell脚本的case可以匹配字符串和Wildcard，每个匹配分支可以有若干条命令，末尾为;;
echo "Is it morning? Please answer yes or no."
read YES_OR_NO
case "$YES_OR_NO" in
yes|y|Yes|YES)
echo "Good Morning!";;
[nN]*)
echo "Good Afternoon!";;
*)
echo "Sorry, $YES_OR_NO not recognized. Enter yes or no."
exit 1;;
esac

#FRUIT是一个循环变量，第一次循环$FRUIT的取值是apple，第二次取值是banana，第三次取值是pear
for FRUIT in apple banana pear; do
echo "I like $FRUIT"
done

#while的用法和C语言类似
echo "Enter password:"
read TRY
while [ "$TRY" != "secret" ]; do
echo "Sorry, try again"
read TRY
done

COUNTER=1
while [ "$COUNTER" -lt 10 ]; do
echo "COUNTER is $COUNTER"
COUNTER=$(($COUNTER+1))
done

#$0     相当于C语言main函数的argv[0]
echo "The program $0 is now running"
#$1、$2...       这些称为位置参数（Positional Parameter），相当于C语言main函数的argv[1]、argv[2]...
echo "The first parameter is $1"
echo "The second parameter is $2"
$@      表示参数列表"$1" "$2" ...，例如可以用在for循环中的in后面
for PARAM in $@; do
echo "PARAM $PARAM"
done

echo "The parameter list is $@"
#位置参数可以用shift命令左移
shift
echo "The first parameter is $1"
echo "The second parameter is $2"
echo "The parameter list is $@"

#数就像是迷你脚本，调用函数时可以传任意个参数，在函数内同样是用$0、$1、$2等变量来提取参数
#函数中的位置参数相当于函数的局部变量，改变这些变量并不会影响函数外面的$0、$1、$2等变量
#函数中可以用return命令返回，如果return后面跟一个数字则表示函数的Exit Status

is_directory()
{
DIR_NAME=$1
if [ ! -d $DIR_NAME ]; then
return 1
else
return 0
fi
}

for DIR in "$@"; do
if is_directory "$DIR"
then :
else
echo "$DIR doesn't exist. Creating it now..."
mkdir $DIR > /dev/null 2>&1
if [ $? -ne 0 ]; then
echo "Cannot create directory $DIR"
exit 1
fi
fi
done

sh与bash区别

[wwy@sf-watch test]$ cat t2.sh
#!/bin/bash
diff <(echo xxx) <(echo yyy) # 此语法包含bash的特性，不属于sh的POSIX标准

[wwy@sf-watch test]$ bash -x ./t2.sh # 使用bash 调用，不会出问题
+ diff /dev/fd/63 /dev/fd/62
++ echo xxx
++ echo yyy
1c1
< xxx
---
>    yyy
[wwy@sf-watch test]$ sh ./t2.sh    # 而用sh调用，报错如下
./t2.sh: line 3: syntax error near unexpected token `('
./t2.sh: line 3: `diff <(echo xxx) <(echo yyy)'
[wwy@sf-watch test]$ echo $?

Things bash has that sh does not:

long invocation options
[+-]O invocation option
-l invocation option
`!' reserved word to invert pipeline return value
`time' reserved word to time pipelines and shell builtins
the `function' reserved word
the `select' compound command and reserved word
arithmetic for command: for ((expr1 ; expr2; expr3 )); do list; done
new $'...' and $"..." quoting
the $(...) form of command substitution
the $(<filename) form of command substitution, equivalent to
$(cat filename)
the ${#param} parameter value length operator
the ${!param} indirect parameter expansion operator
the ${!param*} prefix expansion operator
the ${param:offset[:length]} parameter substring operator
the ${param/pat[/string]} parameter pattern substitution operator
expansions to perform substring removal (${p%[%]w}, ${p#[#]w})
expansion of positional parameters beyond $9 with ${num}
variables: BASH, BASH_VERSION, BASH_VERSINFO, UID, EUID, REPLY,
TIMEFORMAT, PPID, PWD, OLDPWD, SHLVL, RANDOM, SECONDS,
LINENO, HISTCMD, HOSTTYPE, OSTYPE, MACHTYPE, HOSTNAME,
ENV, PS3, PS4, DIRSTACK, PIPESTATUS, HISTSIZE, HISTFILE,
HISTFILESIZE, HISTCONTROL, HISTIGNORE, GLOBIGNORE, GROUPS,
PROMPT_COMMAND, FCEDIT, FIGNORE, IGNOREEOF, INPUTRC,
SHELLOPTS, OPTERR, HOSTFILE, TMOUT, FUNCNAME, histchars,
auto_resume
DEBUG trap
ERR trap
variable arrays with new compound assignment syntax
redirections: <>, &>, >|, <<<,
<&word-,
>&word-
prompt string special char translation and variable expansion
auto-export of variables in initial environment
command search finds functions before builtins
bash return builtin will exit a file sourced with `.'
builtins: cd -/-L/-P, exec -l/-c/-a, echo -e/-E, hash -d/-l/-p/-t.
export -n/-f/-p/name=value, pwd -L/-P,
read -e/-p/-a/-t/-n/-d/-s/-u,
readonly -a/-f/name=value, trap -l, set +o,
set -b/-m/-o option/-h/-p/-B/-C/-H/-P,
unset -f/-v, ulimit -i/-m/-p/-q/-u/-x,
type -a/-p/-t/-f/-P, suspend -f, kill -n,
test -o optname/s1 == s2/s1 < s2/s1 > s2/-nt/-ot/-ef/-O/-G/-S
bash reads ~/.bashrc for interactive shells, $ENV for non-interactive
bash restricted shell mode is more extensive
bash allows functions and variables with the same name
brace expansion
tilde expansion
arithmetic expansion with $((...)) and `let' builtin
the `[[...]]' extended conditional command
process substitution
aliases and alias/unalias builtins
local variables in functions and `local' builtin
readline and command-line editing with programmable completion
command history and history/fc builtins
csh-like history expansion
other new bash builtins: bind, command, compgen, complete, builtin,
declare/typeset, dirs, enable, fc, help,
history, logout, popd, pushd, disown, shopt,
printf
exported functions
filename generation when using output redirection (command >a*)
POSIX.2-style globbing character classes
POSIX.2-style globbing equivalence classes
POSIX.2-style globbing collating symbols
egrep-like extended pattern matching operators
case-insensitive pattern matching and globbing
variable assignments preceding commands affect only that command,
even for builtins and functions
posix mode and strict posix conformance
redirection to /dev/fd/N, /dev/stdin, /dev/stdout, /dev/stderr,
/dev/tcp/host/port, /dev/udp/host/port
debugger support, including `caller' builtin and new variables
RETURN trap
the `+=' assignment operator

Things sh has that bash does not:
uses variable SHACCT to do shell accounting
includes `stop' builtin (bash can use alias stop='kill -s STOP')
`newgrp' builtin
turns on job control if called as `jsh'
$TIMEOUT (like bash $TMOUT)
`^' is a synonym for `|'
new SVR4.2 sh builtins: mldmode, priv

Implementation differences:
redirection to/from compound commands causes sh to create a subshell
bash does not allow unbalanced quotes; sh silently inserts them at EOF
bash does not mess with signal 11
sh sets (euid, egid) to (uid, gid) if -p not supplied and uid < 100
bash splits only the results of expansions on IFS, using POSIX.2
field splitting rules; sh splits all words on IFS
sh does not allow MAILCHECK to be unset (?)
sh does not allow traps on SIGALRM or SIGCHLD
bash allows multiple option arguments when invoked (e.g. -x -v);
sh allows only a single option argument (`sh -x -v' attempts
to open a file named `-v', and, on SunOS 4.1.4, dumps core.
On Solaris 2.4 and earlier versions, sh goes into an infinite
loop.)
sh exits a script if any builtin fails; bash exits only if one of
the POSIX.2 `special' builtins fails

/etc/profile
~/.bash_profile（ ->  ~/.bashrc  ->  /etc/bashrc）
~/.bash_login
~/.profile

/etc/profile
~/.bash_profile （ ->  ~/.bashrc  ->  /etc/bashrc）
~/.bash_login
~/.profile
$BASH_ENV

~/.bashrc （ ->  /etc/bashrc）

$BASH_ENV

SH：

/etc/profile
~/.profile

/etc/profile
~/.profile

$ENV

[wangweiyu@ComSeOp ~]$ grep ulimit /etc/profile
ulimit -S -c unlimited > /dev/null 2>&1

[wangweiyu@ComSeOp ~]$ ssh wangweiyu@127.0.0.1 'echo $-'
wangweiyu@127.0.0.1's password:
hBc
[wangweiyu@ComSeOp ~]$ echo $-
himBH
[wangweiyu@ComSeOp ~]$ ssh wangweiyu@127.0.0.1 'echo $0'
wangweiyu@127.0.0.1's password:
bash
[wangweiyu@ComSeOp ~]$ echo $0
-bash

[wangweiyu@ComSeOp ~]$ cat .bashrc
# .bashrc
# User specific aliases and functions
# Source global definitions
if [ -f /etc/bashrc ]; then
. /etc/bashrc
fi
echo 'xxx'

[wangweiyu@ComSeOp ~]$ ssh wangweiyu@127.0.0.1 'echo $-'
wangweiyu@127.0.0.1's password:
xxx
hBc

[wangweiyu@ComSeOp ~]$ cat .bashrc
# .bashrc
# User specific aliases and functions
# Source global definitions
if [ -f /etc/bashrc ]; then
. /etc/bashrc
fi
echo 'xxx'
[wangweiyu@ComSeOp ~]$ scp file wangweiyu@127.0.0.1:/tmp
wangweiyu@127.0.0.1's password:
xxx
[wangweiyu@ComSeOp ~]$ echo $?
1
[wangweiyu@ComSeOp ~]$ ls /tmp/
[wangweiyu@ComSeOp ~]$