BASH名字的由来

Bourne Shell是unix上最早的shell, 它叫这个名字是因为最早作者的名字里有Bourne这个词，它写于1970s。 由于它是最早的shell,所以缩写是sh。而bash则是另一个家伙在1989年写的，里面加入了一些增强特性。 为了表达对Bourne Shell作者的敬意，这个新shell的名字叫Bourne Again Shell, 尽管新作者的名字和Bourne没有半毛钱关系。 而这个新shell的缩写则是我们现在广泛使用的bash

Shell 和 Terminal的区别

shell 是用来帮助人和机器交流的中间层。它将人的命令转化为机器能听懂的语言。bash, zsh, sh 他们都是shell. 最开始的terminal是物理的带有显示屏的一个机器，一台机器是一个terminal，上面只能跑一个shell. 现代的terminal可以被emulate出来，你可以在你的电脑上开好多个terminal.

一些特殊变量

echo $$可以打印出当前shell的进程id

$?是上一条命令或脚本的执行状态，0是成功，1是报错。 bash最多支持256种退出状态，即0到255。可以用awk 'BEGIN {exit 258}'; echo $?命令来验证这一点。

$*: All arguments on command line as one string (“$1 $2…”). The values are separated by the first character in $IFS.

$@ All arguments on command line, individually quoted (“$1” “$2” …).

在Bash脚本里，没有双引号的情况下，$*和$@作用相同。而加了双引号后，"$*"会把所有的variable用引号包起来，"$@"则会把接受的variable一个个用引号包起来, 试看下例：

vim t.sh

for i in "$*"
    do
        echo $i
    done

for i in "$@"
    do
        echo $i
    done

接着，你输入sh t.sh WJB I, 你会得到:

WJB I
WJB
I

看到区别了？

数据结构

Shell里有arrary,可以用来存储各类value, 但index只能是数字。

初始化一个array，我们用Hero=(Illidian Arthas Lich-King)

打印array里某一个序号的值, 比如2，你使用echo ${Hero[2]}

打印array里所有value, 你使用 echo ${Hero[*]} 或 echo ${Hero[@]}

Conditional Expression

Conditional expression should be wrapped in [], like [ $a != $b ]

Relational Operators only works between variables have numeric value, or number like string value, such as "32", here is a table, in which $a=10 and $b=20:

Operator Description Example 

-eq              equal                            [ $a -eq $b ]     False  
-ne              not equal                        [ $a -ne $b ]     True  
-gt              greater than                     [ $a -gt $b ]     False
-lt              less than                      [ $a -lt $b ]     True
-ge              greater than or equal          [ $a -ge $b ]     False
-le              less than or equal             [ $a -le $b ]     True

在所有的Conditional Expression里，变量和[之间必须有一个空格，像[ $a >= $b ].
而[$a >= $b]则会导致错误发生。

In shell there are also Boolean Operators, they can be used in condition judge.

Operator       Description          Example
!                not                [ ! false ]                  True
-o               or                 [ $a -gt 35 -o $b -eq 15 ]   False
-a               and                [ $a -ge 9 -a $b -le 20 ]    True

There are also operators works for strings, imagine $a holds "abc" and $b holds "efg", then:

Operator        Description             Exampele
=                  equal                [ $a = $b ]         False
!=              not equal               [ $a != $b ]        True
-z              str is zero byte?       [ -z $a ]           False
-n              str is none-zero byte?  [ -n $b ]           True
[str]             is empty string       [ $a ]              True

In unix we even have operators for files! Let's assume our file has size of 100 bytes with read, write, and execute permissions.

Operator        Description                                 Example
-b file         Is block speicial file?                     [ -b $file ]    False
-c file         Is character special file?                  [ -c $file ]    False
-d file         Is a directory?                             [ -d $file ]    False
-f file         Is an ordinary file?                        [ -f $file ]    True
-g file         Has group ID set on?                        [ -g $file ]    False
-k file         Has sticky bit set on?                      [ -k $file ]    False
-p file         Is named pipe file?                         [ -p $file ]    False
-t file         File descriptor open & bind to terminal?    [ -t $file ]    False
-u file         Has user id set on ?                        [ -u $file ]    False
-r file         Readable?                                   [ -r $file ]    True
-w file         Writable?                                   [ -w $file ]    True
-x file         Executable?                                 [ -x $file ]    True
-s file         Size greater than 0?                        [ -s $file ]    True
-e file         Exists?(works for directory, too)           [ -e $file ]    True

if stick bit set is on for a file, except the file owner and root, no one else can delete other users data in this file.

Normally, we use condition judgement to do decsion making. There are two ways to do so, the first one is if ... then ... n*([elif] ... [then] ...) [else] ... fi structure

Belowing is an oneline if statement:

a=10;b=20
if [ $a -le $b ]; then echo "yes"; fi

If, CASE

You can add multiple elif ... then ... block to conditional evaluation structrue, with an optional else ... block. And, don't forget the fi at the end. Belowing are 2 examples. You should run them as shell script.

# example 1
a=10;b=20
if [ $a -ge $b ]
    then
        echo "A is greater than or equal to B"
elif [ $a -lt $b ]
    then
        echo "A is less than B"
fi

# example 2
a=10;b=20
if [ $a -ge $b ]
    then
        echo "A is greater than or equal to B"
elif [ $a == 8 ]
    then
        echo "A is 8"
elif [ $b == 18 ]
    then
        echo "B is 18"
else
    echo "All the previous conditions does not match"
fi

If you wanna do something to multiple branches, you may using a nested if ... else ... structure, but shell has another grammer for this: case ... esac
case structure is usually used to judge if a variable value in a particular pattern, which is hard to do in shell if structure. The general pattern is below:

case $var in
    pattern1)
        command1
        ;;
    pattern2)
        command2
        command3
        ;;
    pattern3|pattern4|pattern5)
        command4
        ;;
    *)
        command5
        ;;
esac

In the example above, the part starts from pattern2 to command5 is optional. ;; means that the program flow would jump to the end of the case statement. so the case block will finish execution once find a match pattern.
Let's see a usable one-line example:

a=5
case $a in [357]) echo "yes";; esac

And a multiple version: a=5; case $a in [368]) echo "var satisfy condition 1" ;; [37] | [56]) echo "var satisfy condition 2" ;; *) echo 'finish!' esac

the | above means or

LOOP

In shell, we got 4 types of loop tools: while loop, for loop, until loop and select loop, while loop will excute the commands as the condition is ture, for loop will loop over the given var range, until loop will execute the commands until the belowing are 4 basic examples of these loops.

# WHILE LOOP
a=1
while [ $a -lt 5 ]
    do
        echo $a
        a=$(expr $a + 1)
    done


# FOR LOOP
for i in {1..9}
    do
        echo $i
    done


# UNTIL LOOP
a=1
until [ $a -gt 5 ]
    do
        echo $a
        a=$(expr $a + 1)
    done

# SELECT LOOP
select NUM in 2 3 5 7

do case $NUM in [2345]) echo $NUM'235' ;; *) echo $NUM'other' ;; esac

# SELECT LOOP 2
select TOY in 2 3 5
do
    if [ $TOY -gt 3 ]
        then
        echo "greater than 3"
    else
        echo "Less or equal to 3"
    fi
done

All these four loops listed above can be nested together to produce more variety.

You can use continue and break to do loop contral, in shell they act just like they do in python. Except you can use continue n and break n to get out of nth loop. Try the following two examples, you'll know what I mean.

#!/bin/bash

echo "break"

for var1 in 1 2 3
do
    for var2 in 0 5
    do
        if [ $var1 -eq 2 -a $var2 -eq 0 ]
        then
            break 1
        else
            echo "$var1 $var2"
        fi
    done
done


echo "continue"

for i in 6 7 8 9 10
do
    for j in 1 2 3 4 5
    do
        if [ $(expr $i % 2) -eq 0 -a $(expr $j % 2) -eq 0 ]
        then
            continue 2
        else
            echo "$i $j"
        fi
    done
done

Substitution

To assign the output of a command to a variable, simply use:

var=`command`

And follows are different substituion of variables.

Form                Description
${var}              substitute the value of var

${var:-word}        if var is null or unset, word is used as the substituted string, the value of var won't change
${var:=word}        if var is null or unset, var is set to word, then substitue begins
${var:+word}        if var is set, word is used as the substituted string, value of var won't change

${var:?message}     if var is null or unset, message will be printed to standard error.

MetaCharacters

* ? [ ] ' " \ $ ; & ( ) | ^ < > new-line space tab

These are special characters in shell, also named metacharacters. To convert metacharacters to normal character, you got four way to do it

  1. you can add a \ before a single metacharacter.
    echo Hello \; World!

  2. you can wrap any number of any kind of metacharacters in single quote ' to make them normal echo ' *?[]"$;&()|^<> '

single quote can't convert single quote, try echo ' '' ' and echo ' \' ' '

  1. Double quotes " do the similar job, except six characters or combinations below EXCEPTIONS FOR DOUBLE QUOTE:

     `   $   \`  \$  \'  \"  \\

  2. Back quotes ` will treat what inside it as command and execute it.

    echo date

Input Redirection

The symbol for input redirection is <, it reads content of file and feed them as standard input to command. Please see the example below:

$ wc -l test
1 test

$ wc -l < test
1

In the first command, test file was passed as a parameter to wc, so wc knows its name and prints it out.
BUt in the second command, we directly passed the content to wc, so he don't know who is the owner of these content, except they comes from standinput, so no filename prints out.

<< is used to start a Here document, the pattern is like:

command << self_defined_delimiter
    content
    ...
    content
delimiter

It is used to feed multiple lines as text content to command. Below is an example.

cat << 虽然我长得有点奇怪，可我真的是个delimiter，不信你试试
第一行
        来啊
                我是第三行
虽然我长得有点奇怪，可我真的是个delimiter，不信你试试

Write the content above in to a file, then execute it, you'll know what self_defined_delimiter is. Amazing, isn't it?

There are three types of I/O, each have their own identifier, called a file descriptor: standard input: 0 standard output: 1 standard error: 2

With no file descriptor number, < refers to the standard input(0), > and >> refers to standard output(1). And we can specify the part we want to redirect, just like below.

ls non_exist_file 2> tmp

The command above will redirect the error message of ls command to tmp file, 2> means pass standard error to target

Functions

The basic function structure is like follow:

my_func_name () {
                echo "$1 eats $2"       # Write your own command here
                }

You can return number from 0 to 255 in a function, like:

return_status () {
                 return 119
                 }
return_status
echo $?

Function can be nested, like: record_time() { CUR_TIME=DATE +%y%M%D-%h:%m:%s echo $CUR_TIME }

speak()
{
    echo "I'm the king of the world!"
    record_time
}

speak

When you want to remove the definiton of a function, you use:

unset function_name

MAN-PAGE

You use man [section_number] command to view the Manual of a command.
Where the section_number is optional, the default value is 1.

Belowing are some comman sections of a man page:

Section                     Description
NAME                    Name of the command
SYNOPSIS                General usage parameters of the command.
DESCRIPTION             Generally describes of the command and what it does
OPTIONS                 Describes all the arguments or options to the command
SEE ALSO                Lists other commands that are directly related to the command in the man page or closely resembling its functionality.
BUGS                    Explains any known issues or bugs that exist with the command or its output
EXAMPLES                Common usage examples that give the reader an idea of how the command can be used.
AUTHORS                 The author of the man page/command.

USEFUL COMMAND

Belowing are a bunch of useful commands that wait me for explore:

# Original link:  http://www.tutorialspoint.com/unix/unix-useful-commands.htm

Files and Directories: These commands allow you to create directories and handle files.

Dealing with file

chgrp   change file group
file    Determine file type
ln  Create softlink on oldname
rmdir   Delete an existing directory provided it is empty.

Manipulating data:
The contents of files can be compared and altered with the following commands.

cmp Compare the contents of two files
expand  Expand tabs to spaces
join    Join files on some common field
split   Split file into smaller files
tr  Translate characters
fmt Simple text formatter
spell   Check text for spelling error
ex, edit    Line editor

Compressed Files:
Files may be compressed to save space. Compressed files can be created and examined:

compress    Compress files
gunzip  Uncompress gzipped files
gzip    GNU alternative compression method
uncompress  Uncompress files
zcat    Cat a compressed file
zcmp    Compare compressed files
zdiff   Compare compressed files

Getting Information:
Various Unix manuals and documentation are available on-line. The following Shell commands give information:

apropos Locate commands by keyword lookup
info    Displays command information pages online
whatis  Search the whatis database for complete words.
yelp    GNOME help viewer

Network Communication:
These following commands are used to send and receive files from a local UNIX hosts to the remote host around the world.

tftp    Trivial file transfer program
telnet  Make terminal connection to another host
ssh Secure shell terminal or command connection
scp Secure shell remote file copy
sftp    secure shell file transfer program
Some of these commands may be restricted at your computer for security reasons.

Messages between Users:
The UNIX systems support on-screen messages to other users and world-wide electronic mail:

evolution   GUI mail handling tool on Linux
mail    Simple send or read mail program
mesg    Permit or deny messages
parcel  Send files to another user
pine    Vdu-based mail utility
talk    Talk to another user
write   Write message to another user

Programming Utilities:
The following programming tools and languages are available based on what you have installed on your Unix.

Command Description
dbx Sun debugger
gdb GNU debugger
make    Maintain program groups and compile programs.
nm  Print program's name list
size    Print program's sizes
strip   Remove symbol table and relocation bits
cb  C program beautifier
cc  ANSI C compiler for Suns SPARC systems
ctrace  C program debugger
gcc GNU ANSI C Compiler
indent  Indent and format C program source
bc  Interactive arithmetic language processor
gcl GNU Common Lisp
g++ GNU C++ Compiler
javac   JAVA compiler
mysql   Run the mysql SQL interpreter

Misc Commands:
These commands list or alter information about the system:

Command Description
chfn    Change your finger information
determin    Automatically find terminal type
finger  Print information about logged-in users
groupadd    Create a user group
homequota   Show quota and file usage
iostat  Report I/O statistics
last    Show last logins of users
logout  log off UNIX
lun List user names or login ID
netstat Show network status
passwd  Change user password
printenv    Display value of a shell variable
quota -v    Display disk usage and limits
reset   Reset terminal mode
script  Save the output of a command or process
setenv  Set environment variables
stty    Set terminal options
time    Time a command
tset    Set terminal mode
tty Print current terminal name
umask   Show the permissions that are given to view files by default
uname   Display name of the current system
useradd Create a user account
users   Print names of logged in users
vmstat  Report virtual memory statistics

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