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if command
then
    commands
fi

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if [ condition ] # 第一个方括号之后和第二个方括号之前必须加上一个空格。
then
    commands
fi

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device0="/dev/sda2"   # /   (root directory)
# -b表示块设备
if [ -b "$device0" ]
then
  echo "$device0 is a block device."
fi

device1="/dev/tty$1"   # PCMCIA modem card
# -c表示字符设备
if [ -c "$device1" ]
then
  echo "$device1 is a character device."
fi

function show_input_type()
{
    # -p表示管道
    [ -p /dev/fd/0 ] && echo PIPE || echo STDIN
}
show_input_type "Input"
echo "Input" | show_input_type

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与： [ condition1 ] && [ condition2 ]
或： [ condition1 ] || [condition2]

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if [ "$expr1" -a "$expr2" ]
then
  echo "Both expr1 and expr2 are true."
else 
  echo "Either expr2 or expr2 is false."
fi

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case variable in
pattern1 | pattern2) command1;;
pattern3) command2;;
*) default command;;
esac

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for var in list
do 
    commands
done

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$ cat test1b
#!/bin/bash
# testing the or variable after the looping

for test in Alabama Alaska Arizona Arkansas California Colorado
do 
    echo "The next state is $test"
done
echo "The last state we visited was $test"
test=Connecticut
echo "Wait, now we're visiting $test"
$ ./test1b
The next state is Alabama
The next state is Alaska
The next state is Arizona
The next state is Arkansas
The next state is California
The next state is Colorado
The last state we visited was Colorado
Wait, now we're visiting Connecticut

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$ cat badtest1
#!/bin/bash
# another example of how not to use the for command

for test in I don't konw if this'll work
do 
    echo "word:$test"
done
$ ./badtest1
word:I
word:dont know if thisll
word:work

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$ cat test2
#!/bin/bash
# another example of how not to use the for command

for test in I don\'t konw if "this'll" work
do 
    echo "word:$test"
done
$ ./test2
word:I
word:don't
word:konw
word:if
word:this'll
word:work

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$ cat test3
#!/bin/bash
# an example of how to properly define values

for test in Nevada "New Hampshire" "New Mexico" "New York"
do 
    echo "Now going to $test"
done
$ ./test3
Now going to Nevada
Now going to New Hampshire
Now going to New Mexico
New goint to New York

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$ cat test6
#!/bin/bash
# interate through all the files in a directory

for file in /home/rich/test/*
do
    if [ -d "file" ] #在linux中，目录名和文件名中包含空格当然是合法的。要适应这种情况，应该将$file变量用双引号圈起来。如果不这么做，遇到含有空格的目录名或文件名时就会有错误产生。
    then
        echo "$file is a directory"
    elif [ -f "$file" ]
    then 
        echo "$file is a file"
    fi
done

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for (( i=1; i <= 10; i++))
do 
    echo "The next number is $i"
done

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$ cat test20
#!/bin/bash
# breaking out of an outer loop

for (( a = 1; a < 4; a++))
do 
    echo "Outer loop: $a"
    for (( b = 1; b < 100; b++))
    do
        if [ $b -gt 4 ]
        then
            break 2 #跳出两层循环，跳出本循环直接break
        fi
        echo "    Inner loop: $b"
    done
done

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while test command
do
    other command
done

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$ cat test10
#!/bin/bash
# while command test

var1=10
while [ $var1 -gt 0 ]
do
    echo $var1
    var1=$[ $var1 - 1 ]
done

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until test commands
do
    other commands
done

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$ cat test25
#!/bin/bash
# finding files in the PATH

IPS=:
for folder in $PATH
do 
    echo "$folder:"
    for file in $folder/*
    do
        if [ -x $file ]
        then
            echo "    $file"
        fi
    done
done

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# read命令会自动读取.csv文本文件的下一行内容，所以不需要专门再写一个循环来处理。当read命令返回FLASE时(也就是读取完整个文件时)，while命令就会退出。
$ cat test26
#!/bin/bash
# process new user accounts

input="users.csv"
while IFS=',' read -r userid name
do
    echo "adding $userid"
    useradd -c "$name" -m $userid
done < "$input"

$ cat users.csv
rich,Richard Blum
christine,Christine Bresnahan
barbara,Barbara Blum
tim,Timothy Bresnahan