Wildcards and special symbols for shell scripts

wildcard ：

Special symbols ：

（1） Data flow redirection ： Data that should appear on the screen after an instruction is executed (stdout/stderr) Transfer to other places

• The standard input (0)： Use < or <<
• standard output (1)： Use > or >>
• The standard error (2)： Use 2> or 2>>（ If you don't write numbers, the default is 1）

Example ：

 example 1：
    ll / > ~/rootfile    # Output the contents of the root directory to the file 
        >： If the file does not exist, create , Existence covers 
        >>: If it exists, it adds 
 example 2:
    find /home -name .bashrc>list_right 2>list_error  # take stdout And stderr Save to different files 
 example 3: Write both correct and wrong information to list in 
    find /home -name .bashrc>list 2>list  # error 
    find /home -name .bashrc>list 2>&1    # correct 
    find /home -name .bashrc &>list       # correct 
 example 4:
    cat > catfile               # User input characters , Then save to catfile in 
    cat > catfile < ~/.bashrc   # The characters entered are composed of bashrc Provide storage in catfile in （ Copy ）
 example 5:
    cat > catfile << "eof"      #<< The input character representing the end , Input eof Then end the input 

Be careful ：

• /dev/null Black hole device , You can output unwanted information to /dev/null in

find /home -name .bashrc 2>/dev/null  # Discard the wrong data , The correct data is displayed on the screen 

（2） Pipe symbol “|” Use ：

ls -al /etc|less    #ls Of stdout As less Ordered stdin

• stay "|" The following designation must be a pipeline command ( Acceptable stdin), Such as less、more、head、tail
• Route commands only process stdout, about stderr Will ignore

Examples of common pipeline commands ：

1.cut
cut -d ' Separator character ' -f fields    #-f To take out the paragraphs , Result subscript from 1 Start 
cut -c  Character range                 # Take the fixed character interval as the unit of characters 
 example ：
    echo $PATH | cut -d ':' -f 5    # Take out No 5 individual 
    echo $PATH | cut -d ':' -f 3,5  # Take out No 3 With the first 5 individual 
    export | cut -c 12-             # Take out No 12 String after characters 
    export | cut -c 12-20           # Take out 12-20 String 
2.grep
 Options :--color=auto  You can highlight keywords in the results 
 example :
    last | grep 'root'                 # Take out yes root The line of 
    last | grep -v 'root'              # Take out no root The line of 
    last | grep 'root' |cut -d ' ' -f1 # Take out yes root The first column of the row 
3.sort: Sort 
-t: Separator  -k: Sort by the interval separating the results  -r: Reverse sorting  -f: Ignore case  -b: Ignore the first space  -n In numerical order 
 example : Sort the account numbers 
    cat /etc/passwd | sort
    cat /etc/passwd | sort -t ':' -k 3     # Sort in the third column 
    cat /etc/passwd | sort -t ':' -k 3 -n  # The above sort is sorted by string ,1 It will appear in 11 Back ,-n Point out sorting by number 
4.uniq: Remove duplication 
 example :
    last | cut -d ' ' -f1 | sort | uniq [-c]    # Remove duplication ,-c For count 
5.wc: Statistics 
 example :
    cat /etc/man.config | wc      # The three numbers output represent : That's ok 、 Number of words 、 Number of characters 
6.tee: As input to the pipeline , Also output to the screen 
    last | tee last.list          # Output to screen and last.list
7.tr: Delete or replace text 
-d: Delete the following string   -s: Replace duplicate characters 
 example :
    last | tr '[a-z]' '[A-Z]'
    cat /etc/passwd | tr -d ':'   # Remove the colon from the result 

minus sign - Use of

In the pipeline command , Will use the previous instruction stdout As this time stdin, When some instructions need to be processed with file names , The stdin And stdout You can use the minus sign “-” Instead of . example ：

tar -cvf - /home | tar -xvf -

• use tar pack /home, Fill in the file name and location “-”, The packed data is transferred to stdout, Then the back tar Of “-” Is the of the previous instruction stdout.