Simple record of shell grammar | 7 clock

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Common commands

Echo for output
```
Echo 'AymFX most Handsome'Copy the code
```

Variable used

your_name="aymfx"
echo $your_name
echo ${your_name}
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A read-only variable

Myname ="aymfx" echo $myname readonly myname myname=" #myname: readonly echo $mynameCopy the code

To delete a variable

myname="aymfx"
echo $myname
unset myname
echo $myname # no output
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Single quotes
- Any character in a single quote is printed as is, and variables in a single quote string are invalid;
- A single quotation mark cannot appear in a single quotation mark string (even with an escape character), but can appear in pairs as string concatenations.
Double quotation marks
- You can have variables in double quotes
- Escape characters can appear in double quotes
```
myname="aymfx"
str="hello \"$myname\""
echo $str hello "aymfx"
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```

Get the length of the string

String ="aymfx" echo ${#string} #Copy the code

Extract substring

String ="aymfx" echo ${string:1Copy the code

Find substrings

Using the “expr” command in this case is only for Linux commands

STRING="this is a string"
SUBSTRING="hat"
expr index "$STRING" "$SUBSTRING"     # 1 is the position of the first 't' in $STRING
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Shortest substring matching

${string#substring}
${string%substring}

filename="bash.string.txt"

echo ${filename#*.} #string.txt 
echo ${filename%.*} #bash.string
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Shortest substring matching

${string##substring}
${string%%substring}

filename="bash.string.txt"
echo "After deletion of longest match from front:" ${filename##*.} # txt
echo "After deletion of longest match from back:" ${filename%%.*}# bash
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Substring to replace

Replace the first occurrence of a substring with a substitution

str='to be or not to be'
echo ${str[@]/be/cat} #to cat or not to be
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Replace all occurrences of substrings

str='to be or not to be'
echo ${str[@]//be/cat} #to cat or not to cat
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Delete all occurrences of substrings (replace with empty strings)

str='to be or not to be'
echo ${str[@]// not/} #to be or not to be
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If you replace the occurrence of a substring at the beginning of $STR

str='to be or not to be'
echo ${str[@]/#to be/eat now} #eat now or not to be
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If it ends at $STR, it replaces the occurrence of the substring

str="to be or not to be"
echo ${str[@]/%be/eat}  # to be or not to eat
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An array of
- Initialization (An array can hold multiple values with a single name. Array names are the same as variable names. Initialize arrays by allocating space-separated values contained in ().
```
apple=1
my_array=($apple banana "fruit basket" orange)
echo ${#my_array[@]} # 4
echo ${my_array[3]} #orange
echo aymfx #aymfx
echo ${my_array[0]} # 1
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```
- Array members need not be contiguous or contiguous. Some members of the array can be left uninitialized.
- The total number of elements in the array is referenced by ${# arrayName [@]}
Arithmetic operator
- A plus b plus A plus B
- A minus b minus a minus b.
- A times b times a times b.
- A/B division (integer)
- Take the modulo of a%b (divided by the remainder of the integer b)
- **a*** * * *bExponentiation (a equals a power of B)

Conditional statements

# the first

name='aymfx'
if [ $name = 'aymfx' ];then
    echo "${name}Really handsome" # aymfx really handsome
fi

# the second

name='aymfx'
if [ $name = 'ly' ];then
    echo "${name}Really handsome" # aymfx really handsome
else
    echo "There is no more${name}Never better." 
fi

# the third

name='aymfx'
if [ $name = 'ly' ]; then
    echo "${name}Really handsome" # aymfx really handsome
elif [ $name = 'aymfx' ]; then
    echo "${name}Handsome burst" # aymFX is awesome
else
    echo "There is no more${name}Never better." 
fi

# value comparison type
comparison    Evaluated to true when
$a -lt $b    $a < $b
$a -gt $b    $a > $b
$a -le $b    $a< =$b
$a -ge $b    $a> =$b
$a -eq $b    $a is equal to $b
$a -ne $b    $a is not equal to $b

Type of string comparison

comparison    Evaluated to true when
"$a" = "$b"     $a is the same as $b
"$a"= ="$b"    $a is the same as $b
"$a"! ="$b"    $a is different from $b
-z "$a"         $a is empty


# case

mycase=1
case $mycase in
    1) echo "You selected bash";;
    2) echo "You selected perl";;
    3) echo "You selected phyton";;
    4) echo "You selected c++";;
    5) exit
esac
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= must be surrounded by Spaces using “” around string variables to avoid shell extension of special characters to *

Loop structure


# for loop

# for
for arg in [list]
do
 command(s)...
done

#! /bin/bash
names=(aymfx liya zhangmeng)
for name in ${names[@]};do
    echo $name
done

# while
while [ condition ]
do
 command(s)...
done

COUNT=4
while [ $COUNT -gt 0 ];do
    echo "value of count ${COUNT}"
    COUNT=$(($COUNT-1))
done

#bash until loop

until [ condition ]
do
 command(s)...
done

COUNT=1
until [ $COUNT -gt 5 ];do
    echo "value of count ${COUNT}"
    COUNT=$(($COUNT+1))
done

NUMBERS=(951 402 984 651 360 69 408 319 601 485 980 507 725 547 544 615 83 165 141 501 263 617 865 575 219 390 237 412 566 826 248 866 950 626 949 687 217 815 67 104 58 512 24 892 894 767 553 81 379 843 831 445 742 717 958 609 842 451 688 753 854 685 93 857 440 380 126 721 328 753 470 743 527)
for value in ${NUMBERS[@]};do
    if [ $value -eq 237 ]; then
        break;
    elif[$(($value % 2)) == 0 ]; then
        echo $value
    fi
done

# break and continue can be used to control the execution of loops for,while, and until constructs. Continue is used to skip the rest of a particular loop iteration, while break is used to skip the rest of the entire loop.

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The shell function
- Function calls are equivalent to commands. You can pass parameters to a function by specifying parameters after the function name. The first argument in the function is called 1, the second argument is called 1, the second argument is called 1, the second argument is called 2, and so on
```
function function_B {
  echo "Function B."
}
function function_A {
  echo "$1"
}
function adder {
  echo "$(($1 + $2))"
}


function_A "Function A."     # Function A.
function_B                   # Function B.
adder 12 56                  # 68
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```

Some explanation

#! Is a convention tag that tells the system what interpreter the script needs to execute, that is, which Shell to use.
- #! /bin/bash
The method to run the script
- Chmod +x./test.sh # enables the script to execute
- /test.sh # Execute the script
- /bin/sh test.sh
Variable types
- Local variables Local variables are defined in a script or command and are only valid in the current shell instance. Programs started by other shells cannot access local variables.
- Environment variables All programs, including those launched by the shell, have access to environment variables, and some programs need them to run properly. Shell scripts can also define environment variables if necessary.
- Shell variables Shell variables are special variables set by the shell program. Some shell variables are environment variables and some are local variables, which ensure the normal operation of the shell
Value of the command output
- You can do this by wrapping the command with ‘ ‘(called backquotes) or $()
```
FILELIST=`ls`

FileWithTimeStamp=/tmp/my-dir/file_$(/bin/date +%Y-%m-%d).txt
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```
Variable 1 refers to the first argument on the command line, variable 1 refers to the first argument on the command line, variable 1 refers to the first argument on the command line, variable 2 refers to the second argument, and so on. The variable $0 refers to the current script.
```
Sh apple banner echo $0 #auto. Sh echo $1 #apple echo $# #2Copy the code
```

Special variables

$0– the current script file name |
$n– call the first N parameters passed to the script or calling function |
$#– the number of parameters passed to the script or function |
$@– | all passed to the script or function parameters
$*– | all passed to the script or function parameters
$?– the last execution of command exit status |
$$– ID of the current shell process. For a shell script, this is that they are performing the process ID. |
$!– the last process of background command |

echo "Script Name: $0"
function func {
    for var in $*
    do
        let i=i+1
        echo "The \$${i} argument is: ${var}"
    done
    echo "Total count of arguments: $#"
}
func We are argument

#Script Name: auto.sh
#The The $1 argument is: We
#The $2 argument is: are
#The $3 argument is: argument
#Total count of arguments: 3

function func {
    echo "--- \"\$*\""
    for ARG in "$*"
    do
        echo $ARG
    done

    echo "--- \"\$@\""
    for ARG in "$@"
    do
        echo $ARG
    done
}
func We are argument

#--- "$*"
#We are argument
#--- "$@"
#We
#are
#argument


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Common signal types
- SIGINT: User sends interrupt signal (Ctrl + C)
- SIGQUIT: User sends exit signal (Ctrl + C)
- SIGFPE: An illegal math operation was attempted
HUP INT QUIT ILL TRAP ABRT EMT FPE KILL BUS SEGV SYS PIPE ALRM TERM URG STOP TSTP CONT CHLD TTIN TTOU IO XCPU XFSZ VTALRM PROF WINCH INFO USR1 USR2
```
trap "echo Booh!" SIGINT SIGTERM
echo "it's going to run until you hit Ctrl+Z"
echo "hit Ctrl+C to be blown away!"

while true        
do
    sleep 60       
done

trap booh 2 15

trap "rm -f folder; exit" 2

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```

File tests (In general, you will need to do some file tests on a running file system. In this case, the shell will give you some useful commands to implement it.)

-<command> [filename]

[filename1] -<command> [filename2]

# Use "-e" to test whether the file exists

#! /bin/bash
filename="package.json"
if [ -e "$filename" ]; then
    echo "$filename exists as a file"
fi

# Use -d to test whether the directory exists
directory_name="public"
if [ -d "$directory_name" ]; then
    echo "$directory_name exists as a directory"
fi

Use "-r" to test whether the file has read permission for the user running the script/test
filename='README.md'
if [ ! -f "${filename}" ];then
    touch "${filename}"
fi
if [ -r "${filename}" ];then
    echoHave read permissionelse
    echoHave no legal powerfi
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Simple record of shell grammar | 7 clock

Common commands

Some explanation

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