Bo2SS

High-level languages, scripting languages, interpreted languages: xxx.sh, generally based on bash

Course Content#

First Shell Script#

#!/bin/bash
# Comment
echo "Hello"

• The first line #! specifies the interpreter, valid only on the first line
• Comment: "#"

Variables#

a=123
a=hello
a=`pwd`
PATH=${PATH}:path  # A string can be directly concatenated after a variable
# 【Note】":a" has a special meaning, pointing to the current path

• No need to define variable types【Weakly typed language】
  • For example, a=123 can be a string or an integer
• "" has a special meaning
  • 
  • You can enclose a in double quotes, such as a=$a:"a" or a=$a"", resulting in a=123

Special Variables#

Positional Variables

Result:

• 

• $n: If n > 9, it needs to be enclosed in braces

• $@ and $* differ in that: when passed parameters are enclosed in double quotes, and the $ variable is also enclosed in double quotes, "$@" will separate all parameters【general case】, while "$*" will treat all parameters as a whole

  • This can be seen when using for...in, see Shell special variables: Shell $0, $#, $*, $@, $?, $$ and command line parameters——C Language Chinese Network

• $# : does not count $0

Status Variables

• $? : [The result of the last] command execution, 0——success, non-0——failure
  • 
  • Facilitates automatic judgment of command success in scripts
• $$ and $! : PID of the current process and the last running background process, generally used in automated testing and multi-script interaction scenarios

Input and Output#

• Input: read
  • 
  • -p: Displays friendly prompts, needs to be used in bash
  • -s: Silent mode
  • -t: Input wait duration, timeout ends (unit: s)
• Output: echo
  • -e: Enable escape
• Output: printf
  • Very similar to C language's printf!
  • 
  • [PS] ! and \n have special meanings when used together, need to be separated
    • Needs to be separated in bash
    • Can be separated in zsh, or escaped with \
    • Generally for special symbols——Beginner's Tutorial, need to be cautious!

Functions#

• 

• No parameters

• Definition: Many ways to write, function, () and {} combinations, the third way is more similar to C language writing

• Call: function name parameters...

  • 
  • Adding function for readability

• ❗【Note】The return value of functions in shell is limited, the range is 0-255, overflow will cycle and recalculate

Control Flow#

do——done, if——then——fi, case——esac

【Key Point】TEST Expression⭐#

• Can judge types: string, integer, file
• ❗ 【Note】When the condition is true, it returns 0, otherwise returns non-0
  • The return value of commands in Linux is also like this, 0 represents success
• [PS]
  • STRING1 = STRING2 can also use == 【Recommended later, two brackets [[ ]] support】
  • -G FILE: The file exists and is owned by a valid group ID
    • If the group is dissolved, the group ID of the file controlled by the group becomes invalid
  • For specifics, refer to the man manual: man test

Branch Structure#

if

• 

• Using two brackets [[ ]] for if is more advanced and has better compatibility with TEST expressions

  • One bracket [ ] is an older writing style, not recommended, use two brackets
  • Refer to The role of ()、(())、[]、[[]]、{} in Linux Shell——Blog

case

• 

• The semicolon ;; cannot be omitted! Equivalent to break, and if break is added, it must also be present

• Default case can use *)

• case is used less often, generally for menus, more aesthetically pleasing than if

Loop Structure#

for

• Use seq to generate sequences

• 

• Use ls matching rules

• 

• Two forms of for

• 

• The content in double parentheses (()) only needs to comply with C language operation rules, variables can be written without the variable prefix $, can write i++, normally cannot use ++

while

• 

• Also applicable to test expressions

• Initialize num variable, otherwise

  • The first time echo $num, $num is empty, resulting in an empty line
  • When encountering the subsequent +1, the system determines it as an integer type, and only then is it treated as an integer

until

• 

• The only difference from while is: until writes the stop condition, while writes the loop condition

Arrays#

• 

• Array assignment【Method ①】 and calling

• 

• 【Common array operations】 can also output the index of array elements, see below

• 

• Declare array: declare -a num, for readability, can also be done without declaration [Weakly typed language]

• Assignment【Method ②、③】

  • num[2]=10 num[5]=7 num[100]=3.2.4
  • num=(1 a b 10)

• Array elements do not have to be assigned continuously and can be of any type

  • 【Common array operations】 add !: output all array elements' corresponding indices, can see they are not continuous

【Other Array Operations】

• 

• +=: Append

• unset: Delete array or element [delete by index]

• [PS] Assignment method ③ treats multiple spaces as a single separator

Application

• 

• Can be used to store a list of file names

• 【Note】Assignment method ③ assigns values to arrays using spaces as separators, so if file names contain spaces, special handling is needed

• 【PS】Prime sieve, linear sieve

In-Class Exercises#

Sum of Even Numbers from 1 to 100#

• 

• $[ ] can be used for integer operations

• seq is slightly less efficient

Bruteforce Prime Calculation#

• 

• The return value of functions

  • Obtained through $?, after calling the function echo $? can get it, but the return value has a range limit of 0~255!
  • Obtained through command substitution ``

• Note the conflict issue with the loop variable i: define local variables

• 🆒 Debugging the program

  • Habit: Appropriate echo output
  • Global debugging: bash -x *.sh
  • Local debugging: set -x [debug code area] set +x

Prime Sieve#

• 

• Method one is convenient for operating primes and can reduce judgment during output

• Method two can also operate composite numbers

【Comparison of two methods: Bruteforce and Prime Sieve】Calculate the sum of primes from 2 to 10000

• 9.prime.sh Bruteforce —— 10.prime.sh Prime Sieve

Additional Knowledge Points#

• .sh scripts can be executed directly with bash or source, if using ./ it requires executable permissions
  • What are the differences between source, sh, bash, ./——cnblogs
• set -x can enable shell debugging
• Shell Command Substitution: Assigning the output of a command to a variable——C Language Chinese Network
  • $() supports nesting, backticks `` do not
  • $() is only valid in Bash [seems zsh and sh also support], while backticks `` can be used in various shells
• Content in double parentheses (())
  • Only needs to comply with C language operation rules
  • Variables can be written without the variable prefix $
  • Can write i++, normally cannot use ++
• ⭐ In Shell, when a variable is not yet defined, its value is empty, echo output appears as an empty line
• ⭐ Space issues【Strict】
  • In assignment statements: = cannot have spaces on either side
  • Related to TEST expressions: [[ ]] must have spaces on both sides
• For variable name i, if you want to concatenate $i with _i, using $i_i will look for variable i_i, so use ${i}_i

Tips#

• Writing Shell scripts
  • Do not worry too much about performance, purely doing mathematical calculations is inefficient
  • The goal is to quickly solve a problem and plan the flow of all tasks
  • Remember to add【backup】 operations before performing actions
  • Generally, it is about letting the system do things, which is harder than making specific programs do things
    • Programs generally have their own parameter settings but lack universality
• API generally refers to services
• ⭐Shell Style Guide——Google Open Source Project Style Guide——Shell Programming Specifications
• Methods for multi-line comments and single-line comments in Shell scripts——Blog

:<<!
[Code to be commented]
!

• You can learn about let——Beginner's Tutorial, convenient syntax

Course Summary#