Introduce to Shell Programming

Linux
26

Shell is a program that infinite looped to execute and interpreter command.

It likes: id1([Print Prompt]) -> id2([Read a Command]) -> id3([Parse Command]) -> id4([Execute the Command]) -->id1

There are many types of sh: zsh, csh, sh, bash, etc. Now the default shell for most Linux distributions is bash. This article will also be based on shell programming using bash.

Basics

Shell Programming basically combine most Linux Command and write it in some way in a sh file so that bash can interpreter and execute it. Some common Bash commands:

  • echo, ls, cd, cat, grep, mkdir, touch, pwd and so on.

  • use helpto list all bash command

  • use man to view detailed documentation on various commands, library functions, configuration file formats, etc.

Quotes

Sometimes we need quotes to wrap sentences or commands to make them together.

  • '': show all string and do nothing with special symbols

  • "": show all string and show the result of $var, original symbol after \, execution result in ``

Variable

Initializing variables in sh is very easy because there is no type checking. variable=whateveriwant is all you need. But be aware that it is space sensitive. Bash use space to separate variables. variable = will execute command variable and = .

  • To execute your variable, you should add $ before your variable name, i.e. $variable

  • If space is needed, use '' or "" to wrap them.

  • If want to assign the result of some command use `` or $() to wrap them, or "" for sentence and result combination.

  • use unset to delete assign

  • use local to declare variable in the same scope

Operators using default values:

  • ${VARIABLE:=default} If VARIABLE is unset or empty, assigns default to VARIABLE and returns the value of default.

  • ${VARIABLE:-default} If VARIABLE is unset or empty, returns the value of default but does not change the value of VARIABLE.

  • ${VARIABLE:+alternate} If VARIABLE is set and non-empty, set the value of alternate. Otherwise return the empty string.

  • ${VARIABLE:?error_message} If VARIABLE is not set or empty, print error_message and exit the script. If it is set, return its value.

Symbols

  • | is the pipe symbol. The output result on the left side of | will be used as the input of the command on the right side

  • ||is the logic or symbol. If and only if the command on the left side of || fails or ends with a non-zero status, the command on the right side will be executed.

  • & is the background run symbol. The command before & will run background.

  • && is the logic and symbol. The command on the right side will be executed if and only if the command on the left side of && is executed successfully.

  • ; is the command separator. Used to separate multiple commands so that they can be executed one after another in one line.

  • ;; Used to end a pattern in a case statement.

  • < Use the contents of a file as input to a command

  • << Takes multiple lines of text as input to a command, start and end with customized eof.

  • <<< Single line input

  • > Redirects the output of a command to a file, overwriting the contents of the file.

  • >> Append the output of the command to the end of the file instead of overwriting the file contents.

  • if you want to use those symbol, add \ before each single character

  • : used when a valid command placeholder is expected without performing any action

Some symbols are executable followed by $

  • $? : a special variable to store last command status. If 1 fail or 0 success

  • $#: the number of arguments from current execution

  • $num: the numth arg, if no specifical definition of num

  • $*：all args from current execution

  • $@: target, or remain args

Control Flow

Conditional Branching

Conditions

Some common conditions need to be compared using test or use [] to wrap them.

  • There are -lt, -gt, -eq, -ge, -le, -ne for comparing numbers.

  • For string, you can use = or != for direct comparison

  • For file, you can use -arg filename

    • Use -f to compare whether the file exists and is a file instead of a folder

    • Use -d to compare whether the file exists and is a folder instead of a file

    • Use -s to compare whether the file exists and is a file instead of a folder, and the file is not empty

if/elif/else Statements

if conditions
then	
  commands
elif conditions
then
  commands
else
  commands
fi

switch-case statement

case arg in
    opt1)
        commands
        ;;
    opt2)
        commands
        ;;
    *)
        commands
        ;;
esac

For Loops

for xx in arg1 arg2 xxx
do
    command
done

If you want to generate a sequence of number, you can use `seq start step end` or `seq start end`, end is inclusive. You can also use $() wrap them as mentioned above.

While Loops

while conditions
do
  commands
done

Function

func(){
}

For arguments pass into function, use executable symbols followed by $

i.e. you can use local var=${1:-arg1} to initialize a local variable var with $1 or default value arg1 if there is not first arguments.