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Shell Programming

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

flowchart LR
	id1([Print Prompt]) --> id2([Read a Command]) --> id3([Parse Command]) --> id4([Execute the Command]) -->id1

We have two main kinds of sh: csh and sh, csh is far behind . Thus we will use bash from sh to do shell programming

Basic Command and Workflow​

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.

If a command produces output that we want to discard, we redirect it to /dev/null.

Some Command we always use like:

  • echo: we can followed string, to print this string in the shell window
    • Notice: to use some symbol like ? we should write '?' to avoid do specific action for ?, similar as ?, if we write *.c it will not print *.c, instead of it, it will print all files who with name .c followed
  • exec

Variables​

For programming, we always need many variables to help us work. In shell programing, we declare a variable:

  • <Variable Name>=<Value>: where we can pick name to replace <>(include all inside) to create variable and assign value in it

    • For example, i=1 is set variable i and it has a value of 1
    • Notice, we can not do any operation for assign value to variable, e.g. i=i+1 has a value of i+1 not the value of i + 1

  • $<Variable Name>: just like the macro, we add $ and sh will know the following to be replaced as variables value

    • For example, run i=vim and then run $i test is just the same as vim test
    • another example, run i=vim and then run echo $i will print out vim

  • To do operation assign, we need to use `expr` in the given assign

    i=`expr 4 + 2`
    $i

    In the example above the i will have a value of 6. but notice it will only work for integer

    • Notice, some symbol need use \ to declare as operation not a command

  • read : the input function of shell

    • e.g. run read name, then we have a variable name with the value of input
    • Since Linux uses white spaces to separate argument, then read also can read many variables. The input also should be separate by white space, if there is not enough value to assign, then the extra variables will be empty. Similarly, if there are more input, then the last variables will be assign all of the remain input.

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

  • $#: the number of arguments

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

  • $*： all args

  • $@: target, or remain

  • Difference between '', "" and no quoting:

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

    • "": show all string and show the value of variables if has $ before variable name, symbol after\, content in ``

    • no quoting: interpreter all symbols, e.g. > will redirect output into followed name file

    • quoting will make the whole string as an argument, but no quoting will see each token separated by white space as different argument. An example following showing the difference

f='a b'
cat $f
cat: a: No such file or directory
cat: b: No such file or directory
cat "$f"
xxxx
cat '$f'
cat: a: No such file or directory
cat: b: No such file or directory

if ... else ...​

if conditions
then
	commands
elif conditions
then
	commands
else
	commands
fi

Shell has if statement like above.

For conditions:

  • we can use the success status of commands as conditions
  • If we want comparation for variables, we should use test command before comparation
    • e.g. test 3 -lt 4
    • Notice we should use argument like -gt, -lt, -eq, -ge, -le, -ne to compare integer
    • use argument =, != to compare strings
    • We can use test to test file
      • -f filename : file exists and is a plain file
      • -d filename: file exists and is a directory
      • -s filename: file exists and is a plain file with size larger than 0
    • More look at the man page of test
  • we can also use [ and ] instead of test, but be careful with the white space
  • we can use : as pass in python

while​

while conditions
do
	commands
done

A basic shell while statement

for loop​

for var in arg1 arg2 ...
do
	commands
done

A basic shell for statement

seq: Linux command create sequence by following 2/3 args

  • 2 args: start end
  • 3 args: start step end

we also can write

for i
do
	commands
done

we will loop all arguments

Switch​

for i
do 
	case $i in
		opt1)
			commands
			;;
		opt2)
			commands
			;;
		*)
			commands
			;;
	esac
done

basic switch statement

Redirection​

A > B: overwrite A's output from stdout into B

A < B: use B as input

A >> B: write A's output follow original B's data into B

if we want to use the stdin, stdout, stderr, write the representative number just before the symbol without white space or &num after the symbol

Notice: 2 >B is different with 2>B, 2 >B will exec a file named 2, but 2>B will write the stderr into B. And >&2 mean output to stderr