Introduction

Shell provides users an interface to interact with the operating system. It interprets the shell script and handles the output from the operating system.

In fact, shell itself is an application. The most common shell in UNIX-like system is bash. Another popular shell is zsh with enhanced functions. For SSL telnet connection, the ssh shell is usually used.

Header

The shell script can be run by using

bash script-name

You can replace bash with any shell you want. However, usually the shell scrip start with an directive statement, so that it can be run without specifing the shell name again.

#!/bin/bash

It should be noted that other than the first line, lines start with # are comment lines.

Variable

Shell variables are created once they are assigned a value.

• Value assignment is done using the = sign. No space permitted when initializing variables.

• Variables are used by $. Usually use ${var} to avoid ambiguity.

Script argument

Arguments can be passed to the script when it is executed.

Inside the script, the $1 variable references the first argument in the command line, $2 the second argument and so forth. The variable $0 references to the current script.

Statement

• Command

  Variables can be assigned with the value of a command output. var=$(command)

• Perform arithmetic operations

  $((expression))

  ans=$(( x + y ))

Condition control

if [ condition ]; then
  statement
elif [ condition ]; then
  statement
else
  statement
fi

Loop

for var in 1 2 3 4 5
do
    echo $var
done

Other kinds of for loops

for output in $(command)
for var in {1..5}
for var in {0..10..2}
for (( c=1; c<=5; c++ ))

break and continue can be used inside the loop

Function

function_name {
  command
}

Do not have to specity the number of arguments

Output Input

printf “%s: %d\n” “a string” 12