Linux Learning 3 - Shell Script - Variables, Assignments, Parameters and I/O

Made by Mike_Zhang

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1 Introduction

• A script is a program that is interpreted instead of compiled. An interpreter takes each instruction in the script one at a time, translates it to an executable statement, and executes it;
• Shell Scripting is to produce code to automate Linux system tasks.

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2 Create and Run

1. Create a script file

vi scriptexample.sh

1. Write the script

• Every shell script must start with a comment that specifies the interpreter which should run the script.
  • Bash: #!/bin/bash
  • Csh: #!/bin/csh
  • Tcsh: #!/bin/tcsh

#!/bin/bash
date
ls

When in the vi window, press key I to edit, press Esc to exit edit mode, then type :wq and press Enter to save and exit.

1. Change the file permission

• The script file must also be executable. We will use permission 745, or alternatively 755.

chmod +x ./scriptexample.sh

1. Run the script

• execute this script by entering:

./scriptexample.sh

• The start script outputs information to your terminal window.

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echo: output statement;
#: comment

#!/bin/bash
date
echo # output a blank line
ls

Redirect the script’s output to a file:

./scriptexample.sh >> output.txt

output.txt file:

at Jan 28 10:11:56 HKT 2023

linux_basic
linux_basic.md
output.txt
scriptexample.sh

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3 Variables Assignments Parameters

3.1 Variables in Bash

Environment variables:

• because they have been exported, are accessible from within your script or from the command line and variables defined within your script that are only available in the script.

Variables:

• letters, numbers, and underscores(_);
• starts with a letter or underscore;
• case sensitive
• reserved words (e.g., if, then, while) are only recognizable if specified in lower case.
• variable names can be the same as reserved words such as if, but not recommended.
• Variables in Bash store ONLY strings. Numbers are treated as strings unless we specify that we want to interpret the value as a number;

3.2 Assignment Statements

VARIABLE = VALUE

• No spaces are allowed around the equal sign;
• Variable does NOT need to be declared;
• Value on the right hand side can be one of four things:
  • A literal value such as Hello, “Hi there,” or 5
  • The value stored in another variable, in which case the variable’s name on the right hand side of the assignment statement is preceded by a dollar sign as in $FIRST_ NAMEor $X
  • The result of an arithmetic or string operation
  • The result returned from a Linux command

Quote mark:

• If the value is a string with no spaces, then quote marks may be omitted;
• If the value is a string with spaces, quote marks must be included.

[Example]

X=5

• X stores the string 5

NAME = Frank

• NAME stores Frank

NAME = "Frank Zappa"

• NAME stores Frank Zappa, the quote marks are required because of the space between Frank and Zappa

NAME = Frank Zappa

• An error arises because of the space, the Bash interpreter thinks that Zappa is a command, which does not exist

NAME = FrankZappa

• No quote marks are required here because there is no space, so in fact the literal value is really one string of 10 characters

NAME = ls *

• An error should arise as Bash tries to execute *, which is replaced by all of the entries in the current directory. If all of the entries in the directory are executable programs, this will result in all of these programs executing!

X = 1.2345

• X stores string 1.2345

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To retrieve the value stored in a variable:

• Must precede the variable name with a $ as in $NAME or $X .
• However, the $ is NOT applied if the variable is enclosed in single quote marks (‘ ’).
• It is applied if the variable is enclosed in double quote marks (“ ”)

[Example]

NAME = "$FIRST_NAME $LAST_NAME"

• NAME stores Frank Zappa

NAME = '$FIRST_NAME $LAST_NAME'

• NAME stores $FIRST_NAME $LAST_NAME

NAME = $FIRST_NAME$LAST_NAME

• Name stores FrankZappa, notice that since there is no space on the right hand side of the assignment statement, we did not need to place $FIRST_Name $LAST_NAME in quote marks

NAME = $FIRST_NAME $LAST_NAME

• Results in an error because there is a space and no quote marks

NAME = "FIRST_NAME LAST_NAME"

• NAME stores FIRST_NAME LAST_NAME because there is no $ preceding the variable names

NAME = 'FIRST_NAME LAST_NAME'

• NAME stores FIRST_NAME LAST_NAME

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To remove a value from a variable

• Assign it the NULL value: VARIABLE = NULL
• Having nothing on the right hand side of the equal sign: VARIABLE =
• Use the unset command: unset VARIABLE

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3.3 Executing Linux Commands

assignment statement: DATE = date

• sets the variable DATE to store the string date
• and sending the output to the terminal window

To execute and store the command ONLY, not output to the terminal window:

• place command within either ` ` marks or $() marks;

[Example]

DATE = `date` # or
DATE = $(date)

• This instruction causes the Linux date command to execute
• NOT sending the output to the terminal window,
• the output is stored in the variable DATE.

[Example]

DATE = "Hello $FIRST_NAME, today’s date and time is $(date)"

• embed the entire right hand side in “” or ‘’ marks

If the item enclosed inside ` ` or $() is not a Linux command, an error will arise.

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To declare a variable

declare [options] name[=value]

• options and the initial value are optional
• Options are denoted
  • + to turn an attribute of the variable off
  • and − to turn it on.
  • Option:
    • a (array), f (function), i (integer), r (read-only), and x (exported).

[Example]

declare –rx ARCH = 386

• create a read-only variable, ARCH, with the value 386 and export it beyond the current shell

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3.4 Arithmetic Operations

Two ways:

1. precede the assignment statement with the word let
   • let a = n + 1
2. embed the arithmetic operation inside the notation $(())
   • a = $((n + 1))

     $ before n in either case is allowed but not necessary.

[Example]

N=1
N=$((N+1))

• N is now 2

[Example]

let N+=1

• Reassign N to be 1 greater, N is now 3, note that without the “let” statement, N+=1 is interpreted instead as string concatenation

let Y=$N+1

• Y is 4, the $ in front of N is not needed so this could be let Y = N + 1

let Z=Y%N

• Z is 1 (4/3 has a remainder of 1)

Y=$(($Y+1))

• Y is now 5, we can omit the $ before Y

let Y=$((Y << 2))

• Y is now 20. << is a left shift which results in a number being doubled. The number 2 in this instruction indicates that the left shift should be done twice. As Y was 5, the left shift doubles Y to 10 and then 20.

((Y++))

• Y is now 21

((Q––))

• As Q had no value, it is now −1

N=$N+1

• N has the value of 3 + 1 (literally a 3, a plus, and a 1) because $N+1 is treated as a string concatenation operation instead of an arithmetic operation

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Bash Arithmetic Operations

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Can specify multiple assignment statements on the same line:

X = 1 Y = 2 Z = 3

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Command expr:

• performs and returns the result of a string or arithmetic operation;
• print to the terminal window.

[Example]

N=3

expr $((N+1))

• This returns 4 without changing N

expr $N+1

• This is treated as string concatenation and returns the string 3 + 1

X=‘expr $((N+1))‘

• The expr command computes and returns 4, which is then stored in X, so X stores 4 when this instruction concludes

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3.5 String Operations Using expr

expr command string params

command: substr, index, length;

1. substr

• a substring operation;

expr substr string index length

• string: the string;
• index: an integer indicating the position in the string to start the substring (base-1 index);
• length: an integer indicating the length of the substring

expr substr abcdefg 3 2: return the string cd.

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1. index

• Given two strings and it returns the location within the first string of the first character in the second string found;

expr index str1 str2

• The index returned is the location in the master string (str1) of the first matching character of the search string (str2);

expr index abcdefg qcd: returns 3

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1. length

• returns the length of a string

expr length str

expr length abcdefg: return 7

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• only work if it is executed in an expr command.
• these can be used as the right hand side of an assignment statement in which case the entire expr must be placed inside ` ` marks or inside the notation $().

[Example]

PHRASE stores “hello world”

X=‘expr substr "$PHRASE" 4 6‘

• X would store “lo wor”

X=‘expr index "$PHRASE" nopq‘

• X would store 5

X=‘expr length "$PHRASE"‘

• X would store 11

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3.6 Parameters

• Specified by the user at the time that the script is invoked.
• Parameters are listed on the command line after the name of the script.

./scriptexample.sh 2 3

• 2, 3 are parameters;
• they are not variables. They cannot change value while the script is executing;
• They are accessed using the notation $n where n is the index of the parameter

[Example]

./scriptexample.sh:

#!/bin/bash
STR1="Hello World"
STR2="Frank Zappa Rocks!"
STR3="abcdefg"
expr substr "$STR1" $1 $2
expr substr "$STR2" $1 $2
expr substr $STR3 $1 $2

Run:

./scriptexample.sh 2 3

Output:

ell
ran
bcd

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4 Input and Output

4.1 Output with echo

output statement: echo

[Example]

echo Hello $NAME, how are you?
echo Your current directory is $PWD, your home is $HOME
echo Your current directory is ‘pwd‘, your home is ~
echo The date and time are $(date)
echo The number of seconds in a year is $((365*24*60*60))

Accept escape characters:

• –e option: echo to interpret these characters;

[Example]

echo –e "Hello World!\nHow are you today?"

• Hello World
• How are you today?

echo –e "January\tFebruary\tMarch\tApril"

• January February March April

echo –e "\x40\x65\x6C\x6C\x6F"

• Hello

echo –e Hello\\World

• Hello\World (recall that the quote marks are not needed for \\)

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• The use of “” and ‘’ in echo statements is slightly different from their use in assignment statement;
• The “” return values of variables and Linux commands whereas in ‘’, everything is treated literally.

[Example]

echo $FIRST $LAST

• Frank Zappa

echo "$FIRST $LAST"

• Frank Zappa

echo '$FIRST $LAST'

• $FIRST $LAST

echo "$FIRST \n$LAST"

• Frank \nZappa

echo –e $FIRST \n$LAST

• Frank nZappa

echo –e "$FIRST \n$LAST"

• Frank
• Zappa

echo –e "abc\bdef"

• abdef − the \b backs up over the ‘c’

echo –e "\0106\0162\0141\0156\0153"

• Frank

echo –e "$FIRST\t$LAST"

• Frank Zappa

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4.2 Input with read

input statement: read

• The read statement is then followed by one or more variables;
• If multiple variables are specified they must be separated by spaces;
• The variable names are listed without $ preceding them;
• The read can be used to input any number of variable values.
• If the user does NOT provide the number of values as expected, any remaining variables are given NULL values.

[Example]

scriptexample.sh:

echo Enter your name
read NAME
echo $NAME

Output:

Enter your name
|

Input (and press Enter):

Mike<Enter>

Output:

Mike

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–n option for echo:

• causes the output to not conclude with a new line

echo -n Enter your name
read NAME
echo $NAME

Output:

Enter your name|

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-p option for read:

• outputs a prompting message prior to the input

read –p "message" var1 var2 ...

[Example]

read -p "Enter your name: " NAME
echo $NAME

Output:

Enter your name: |

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–N option for read:

• Specify an integer number;
• Accept only N number of inputs;
  • read –N 10 will only read up to the first 10 characters;
• The –N option will only store the input into one variable, so you would not use it if your input was for multiple variables.

–t option for read:

• Specify a number of seconds.
• If the input is not received from keyboard within that number of seconds, the instruction times out and no value is stored in the variable

-s option for read:

• Read statement execute in silent mode;
• Any input characters are not echoed to the terminal window;
• Used when inputting a password or other sensitive information;
• If read with no variables, the Bash interpreter uses the environment variable REPLY to store the input in to.

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References

R. Fox, Linux with operating system concepts. Boca Raton, Fl: Crc Press, 2015.

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Outro

尾巴
有关Linux的学习，马上继续更新。
最后，希望大家一起交流，分享，指出问题，谢谢！

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