Seyeon

Seyeon

Hello, I am Seyeon :)

[Unix-Linux] Shell Scripting - Part 1

Date:     Updated:

Categories:

Tags:

📋 This is my note-taking from what I learned in the Unix/Linux course!


What is a Shell Script?

  • User interacts with the kernel via a command interpreter – shell
  • We enter commands and the shell executes it.
  • If you have to do the same tasks often, it is better to accumulate the commands into a file and then execute that file.
  • The first line of the file must be the pathname of the shell.
  • You may use the echo command to send output to the screen and the read command that read input from the keyboard.
  • You may use variable to store values.
  • Control structure are supported.
  • You may put any command that you would normally execute at the prompt into the shell script.
  • A shell program is normally assembled line by line.


First Shell Script

#!/bin/bash

# Author: Narendra
# Name: shell00.sh
# Date: April 17, 2023
# Purpose: To demonstrate the following:
# 1) comments
# 2) sending output to the user
# 3) running a simple command

echo Narendra Pershad #this is also a comment
echo COMP301
ls -l

Output:

Narendra Pershad
COMP301
total 56
drwxr-xr-x@  2 seyeonjo  staff     64 16 Jun 13:37 backup
-rw-r--r--   1 seyeonjo  staff     70 12 Jul 08:41 file
-rwxr-xr-x   1 seyeonjo  staff    257 18 Jul 21:52 hello.sh
-rw-r--r--@  1 seyeonjo  staff  10194  9 Jun 16:51 lab.txt
drwxr-xr-x@ 15 seyeonjo  staff    480 15 Jul 19:34 lab3
drwxr-xr-x@  4 seyeonjo  staff    128  9 Jun 16:33 lab4
drwxr-xr-x@  4 seyeonjo  staff    128 23 Jun 13:04 lab6
-rw-r--r--@  1 seyeonjo  staff    854 23 Jun 13:07 lab6.txt
drwxr-xr-x@  6 seyeonjo  staff    192 11 Jul 22:40 lab7
-rwxr-xr-x@  1 seyeonjo  staff   1086 16 Jun 13:37 q5.sh


Running the Shell Script

  • Ensure that the first line of the file is the pathname of the shell.
  • Change the permission of the file to be executable.
    • chmod 755 «script_name»
    • chmod +x «script_name»
  • To run the script, invoke it from the command prompt.
    • ./«script_name»
  • Scripts are not compiled, instead they are interpreted by the linux shell.
  • If an error is encountered, execution halts.
(base) seyeonjo@seyeonjos-MacBook-Pro Unix:Linux % chmod +x hello.sh
(base) seyeonjo@seyeonjos-MacBook-Pro Unix:Linux % ./hello.sh


Variables

  • The shell can access Environment variables.
    • Are normally upper-cased
    • Such as: HOME, PATH
  • You may also define your own working variables in shell scripts (shell variables).
  • You can prevent modification of a variable by marking it read-only.
  • You may remove a variable using the unset command.
  • The printenv command will display all the environment variables.

Variable Names

  • Avoid using dollar sign in variable names.
  • Use descriptive names that reflects the purpose of contents of the variable.
  • Use capitalization appropriately and consistently.
  • If a variable name is to consist of two or more words, use underscores between the words.
  • Variables are case sensitive.
  • Prefer snake casing.

More on Variables

  • Omit spaces when you assign a variable without using single/double quotation marks around value.
  • To assign a variable that must contain spaces, enclose value in “” or ‘’.
  • To reference a variable, use a $ in front of it or enclose it in { }.
  • Use [ ] to refer to a specific value in an array.
  • Variables are local to the current sell session or script.
  • Avoid modifying environment variables.
  • You may export a shell variable to make the variable available to other scripts.
  • If you need to make a variable available to sub-process, use the export command.
  • In this course, we will use a pair of single quotes to represent a variable.

img img img


Shell Operators

Bash shell operators are divided into four groups:

  • Assignment Operators: Used to assign values to variables. Example: = (simple assignment), += (append assignment), etc.
  • Comparison Operators: Used to compare values or expressions. Example: -eq (equal to), -ne (not equal to), -lt (less than), -gt (greater than), etc.
  • Logical Operators: Used to perform logical operations. Example: && (logical AND), || (logical OR), ! (logical NOT), etc.
  • Arithmetic Operators: Used for arithmetic calculations. Example: + (addition), - (subtraction), * (multiplication), / (division), % (modulo), etc.
  • String Operators: Used for string manipulation. Example: = (string equality), != (string inequality), + (string concatenation), etc.
  • File Test Operators: Used to test various properties of files. Example: -e (existence), -r (readable), -w (writable), -x (executable), etc.
  • Conditional Operators: Used for conditional expressions and flow control. Example: if, case, [[ ... ]], etc.
  • Redirection Operators: Used to redirect input/output streams. Example: > (output redirection), < (input redirection), >> (append output), | (pipe), etc.


Second Shell Script

#!/bin/bash

# Author: Narendra
#   Name: shell00b.sh
#   Date: April 18, 2023
#
# Purpose: To demonstrate environment variables.
#   Note, most of these variables are uppercased
#   to get the value of the variable, use the
#   $ expansion operator

echo $USER    #prints: npersh
echo $SHELL     #prints: /bin/bash
echo $NAME    #prints: PR-LA230-NARENDRAPERSHAD
echo $PWD     #prints: /mnt/c/progs/COMP301/progs
echo $LOGNAME     #prints: npersh
echo $HOME    #prints: /home/npersh
echo $BASH    #prints: /bin/bash
echo $BASH_VERSION    #prints: 5.1.16(1)-release
echo $OSTYPE    #prints: linux-gnu
echo the machine type is $MACHTYPE    #prints: The machine type is x86_64-pc-linux-gnu

(base) seyeonjo@seyeonjos-MacBook-Pro hello % chmod +x hello1.sh
(base) seyeonjo@seyeonjos-MacBook-Pro hello % ./hello1.sh


Third Shell Script

echo " name of the script: $0" #prints: ./shell00c.sh
echo " first argument to the script: $1" #prints: mary
echo " second argument to the script: $2" #prints: had
echo " number of arguments to the script: $#" #prints: 5
echo " all the arguments to the script: $@" #prints: mary had a little lamb
echo " all the arguments to the script: $*" #prints: mary had a little lamb
echo " process id of the script: $$" #prints: 96
echo "the exit code for the last command: $?" #prints: 0

(base) seyeonjo@seyeonjos-MacBook-Pro hello % chmod +x hello2.sh
(base) seyeonjo@seyeonjos-MacBook-Pro hello % ./hello2.sh Mary had a little lamb


Fourth Shell Script

#!/bin/bash

# Author: Narendra
# Name: shell00d.sh
# Date: April 17, 2023
#
# Purpose: To demonstrate the following:
# 1) no quotes (strongly discouraged)
# 2) single quotes (does not evaluate variables)
# 3) double quotes (Allows you to evaluate a variable)
# 4) back tick (Allows you to runs a command and assigns the result to a variable)
# 5) user-defined variables: names of variable must not start with a digit

greeting=Hello      #no quotes (don’t use)
first='Narendra K.'       #the preferred way of defining a variable
last="Pershad"      #this is possible, but should be reserved for variable expansion

raw='$greeting world from $first $last'       #no shell expansion
echo $raw       #prints: $greeting world from $first $last

name="$greeting world from $first $last"      #shell expansion
echo $name      #prints: Hello world from Narendra K. Pershad

lscript='wc -l shell00d.sh'       #normal variable
echo $lscript       #prints: wc -l shell00d.sh

lscript=`wc -l shell00d.sh`       #the results of a command is stored
echo $lscript       #prints: 28 shell00d.sh

echo `wc -l shell00d.sh`      #also prints: 28 shell00d.sh
  • wc: The command stands for “word count” and is used to count lines, words, and characters in a file or input.
  • -l: This is an option for wc which specifies that we want to count the number of lines.
  • shell00d.sh: This is the file name or path of the file on which we want to perform the line count.

So, when you run the command wc -l shell00d.sh, the output will display the number of lines in the file shell00d.sh.


Code

(base) seyeonjo@seyeonjos-MacBook-Pro ~ % cat /etc/shells
# List of acceptable shells for chpass(1).
# Ftpd will not allow users to connect who are not using
# one of these shells.

/bin/bash
/bin/csh
/bin/dash
/bin/ksh
/bin/sh
/bin/tcsh
/bin/zsh

(base) seyeonjo@seyeonjos-MacBook-Pro week05 % echo #If you type "echo", print empty line

(base) seyeonjo@seyeonjos-MacBook-Pro week05 % ls ; cal ; date
all.txt		shell01d.sh	shell03b.sh	shell03f.sh
shell01a.sh	shell02a.sh	shell03c.sh	shell03g.sh
shell01b.sh	shell02b.sh	shell03d.sh	shell03h.sh
shell01c.sh	shell03a.sh	shell03e.sh	shell03i.sh
     June 2023
Su Mo Tu We Th Fr Sa
             1  2  3
 4  5  6  7  8  9 10
11 12 13 14 15 16 17
18 19 20 21 22 23 24
25 26 27 28 29 30

Wed  7 Jun 2023 08:54:13 EDT

echo $course #print Unix/Linux Operating System
echo '$course' #print $course
echo "$course" #print Unix/Linux Operating System
echo `$course` #print ./week05b.sh: line 9:Unix/Linux: No such file or directory
echo `date` #print Wed Jun 7 09:15:14 EDT 2023
echo `ls shell01?.sh` #print shell01a.sh shell01b.sh shell01c.sh shell01d.sh

(base) seyeonjo@seyeonjos-MacBook-Pro week05 % cal
     June 2023
Su Mo Tu We Th Fr Sa
             1  2  3
 4  5  6  7  8  9 10
11 12 13 14 15 16 17
18 19 20 21 22 23 24
25 26 27 28 29 30

(base) seyeonjo@seyeonjos-MacBook-Pro week05 % cal | tail +3
             1  2  3
 4  5  6  7  8  9 10
11 12 13 14 15 16 17
18 19 20 21 22 23 24
25 26 27 28 29 30

(base) seyeonjo@seyeonjos-MacBook-Pro week05 % cal | tail +3 | wc -w
      31
(base) seyeonjo@seyeonjos-MacBook-Pro week05 %

cal $1 $2 | tail +3 | wc -w

./week05c.sh november 2023 #print num of nov
./week05c.sh february 2023 #print num of feb



Back to Top

See other articles in Category Unix-Linux

Leave a comment

Lastest article 10 :)