Unix Shell Scripting Tutorial - page 4

Question: What are man pages?
Unix's version of Help files are called man pages.


Question: Can we perform regular expression in unix?
Yes, We can do.
We can do using sed.
Full form of SED is stream editor.
For Example(s)

cat /etc/passwd | sed -n '1,3p'

REGEX_DATE='^\d{2}[/-]\d{2}[/-]\d{4}$'
echo "$1" | grep -P -q $REGEX_DATE
echo $?

Question: What are character meaning in regular expression?

1. ^a: Start with a
2. a$: End with a
3. .: Match any single character.
4. *: Match zero OR more occurances.
5. [abcd]: Any character from a,b,c and.
6. \: Skip the effects of special character.
7. [a-z]: Match a-z any character.
8. [a-z0-9]: March a-z OR 0-9 any character.
9. [[:alnum:]]: Alphanumeric [a-z A-Z 0-9]
10. [[:alpha:]]: Alphabetic [a-z A-Z]
11. [[:blank:]]: Blank characters (spaces or tabs)
12. [[:cntrl:]]: Control characters
13. [[:digit:]]: Numbers [0-9]
14. [[:graph:]]: Any visible characters (excludes whitespace)
15. [[:lower:]]: Lowercase letters [a-z]
16. [[:print:]]: Printable characters (non-control characters)
17. [[:space:]]: Whitespace
18. [[:punct:]]: Punctuation characters
19. [[:upper:]]: Uppercase letters [A-Z]
20. [[:xdigit:]]: Hex digits [0-9 a-f A-F]

Question: Can we execute multiple command in single statement?
Yes, We can execute multiple commands in single statement.
For Example

sed -e 'command1' -e 'command2'  -e 'command3'  -e 'command4' 

Question: What are the most comman commands?

1. cat filename: Displays a filename.
2. cd dirname: Moves you to the identified directory.
3. cp file1 file2: Copies one file/directory to the specified location.
4. file filename: Identifies the file type
5. head filename: Shows the beginning of a file
6. less filename:
7. ls dirname: Display the content of directory.
8. mv file1 file2: Rename a file
9. tail filename: Display the end of file
10. touch filename: Create a blank file.
11. whereis filename: Display the location of file
12. :

Question: What is use of df command?
Its used to display the disk space usages.

Filesystem     1K-blocks     Used Available Use% Mounted on
/dev/xvda1      20509288 15036052   5372988  74% /
devtmpfs         2015996       64   2015932   1% /dev
tmpfs            2024996        0   2024996   0% /dev/shm
/dev/xvdf      103081248 44579524  53258844  46% /var/lib/mysql
/dev/xvdg      103081248 89393356   8445012  92% /mnt

Question: What is use of du command?
Its used to display how much space is taking by each directory.


Question: What mount command used for?
mount point is a directory to access your data (files/folders) which is stored in your disks.


Question: What unmount command used for?
To unmount (remove) the file system from your system.
For Example

umount /dev/cdrom

Question: What are the different type of users in unix?

1. Root User: It is super admin who have all the access and no need to any further permission.
2. System accounts: It have access of system-specific components like mail accounts and the sshd accounts.
3. User accounts: User accounts provide interactive access to the system. Users are typically assigned to these accounts and usually have limited access files and directories.

Question: What are the main administrator files?

1. /etc/passwd: Keeps the user account and password info.
2. /etc/shadow: Holds the encrypted password.
3. /etc/group: It have group information.
4. /etc/gshadow: secure group account information

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1. Unix Shell Scripting Tutorial - page 3.
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Unix Shell Scripting Tutorial - page 3

Question: What are special variable in unix?
Special variable are those variable which are reserved by unix shell. For Example $$ is special variable and it return the current process id.


Question: How to use array variable in for loop?
Yes, we can define.

NUMS="1 2 3 4 5 6 7 8 9"
for NUM in $NUMS
do
   Q=`expr $NUM % 2`
   if [ $Q -eq 0 ]
   then
      echo "Number is an even number!!"
      continue
   fi
   echo "Found odd number"
done

Question: Give example of while with break?

a=2
while [ $a -lt 10 ]
do
   echo $a
   if [ $a -eq 5 ]
   then
      break
   fi
   a=`expr $a + 1`
done

Once $a will reach to 5, then it will break the statement.
output

2
3
4
5

Question: How to print the special character as string?
Use the Backslash before the special character, It will print. For Example:

echo \$;

Question: What is Output Redirection?
We use > notation to Output Redirection.

who > users

If a command has its output redirected to a file and the file already contains some data, that data will be lost.

We can use >> to append in file, it will not lost the data.

who >> users

Question: What are the Redirection Commands?

1. pgm > file: Output of pgm is redirected to file.
2. pgm < file: Program pgm reads its input from file.
3. pgm >> file: Output of pgm is appended to file.
4. n > file: Output from stream with descriptor n redirected to file
5. n >> file: Output from stream with descriptor n appended to file
6. n >& m: Merges output from stream n with stream m.

Question: How to create a custom function?

# Create a new function with name of Hello
Hello () {
   echo "Hello World! How are you?"
}

# Call the function
Hello

Question: How to create a custom function with parameter?

# Define your function here
Hello () {
   echo "Hello World! How are you?"
   echo "First parameter: $1";
   echo "Second parameter: $2";    
}

# Invoke your function
Hello first second

Output

Hello World! How are you?
First parameter: first
Second parameter: second

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