Unix Shell Scripting Tutorial - page 4 | Web Technology Experts Notes

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?

^a: Start with a
a$: End with a
.: Match any single character.
*: Match zero OR more occurances.
[abcd]: Any character from a,b,c and.
\: Skip the effects of special character.
[a-z]: Match a-z any character.
[a-z0-9]: March a-z OR 0-9 any character.
[[:alnum:]]: Alphanumeric [a-z A-Z 0-9]
[[:alpha:]]: Alphabetic [a-z A-Z]
[[:blank:]]: Blank characters (spaces or tabs)
[[:cntrl:]]: Control characters
[[:digit:]]: Numbers [0-9]
[[:graph:]]: Any visible characters (excludes whitespace)
[[:lower:]]: Lowercase letters [a-z]
[[:print:]]: Printable characters (non-control characters)
[[:space:]]: Whitespace
[[:punct:]]: Punctuation characters
[[:upper:]]: Uppercase letters [A-Z]
[[: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?

cat filename: Displays a filename.
cd dirname: Moves you to the identified directory.
cp file1 file2: Copies one file/directory to the specified location.
file filename: Identifies the file type
head filename: Shows the beginning of a file
less filename:
ls dirname: Display the content of directory.
mv file1 file2: Rename a file
tail filename: Display the end of file
touch filename: Create a blank file.
whereis filename: Display the location of file
:

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?

Root User: It is super admin who have all the access and no need to any further permission.
System accounts: It have access of system-specific components like mail accounts and the sshd accounts.
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?