linux – Page 2 – Piyush Gupta Blog

How to prompt for User Input in Linux shell script

Author : Piyush Gupta

Command:

# read var

# read -s “Waiting for input: ” var

Uses:

read command is used for getting user input in a Linux shell script. -p switch with read command is used for showing some helpful text on-screen. Create a shell script named input.sh and add following content.

#!/bin/bash

read -p "Enter Your Name: "  username
echo "Welcome $username!"

Lets execute the shell script

# sh input.sh

Enter Your Name: Piyush
Welcome Piyush!

Input Password in Shell Script

If you want to take input of password in shell script. You must want to not to show any character on-screen entered by user. Use -s for silent mode. Using -s input characters are not echoed.

#!/bin/bash

read -s -p "Enter Password: " pswd
echo $pswd

How to extract .gz File in Linux

Author : Piyush Gupta

GZ files are archive files compressed with the “gzip” program, similar to zip files. These archive files contain one or more files, compressed into a smaller file size for faster download times from the Internet. Source code and other software program files for Linux are often distributed in .gz or .tar.gzformat. Unzip a GZ file using the “gunzip” command or a .tar.gz file using the “tar” command in a Linux terminal.

Command:

$ gunzip archive.gz

Uses:

.gz is files are compressed with gzip in linux. To extract .gz files we use gunzip command.

First use following command to create gzip (.gz) archive of access.log file. Keep remember that below command will remove original file.

gzip access.log

Above command will create a archive file named access.log.gz in current directory.

ls -l access.log.gz
-rw-r--r-- 1 root root 37 Sep 14 04:02 access.log.gz

Now use gunzip command to extract access.log.gz file using command. This will extract the file from archive and remove .gz file automatically.

gunzip access.log.gz

How to find a String in all files in Linux

Author : Piyush Gupta

How to Find all Files Containing a String in Linux. This tutorial will help you to search all files containing specific text string recursively. This tutorial uses “find” command to search string in files. Alternatively, You can also use grep command to search text.

Command:

# find / -exec grep -l “string-to-search” {} ;

Uses:

Below is an example command which searches text ‘redhat’ in / filesystem. This command will search for all files containing string ‘redhat’ and list file names only like below.

# find / -exec grep -l "redhat" {} ; 

/lib64/security/pam_oddjob_mkhomedir.so
/lib64/libdevmapper.a.1.02
/lib64/libdevmapper-event.a.1.02
/lib64/libdevmapper.a
/lib64/libdevmapper-event.a
...

To Search in specific file extension files. Like search ‘redhat’ in all php files only, use following command.

# find / -name '*.php' -exec grep -l "redhat" {} ; 

/var/www/projects/index.php
/var/www/projects/new.php
...

How to use DPKG commands in Linux

Author : Piyush Gupta

DPKG is the main package management program in Debian and Debian based System. It is used to install, build, remove, and manage packages. Aptitude is the primary front-end to dpkg.

Some the most commonly used dpkg commands along with their usages are listed in below table:

COMMAND DETAILS

DPKG COMMAND

Install a package

dpkg -i {file.deb}

Update package

dpkg -i {file.deb}

Remove an installed package

dpkg -r {package}

List all installed packages

dpkg -l

List files in an installed package

dpkg -L {package}

Show information about installed package

dpkg -p {package}

Show information about package file

dpkg -I {file.deb}

List files in a package file

dpkg -c {file.deb}

Example:

For installing packages on debian based system we use dpkg (Debian Package Management System) command.

sudo dpkg -i package.deb

Installing Package:

To install a Google chrome from a deabian package file (google-chrome-stable_current_i386.deb) use -i command line switch.

$ sudo dpkg -i google-chrome-stable_current_i386.deb

Removing Package:

In case if we need to remove any package use -r command line switch with package name.

$ sudo dpkg -r google-chrome-stable_current_i386.deb

How to use RPM Commands in Linux

Author : Piyush Gupta

RPM command is used for installing, uninstalling, upgrading, querying, listing, and checking RPM packages on your Linux system. RPM stands for Red Hat Package Manager.

With root privilege, you can use the rpm command with appropriate options to manage the RPM software packages.

In this article, let us review command details of rpm command.

COMMAND DETAILS	RPM COMMAND
Install a package	rpm -i {package.rpm}
Update package	rpm -U {file.rpm}
Remove an installed package	rpm -e {package}

List all installed packages	rpm -qa
List files in an installed package	rpm -ql {package}
Show information about installed package	rpm -qi
Show information about package file	rpm -qpi {file.rpm}
List files in a package file	rpm -qpl {file.rpm}
Verify all installed packages	rpm -Va
Verify installed package	rpm -V {package}

Example:

For installing packages on RedHat based systems, we use rpm (Red Hat Package Manager) command.

rpm -i package-1.2.3.rpm

Installing Package:

To install a rpm package using command line on redhat based system use -i command line switch with rpm command.

rpm -i package-1.2.3.rpm

You can also use YUM or DNF package manager to install downloaded rpm file. Its benefit to resolve dependencies required for the package.

yum localinstall package-1.2.3.rpm     ## CentOS, RHEL systems 
dnf localinstall package-1.2.3.rpm     ## Fedora systems

Removing Package:

In case if we need to remove any package use -e command line switch with the package name.

rpm -e package-1.2.3.rpm

How to Exit Vim Text Editor

Author : Piyush Gupta

VIM is an enhanced version of VI text editor. It is highly configurable text editor to edit files very efficiently. Many of Linux beginners faces issue with exit form vi/vim editor. This tutorial will help you to exit Vim text editor in Linux command line.

How to Exit Vim?

As we have already seen in advanced Vim tips, there are multiple ways to quit Vim. Let me list them one by one along with the procedure.

You have several ways to exit from Vim text editor. But you need to choose the right command to exit from Vim editor based on your decision. Remember that exit command always run from Vim command mode. So you need to press ESC to go to command mode. Now type a colon :to before writing exit command.

ESC + :q – Simple exit from a file, only if no changes made to file.
ESC + :wq – Write changes and quite from Vim editor.
ESC + :q! – Quit Vim editor without saving any changes (discard all changes made after edit file or last saved).

Using Keyboard Shortcuts

You can also use keyboard shortcuts to exit from vim editor. No need to type any thing. Press ESC to make sure you are in command mode.

Shift + z + z – save changes and exit
Shift + z + q – exit without saving changes

Reference: http://vimdoc.sourceforge.net/

How to use cut Command in Linux

Author : Piyush Gupta

Linux cut command is used for extracting file content on fields basis. text files do not have row/column like databases and some times we need the data of single column only. Cut identified columns on basis of separator (eg: colon ‘:’, semicolon ‘;’, comma ‘,’ etc).

For this example we are taking /etc/passwd file. All the rows are stored as below format with colon (:) separated like below. We use -f to specify field number and -d for delimiter (separator).

As per above screen-cast this file has 7 fields. Cut also support to fetch values on character basis suing -c command line switch. Lets take below examples, for which I am using /etc/passwd file.

1. Select Single Field from File –

For example we need the list of usernames from our /etc/passwd file. We know that first column stored username, Entire file is separated by colon (:).

# cut -d":" -f1  < /etc/passwd

root
bin
daemon
adm
lp
sync
shutdown
halt

We can also use pipeline ” | ” for passing the file content as input to cut command, like below –

# cat  /etc/passwd | cut -d":" -f1

2. Select Multiple Columns from File –

We can specify multiple field names with command separated, like below example will show the 1’st, 2’nd and 7’th fields only.

# cut -d":" -f1,2,7 < /etc/passwd

root:x:/bin/bash
bin:x:/sbin/nologin
daemon:x:/sbin/nologin
adm:x:/sbin/nologin
lp:x:/sbin/nologin
sync:x:/bin/sync
shutdown:x:/sbin/shutdown
halt:x:/sbin/halt
mail:x:/sbin/nologin
uucp:x:/sbin/nologin

We can also specify the range of columns with hyphen (-) on fields as well as both option’s together like below example commands.

Here first command will select 1’st, 2’nd,3’rd and 4’th fields.
Second command will select 3’rd, 4’th and 5’th fields.
Last command will show 2’nd, 3’rd, 4’th, 6’th and 10’th fields.

# cut -d":" -f1-4 < /etc/passwd
# cut -d":" -f3-5 < /etc/passwd
# cut -d":" -f2-4,6,10 < /etc/passwd

To get values of all columns except one use following command. For example if we need to select all columns but not 6.

# cut -d":" --complement -s -f6 < /etc/passwd

3. Selecting Single Character’s from File –

Except fields we can also select values from file on basis of single characters, while using characters we don’t need to specify separator.

# cut -c1 < /etc/passwd

r
b
d
a
l
s
s

Similarly fields we can also specify multiple comma separated characters or range of characters.

# cut -c1,2,3,6,7 < /etc/passwd
# cut -c1-3,6,7 < /etc/passwd

How to Use chattr Command in Linux

Author : Piyush Gupta

Chattr – command is useful to change file attributes on Linux second extended file system. It provides more security on your files from unwanted changes and deletes. There are many attributes available to do it.

To Add attributes on a file we use plus (+) and to remove attributes use minus(-) sign . Some impotent attributes are [ASacDdIijsTtu].

How to Use Chattr Command

Add Attribute on File

Create a new file to test this. As example I have crate a new file piyush.txt using touch command and assigned 777 permission on file.

# touch piyush.txt
# chmod 777 piyush.txt
# ls -l piyush.txt
-rwxrwxrwx 1 root root 0 Apr 19 15:10 piyush.txt

Now enable i attribute on file

# chattr +i piyush.txt

As you have enabled i attribute on file, let’s try to remove this file, you we get following error, even file has 777 permissions.

# rm -f piyush.txt
rm: cannot remove `piyush.txt': Operation not permitted

List Attributes of File

To List attributes on a file use lsattr command specified with file name.

# lsattr piyush.txt
----i-------- piyush.txt

Remove Attributes of File

To clear attribute on a file just use (-) sign with attached attributes. for example in piyush.txt.

# chattr -i piyush.txt

# lsattr piyush.txt
------------- piyush.txt

Attributes List in Chattr Command

Attribute [ i ]: A file with i attribute cannot be modified, it cannot be deleted or renamed by any user included root. Only root can clear this attribute. By this attribute you can keep safe your impotent files from other users or accidental delete.

How to read file Line by Line using Linux Shell Script

Author : Piyush Gupta

You can use while..do to read file line by line on a Linux or Unix-like system.

Syntax: Read file line by line on a Bash Unix & Linux shell:

The syntax is as follows for bash, ksh, zsh, and all other shells to read a file line by line
while read -r line; do COMMAND; done < input.file
The -r option passed to read command prevents backslash escapes from being interpreted.
Add IFS= option before read command to prevent leading/trailing whitespace from being trimmed –
while IFS= read -r line; do COMMAND_on $line; done < input.file

Example:-

How to Read a File Line By Line in Bash

Here is more human readable syntax for you:

#!/bin/bashinput="/path/to/txt/file"
while IFS= read -r line
do
  echo "$line"
done < "$input"
The input file ($input) is the name of the file you need use by the read command. The read command reads the file line by line, assigning each line to the $line bash shell variable. Once all lines are read from the file the bash while loop will stop. The internal field separator (IFS) is set to the empty string to preserve white-space issues. This is a fail-safe feature.
How to read a file Line By Line using Shell Script

Some times we required to read file content line by line inside shell a script. For this example this script will read /etc/passwd file line by line and print usernames with there corresponding home directory.

#!/bin/bash

while read line
do
    USERNAME=`echo $line | cut -d":" -f1`
    HOMEDIR=`echo $line | cut -d":" -f6`
    echo "$USERNAME =>  $HOMEDIR"
done < /etc/passwd

How to Find File Creation Time in Linux

Author : Piyush Gupta

File creation time is stored in inode in EXT4 file system. An earlier version of EXT files systems doesn’t support file creation time.

There is a crtime (create time) timestamp in the debugfs stat output. finally EXT4 supports create time just like btime in NTFS windows.

Follow below instructions to how to find file creation time. Select an existing file or create a new file for testing. For this example, I am using an existing file.

Step 1 – Find Inode Number of File

First of all, find the inode number of any file using the following command on terminal.

$ ls -i /var/log/secure

13377 /var/log/syslog

Step 2 – Find File Creation Time (crtime)

After getting the inode number of file, Use debugfs command with inode number stats following by disk path.

$ debugfs -R 'stat ' /dev/sda1

Implementation:

$ debugfs -R 'stat ' /dev/sda1

debugfs 1.41.12 (17-May-2010)
Inode: 13377   Type: regular    Mode:  0600   Flags: 0x80000
Generation: 2326794244    Version: 0x00000000:00000001
User:     0   Group:     0   Size: 223317
File ACL: 0    Directory ACL: 0
Links: 1   Blockcount: 440
Fragment:  Address: 0    Number: 0    Size: 0
 ctime: 0x5230b7ae:55efa068 -- Thu Sep 12 00:04:22 2013
 atime: 0x5230b7ae:55efa068 -- Thu Sep 12 00:04:22 2013
 mtime: 0x5230b7ae:55efa068 -- Thu Sep 12 00:04:22 2013
crtime: 0x4eeacc8a:0948eb58 -- Fri Dec 16 10:13:54 2011
Size of extra inode fields: 28
Extended attributes stored in inode body:
  selinux = "system_u:object_r:var_log_t:s000" (31)
EXTENTS:
(0-24): 35008-35032, (25-54): 164224-164253

Find the entry of crtime in above output. This is the actual file creation time.

References: Read more about ext4 file system

COMMAND DETAILS	DPKG COMMAND
Install a package	dpkg -i {file.deb}
Update package	dpkg -i {file.deb}
Remove an installed package	dpkg -r {package}
List all installed packages	dpkg -l
List files in an installed package	dpkg -L {package}
Show information about installed package	dpkg -p {package}
Show information about package file	dpkg -I {file.deb}
List files in a package file	dpkg -c {file.deb}