Note_Tech

All technological notes.

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Linux - Fundamental: Kernel

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• Linux - Fundamental: Kernel
  • I/O
    • Input/Output Types
    • Null Device
    • Redirection
  • tee: Redirect both display and a file
    • |: Pips
    • Wildcard
  • Lab: I/O Redirection
    • Redirect the Error Output
    • Redirect to Null Device
  • Kernel
    • Kernel Version
    • Kernel dir
    • Installing the Kernel
    • Lab: Download and Install a New Kernel

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I/O

Input/Output Types

I/O Name	Abbreviation	File Descriptor
Standard Input	stdin	0
Standard Output	stdout	1
Standard Error	stderr	2

• Input(stdin) - 0

  • when feeding file contents to a file.
  • < symbol
    • used to redirect input from a file

• Output(stdout) - 1

  • by default, when running a command its output goes to the terminal
  • > symbol
    • used to redirect output to a file.
  • >> symbol
    • used to redirect output append to a file.

• Error(stderr) - 2

  • when the command produced any error.
  • 2> / 2>> can be used to redirect output.

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Null Device

• Null Device

  • a special file that discards all the data written to it.

• >/dev/null: Redirect output to nowhere.

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Redirection

• >
  • Redirects standard output to a file.
  • Overwrites (truncating) existing contents.
• >>
  • Redirects standard output to a file.
  • Appends to any existing contents.
• <
  • Redirects input from a file to a command.
• &
  • Used with redirection to signal that a file descriptor is being used.
• 2>&1
  • Combine stderr and standard output.
• &>file
  • redirect both output and error to alternative locations
• 2>file
  • Redirect standard error to a file.

ls /usr /cdr &>output.out
cat output.out
# ls: cannot access '/cdr': No such file or directory
# /usr:
# bin
# games
# include
# lib
# lib64
# libexec
# local
# sbin
# share
# src
# tmp

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tee: Redirect both display and a file

CMD	DESC
ls /etc \| tee /tmp/output	printed on the screen as well as
redirected to file

# As user1 on server1, run the ls command on /etc, /dvd, and
# /var. Have the output printed on the screen as well as
# redirected to file /tmp/ioutput, and the errors forwarded to
# file /tmp/ioerror. Check both files after the execution of the
# command and analyze the results. (Hint: Input, Output, and
# Error Redirections).

ls /dvd /var > /tmp/output 2> /tmp/error

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|: Pips

• |

  • pipe symbol
  • command-output | command-input
  • take standard output from the preceding command and passes it as standard input to the following command.
    • The error message of the preceding command will not be passed to the second command by default.
    • If error message is needed, using the I/O direction.

• Example:

# the file content from the cat command will be the content of grep command
cat file | grep pattern

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Wildcard

Wildcard	Desc
*	Matches zero or more characters
?	Matches exactly one characters
[aeiou]	Character class, matches exactly one of included characters
[!aeiou]	Exclude exactly one of characters
[a-g],[3-6]	Range
\?,\*	Escape character, match a wildcard character.

• Named Character Classes

  • [[:alpha:]]: Matches alphabetic characters
  • [[:lower:]]: Matches lower-case letters
  • [[:upper:]]: Matches upper-case letters
  • [[:digit:]]: Matches any one digit 0-9
  • [[:alnum:]]: Matches all alphanumeric characters
  • [[:space:]]: Matches white space
  • [[:blank:]]: Matches blank characters, such as space and tab
  • [[:cntrl:]]: Matches control characters
  • [[:graph:]]: Matches graphical characters
  • [[:print:]]: Matches printable characters
  • [[:punct:]]: Matches punctuation characters

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• Example

ll c[aeiou]t
ll [a-d]*
cp *[[:digit:]] /tmp

rm ??

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Lab: I/O Redirection

# output to a file
ls -l > file.txt
ls -l 1> file.txt      # equivalent, no space between 1 and >

cat file.txt

# output and append to a file
ls >> file.txt

# redirect from a file to a command
sort < file.txt     # sort doesn't work on file.txt

sort < file.txt > sorted_files.txt  # redirect from file to sort and output to the new txt

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Redirect the Error Output

# try to ls one existing file and a not existing file, output error
ls file.txt not-here
# ls: cannot access 'not-here': No such file or directory
# file.txt

# output the std output to a file, but display error.
ls file.txt not-here > out
# ls: cannot access 'not-here': No such file or directory
cat out
# file.txt

# output the error to a file, but display std output
ls file.txt not-here 2> out.err
# file.txt
cat out.err
# ls: cannot access 'not-here': No such file or directory

# split std and err into two files
ls file.txt not-here 1>out 2> out.err
cat out
# file.txt
cat out.err
# ls: cannot access 'not-here': No such file or directory

# redirect both into a file
ls file.txt not-here > out.log 2>&1
cat out.log
# ls: cannot access 'not-here': No such file or directory
# file.txt

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Redirect to Null Device

# try to ls one existing file and a not existing file, output error
ls file.txt not-here
# ls: cannot access 'not-here': No such file or directory
# file.txt

# discard error
ls file.txt not-here 2>/dev/null
# file.txt

# display error only, discard std output
ls file.txt not-here 1>/dev/null
# ls: cannot access 'not-here': No such file or directory

# discard all message,including std output and error
ls file.txt not-here >/dev/null 2>&1

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Kernel

# list kernel package
yum list installed kernel*
# Installed Packages
# kernel.x86_64                       5.14.0-362.24.1.el9_3           @rhel-9-for-x86_64-baseos-rpms
# kernel.x86_64                       5.14.0-503.23.2.el9_5           @rhel-9-for-x86_64-baseos-rpms
# kernel-core.x86_64                  5.14.0-362.24.1.el9_3           @rhel-9-for-x86_64-baseos-rpms
# kernel-core.x86_64                  5.14.0-503.23.2.el9_5           @rhel-9-for-x86_64-baseos-rpms
# kernel-modules.x86_64               5.14.0-362.24.1.el9_3           @rhel-9-for-x86_64-baseos-rpms
# kernel-modules.x86_64               5.14.0-503.23.2.el9_5           @rhel-9-for-x86_64-baseos-rpms
# kernel-modules-core.x86_64          5.14.0-362.24.1.el9_3           @rhel-9-for-x86_64-baseos-rpms
# kernel-modules-core.x86_64          5.14.0-503.23.2.el9_5           @rhel-9-for-x86_64-baseos-rpms
# kernel-tools.x86_64                 5.14.0-503.23.2.el9_5           @rhel-9-for-x86_64-baseos-rpms
# kernel-tools-libs.x86_64            5.14.0-503.23.2.el9_5           @rhel-9-for-x86_64-baseos-rpms

Kernel Version

uname -r
# 5.14.0-503.23.2.el9_5.x86_64
# Major version.Major revision.kernel patch version-Redhat version.enterprise linux 9.processor architecture 

# general Linux: 5.14.0

Kernel dir

Installing the Kernel

Lab: Download and Install a New Kernel