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Linux

Table of Contents

Linux Commands

grep

grepis a command under linux which is user for searching text patterns in files.

    grep [options] pattern [file...]
  • options: Optional flags that modify the behaviour of the grep command.
  • pattern: The regular expression or string pattern to search for e.g. "help"
  • file...: Optional arguments specifying the files to search in. If not provided, grep reads input from standard input.

grep options

Option Description Example
-c Count the number of matching lines grep -c "pattern" file.txt
-i Perform case-insensitive matching grep -i "pattern" file.txt
-v Invert the match (select non-matching lines) grep -v "pattern" file.txt
-n Show line numbers along with matching lines grep -n "pattern" file.txt
-l List only the names of matching files grep -l "pattern" file1.txt file2.txt
-r Recursively search directories and subdirectories grep -r "pattern" directory/
-E Use extended regular expressions grep -E "pattern" file.txt
-F Treat the pattern as a fixed string grep -F "pattern" file.txt
-w Match whole words only grep -w "pattern" file.txt
-m <num> Stop reading a file after matching lines grep -m 5 "pattern" file.txt
-q Suppress output; useful for script usage grep -q "pattern" file.txt
--color Highlight the matching pattern in output grep --color "pattern" file.txt

cp

cpis a command under linux which is used to copy files and directories.

  • options: Optional flags that modify the behaviour of the cp command.
  • source: Specifies the source file or directory of which the cp command takes contents from.
  • destination Specifies the destination file or directory in which the cpcommand put content in.

cp options

Option Description Example
-r, -R Copy directories recursively cp -r sourcedir/ destinationdir/
-p Preserve the original file attributes cp -p file.txt backup/
-f Force overwrite existing files without prompt cp -f newfile.txt existingfile.txt
-i Prompt before overwriting existing files cp -i file.txt destination/
-v Verbose output (display copied file names) cp -v file1.txt file2.txt directory/

alias

aliasis a command under linux which is used to define aliases for commands.

Certainly! Here's a cheat sheet for creating and managing aliases in Linux using the alias command, represented in Markdown table format:

The syntax for creating aliases using the alias command in Linux is as follows:

alias <name>=<value>
  • <name>: The name you want to assign to the alias. It can be any valid name without spaces.
  • <value>: The command or set of commands you want the alias to represent. It can be any valid command or a combination of commands.

Here are a few examples to illustrate the alias syntax:

  1. Create an alias named ll that executes ls -l:

    alias ll="ls -l"

  2. Create an alias named gs that executes git status:

    alias gs="git status"

  3. Create an alias named cpv that executes cp -v (verbose copy):

    alias cpv="cp -v"

  4. Create an alias named d that executes docker:

    alias d="docker"

alias options

Command Description Example
alias Lists all defined aliases. alias
alias <name>=<value> Creates an alias with the specified name and value. alias ll="ls -l"
unalias <name> Removes the alias with the specified name. unalias ll
alias <name> Displays the definition of the alias with the given name. alias ll
alias <name>= Clears the definition of the alias with the given name. alias ll=

Aliases are useful for creating custom shortcuts or abbreviations for frequently used commands. By defining aliases, you can save time and simplify your command-line workflow.

Remember that aliases are typically defined in shell configuration files like ~/.bashrc or ~/.bash_profile. After modifying the configuration file, you need to reload it or open a new terminal session for the changes to take effect.

cat

The cat command concatenates (reads) the contents of a file and prints it to the standard output.

cat [OPTION]... [FILE]...

Here's a breakdown of the syntax elements:

  • cat: The command itself.
  • [OPTION]...: Optional flags that modify the behavior of the cat command. Flags are prefixed with a dash (-).
  • [FILE]...: Optional file names or paths. You can specify one or more files to display their contents or concatenate them together.

Here are a few examples illustrating the usage of the cat command:

  1. Display the contents of a single file:

    cat file.txt

  2. Concatenate multiple files and display their contents:

    cat file1.txt file2.txt

  3. Display the contents of a file with line numbers:

    cat -n file.txt

  4. Concatenate multiple files and save the result to a new file:

    cat file1.txt file2.txt > merged.txt

  5. Append the contents of a file to an existing file:

    cat file2.txt >> file1.txt

cat options

Command Description Example
cat Display the contents of a file cat file.txt
cat file1 file2 Concatenate multiple files and display their contents cat file1.txt file2.txt
cat > file Create a new file and enter text (Ctrl+D to save) cat > newfile.txt
cat >> file Append text to an existing file (Ctrl+D to save) cat >> existingfile.txt
cat -n Display line numbers with the file contents cat -n file.txt
cat -b Display line numbers with non-empty lines cat -b file.txt
cat -E Display $ at the end of each line cat -E file.txt
cat -T Display tabs as ^I cat -T file.txt
cat -s Squeeze multiple blank lines into one cat -s file.txt
cat -v Display non-printable characters in a visible format cat -v file.txt

rm

The rm command in Linux is used to remove/delete files and directories. The basic syntax of the rm command is:

rm [OPTION]... FILE...

Here's a breakdown of the syntax elements:

  • rm: The command itself.
  • [OPTION]...: Optional flags that modify the behavior of the rm command. Flags are prefixed with a dash (-).
  • FILE...: One or more file names or paths specifying the files or directories to be removed.

Here are a few examples illustrating the usage of the rm command:

  1. Remove a single file:

    rm file.txt

  2. Remove multiple files:

    rm file1.txt file2.txt

  3. Remove a directory and its contents recursively:

    rm -r directory/

  4. Forcefully remove a file without prompt:

    rm -f file.txt

  5. Prompt for confirmation before removing a file:

    rm -i file.txt

  6. Remove a directory and its contents forcefully and recursively:

    rm -rf directory/

rm options

Command Description Example
rm <file> Delete a file rm file.txt
rm -r <directory> Delete a directory and its contents recursively rm -r directory/
rm -f <file> Forcefully delete a file without prompt rm -f file.txt
rm -i <file> Prompt for confirmation before deleting a file rm -i file.txt
rm -rf <directory> Forcefully delete a directory and its contents recursively rm -rf directory/
rm -d <directory> Delete an empty directory rm -d emptydir/
rm -v <file> Display verbose output (show deleted files) rm -v file.txt
rm --help Display help information about the rm command rm --help

These are some common options and commands for the rm command. Please note that the -f (force) and -r (recursive) options should be used with caution as they can permanently delete files and directories without further confirmation.

Remember to exercise caution when using the rm command as deleted files cannot be easily recovered. Always double-check your commands and ensure you're deleting the intended files or directories.

Additionally, the behavior of the rm command can vary slightly depending on the operating system and version. It's recommended to refer to the command's manual (man rm) for more detailed information and options specific to your environment.

mkdir

The mkdir command in Linux is used to create directories (folders). The basic syntax of the mkdir command is:

mkdir [OPTION]... DIRECTORY...

Here's a breakdown of the syntax elements:

  • mkdir: The command itself.
  • [OPTION]...: Optional flags that modify the behavior of the mkdir command. Flags are prefixed with a dash (-).
  • DIRECTORY...: One or more directory names or paths specifying the directories to be created.

Here are a few examples illustrating the usage of the mkdir command:

  1. Create a single directory:

    mkdir directory_name

  2. Create multiple directories at once:

    mkdir dir1 dir2 dir3

  3. Create a directory with specific permissions (mode):

    mkdir -m 755 directory_name

  4. Create nested directories (with parent directories if they don't exist):

    mkdir -p path/to/directory

  5. Create directories with verbose output:

    mkdir -v dir1 dir2 dir3

mkdir options

Option Description Example
-m, --mode=MODE Set the file mode (permissions) of the created directory to MODE (in octal). mkdir -m 755 directory_name
-p, --parents Create any necessary parent directories. If they already exist, no error is reported. mkdir -p path/to/directory
-v, --verbose Print a message for each created directory. mkdir -v dir1 dir2 dir3
-Z, --context[=CTX] Set the SELinux security context of each created directory to CTX. If CTX is not specified, the default context is used. mkdir -Z httpd_sys_content_t /var/www

touch

The touch command in Linux is used to create new files or update the timestamps (access time and modification time) of existing files. The basic syntax of the touch command is:

touch [OPTION]... FILE...

Here's a breakdown of the syntax elements:

  • touch: The command itself.
  • [OPTION]...: Optional flags that modify the behavior of the touch command. Flags are prefixed with a dash (-).
  • FILE...: One or more file names or paths specifying the files to be created or updated.

Here are a few examples illustrating the usage of the touch command:

  1. Create a new file:

    touch file.txt

  2. Create multiple files at once:

    touch file1.txt file2.txt file3.txt

  3. Update the timestamps of an existing file:

    touch existing_file.txt

  4. Set specific timestamps for a file:

    touch -t 202201011200 file.txt

  5. Update the timestamps of a file using another file as reference:

    touch -r reference_file.txt target_file.txt

touch options

Here's a Markdown table representing the options for the touch command:

Option Description Example
-a Change only the access time touch -a file.txt
-c Do not create a new file if it doesn't exist touch -c file.txt
-d Set the access and modification time to the specified value touch -d "2022-01-01" file.txt
-m Change only the modification time touch -m file.txt
-r <file> Use the timestamp of <file> for the new file touch -r original.txt new.txt
-t <time> Set the access and modification time using <time> touch -t 202201011200 file.txt
-h Change the timestamps of symbolic links instead of the target touch -h link.txt
--help Display help information about the touch command touch --help

These options provide various ways to modify the timestamps (access time and modification time) of files using the touch command. You can use these options to set specific timestamps, change individual timestamps, or reference timestamps from other files.

echo

The echo command in Linux is used to display text or variables on the terminal. The basic syntax of the echo command is:

echo [OPTION]... [STRING]...

Here's a breakdown of the syntax elements:

  • echo: The command itself.
  • [OPTION]...: Optional flags that modify the behavior of the echo command. Flags are prefixed with a dash (-).
  • [STRING]...: Optional strings or variables to be displayed on the terminal. These can be enclosed in quotes for multiple words or containing special characters.

Here are a few examples illustrating the usage of the echo command:

  1. Display a simple string:

    echo "Hello, World!"

  2. Display the value of a variable:

    name="John"
echo "My name is $name"

  3. Display multiple strings:

    echo "Hello," "World!"

  4. Display strings without a trailing newline:

    echo -n "This is a "
echo "sentence."

  5. Interpret and display escape sequences:

    echo -e "This is\ttabbed text"

echo options

Option Description Example
-n Suppresses the trailing newline character echo -n "Hello, World!"
-e Enables the interpretation of escape sequences echo -e "Line 1\nLine 2"
-E Disables the interpretation of escape sequences (default) echo -E "Line 1\nLine 2"
-s Suppresses the echo command itself from being displayed echo -s "This will not be displayed"
--help Displays help information about the echo command echo --help
--version Displays the version information of the echo command echo --version

wc

Under linux the wc command is used to count the number of lines, wordsm and characters in a file or the output of a command. wc stands for word count.

    wc [options] [file]
  • options Optional flags that modify the behaviour of the wc command
  • file Optional argument specifying the file to be analyzed. If not provided, wc reads input from standard input.

The wc command provides three types of counts:

  1. Line count -l option: It displays the number of lines in the given file or input.
  2. Word count -w option: It shows the number of words in the given file or input. A word is defined as a contiguous sequence of characters separated by whitespace.
  3. Character count -c option: It gives the number of characters in the given file or input. This count includes spaces, tabs, and newlines.

A few examples:

  1. Count the numbers of lines, words, and characters in a file:

     wc filename.txt

  2. Count only the number of lines in a file:

     wc -l filename.txt

  3. Count the number of words in the output of a command (piped out):

     cat filename.txt | wc -w

  4. Count the number of characters in a file:

     wc -c filename.txt

wc options

Option Description Example
-l Count the number of lines wc -l file.txt
-w Count the number of words wc -w file.txt
-c Count the number of characters wc -c file.txt
-m Count the number of characters (including multibyte characters) wc -m file.txt
-L Find the length of the longest line wc -L file.txt
--help Display help information about the wc command wc --help
--version Display the version information of the wc command wc --version

These options allow you to customize the output of the wc command and obtain specific information about the input file or stream. You can count lines, words, characters (bytes), or multibyte characters, as well as find the length of the longest line.

It's important to note that the wc command can accept multiple files as arguments, and it will provide separate counts for each file as well as a cumulative count for all files combined.

awk

The awk command in Linux is a versatile text-processing tool used to extract and manipulate data from text files. The basic syntax of the awk command is:

awk 'pattern { action }' [file]

Here's a breakdown of the syntax elements:

  • awk: The command itself.
  • 'pattern { action }': The pattern-action pairs enclosed in single quotes. The pattern determines which lines to process, and the action specifies the action to perform on those lines.
  • [file]: Optional file name or path specifying the input file. If not provided, awk reads from standard input (e.g., piped input).

Here are a few examples illustrating the usage of the awk command:

  1. Print all lines in a file:

    awk '{ print }' file.txt

  2. Print the first column of each line:

    awk '{ print $1 }' file.txt

  3. Print lines where the second column is equal to a specific value:

    awk '$2 == "value" { print }' file.txt

  4. Calculate and print the sum of numbers in a column:

    awk '{ sum += $1 } END { print sum }' file.txt

  5. Print lines with a specific pattern and perform custom actions:

    awk '/pattern/ { print "Found:", $0 }' file.txt

awk options

Option Description Example
-F <value> Specifies the input field separator awk -F ',' '{print $1}' file.csv
-v <var=value> Sets the value of a variable awk -v num=10 '{print $1 + num}' file.txt
-f <file> Specifies a file containing the awk script awk -f script.awk file.txt
BEGIN{} Executes actions before processing the input awk 'BEGIN{print "Start"} {print $0}' file.txt
END{} Executes actions after processing the input awk '{sum += $1} END{print "Total:", sum}' file.txt
NR Represents the current record number awk '{print NR, $0}' file.txt
NF Represents the number of fields in the current record awk '{print NF, $0}' file.txt
/pattern/ Matches lines that contain the specified pattern awk '/keyword/ {print $0}' file.txt
{print} Prints the entire record or selected fields awk '{print $1, $3}' file.txt
if(){} else{} Conditional statement to perform actions based on a condition awk '{if ($1 > 10) print "High"; else print "Low"}' file.txt
length() Returns the length of a string or field awk '{if (length($1) > 5) print $0}' file.txt

Wildcards

Wildcard Description Example
* Matches any sequence of characters (including none) ls *.txt - matches all files ending with .txt
? Matches any single character ls file?.txt - matches file1.txt, file2.txt, etc.
[ ] Matches any single character within the specified range ls file[1-3].txt - matches file1.txt, file2.txt, file3.txt
[! ] Matches any single character not within the specified range ls file[!1-3].txt - matches any file except file1.txt, file2.txt, file3.txt
{ } Expands a sequence of comma-separated values cp file{1,2,3}.txt destination/ - copies file1.txt, file2.txt, file3.txt to the destination/ directory

Here are some examples to illustrate the usage of wildcards:

  1. List all files with the .txt extension:

    ls *.txt

  2. Remove all files starting with backup and ending with .bak:

    rm backup*.bak

  3. Copy all files with a single character name and .txt extension to a directory:

    cp ?*.txt destination/

  4. Find files with names that match a specific pattern:

    find . -name "prefix*suffix"

  5. Move files with names containing a range of numbers:

    mv file[1-5].txt destination/

Aliases

Aliases are shortcuts to commonly used commands. To define an alias, use the alias command.

Command Description Example
alias Displays a list of currently defined aliases alias
alias name=value Creates an alias with the specified name and value alias ll='ls -al'
unalias name Removes the alias with the specified name unalias ll
alias name Displays the definition of the alias with the specified name alias ll
alias name=new_value Modifies the value of an existing alias alias ll='ls -lh'
alias -p or alias --p Displays a list of all defined aliases in a format that can be reused in a shell script alias -p

Here are some examples of commonly used aliases:

  1. Shorten a frequently used command:

    alias ll='ls -al'

  2. Create an alias with command options:

    alias grep='grep --color=auto'

  3. Create an alias to navigate to a specific directory:

    alias mydocs='cd ~/Documents'

  4. Define an alias with multiple commands:

    alias update='sudo apt update && sudo apt upgrade'

  5. Combine an alias with command arguments:

    alias duh='du -sh'

Aliases can significantly enhance your productivity by allowing you to create custom shortcuts or modify the behavior of existing commands. However, please note that aliases are specific to the current shell session and are not permanent. If you want to make an alias permanent, you can add it to your shell configuration file, such as .bashrc or .zshrc.

© Rayan Lee Bopp