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UNIX COMMANDS

1. Getting System Information

  • pwd: Prints the current working directory.

bash pwd

  • touch: Creates a new file or updates the timestamp of an existing file.

bash touch file.txt # Create a new file named file.txt touch -r file1.txt file2.txt # Set the timestamp of file2.txt to be the same as file1.txt touch -t 2024122510530 file.txt # Set a custom timestamp (YYYYMMDDhhmm.ss)

  • uname: Displays information about the system.

bash uname # Shows the OS name uname -m # Displays machine hardware name uname -r # Shows the kernel version uname -a # Displays all system information

  • man: Opens the manual pages for a given command.

bash man <command> # Show the manual for a command

  • apropos: Searches the manual page descriptions for a keyword.

bash apropos <keyword> # Search for commands related to a keyword

  • type: Gives basic information about a command (e.g., whether it's built-in, a function, or an executable).

bash type <command>

  • which: Displays the location of an executable file for a command.

bash which <command>

2. Finding Environment Variables

  • env / printenv: Displays environment variables.

bash env

  • echo: Prints text or variable values to the terminal.

bash echo "Hello, World!" echo $PATH

  • export: Sets an environment variable for the current session and any child processes.

bash export MY_VAR=123

  • unset: Unsets or deletes an environment variable.

bash unset MY_VAR

  • set: Displays all environment variables.

bash set

3. Working with Quoting and Special Characters

  • Single quotes preserve the literal value of all characters.

bash echo '$PATH' # Output will literally show $PATH

  • Double quotes preserve most characters, but variables and escape sequences are expanded.

bash echo "$PATH" # Output will show the value of PATH

4. Redirection and Pipes

  • Redirection:

  • >: Redirects output to a file, overwriting the content.

    bash echo "Hello" > file.txt # Writes "Hello" to file.txt, overwriting its contents

  • >>: Appends output to a file.

    bash echo "World" >> file.txt # Appends "World" to file.txt

  • 2>: Redirects error output (stderr) to a file.

    bash command 2> error.txt

  • 2>/dev/null: Discards stderr (useful when you don’t care about errors).

    bash command 2>/dev/null

  • Pipes (|): Redirects the output of one command as input to another command.

bash ls | grep "file" # List files, then filter to only show those containing "file"

  • diff: Compares two files line by line.

bash diff file1.txt file2.txt

  • tee: Reads from stdin and writes to both stdout and a file.

bash echo "Hello" | tee file.txt # Output goes to both terminal and file.txt

5. File Manipulation

  • cat: Concatenates and displays the content of a file.

bash cat file.txt

  • head: Displays the first ten lines of a file.

bash head file.txt

  • tail: Displays the last ten lines of a file.

bash tail file.txt

  • nl: Numbers the lines in a file.

bash nl file.txt

  • wc: Counts the number of lines, words, characters, or bytes in a file.

bash wc -l file.txt # Line count wc -w file.txt # Word count wc -c file.txt # Byte count

  • cut: Removes sections from each line of files.

bash cut -d':' -f1 file.txt # Extract first field separated by ":"

  • uniq: Filters out repeated lines in a file.

bash uniq file.txt

  • tr: Translates or deletes characters.

bash echo "hello" | tr 'a-z' 'A-Z' # Converts lowercase to uppercase

  • split: Splits a file into multiple smaller files.

bash split -l 100 file.txt # Splits file.txt into 100-line chunks

  • sed: Stream editor for basic text transformations.

bash sed 's/cat/dog/g' file.txt # Replace all instances of "cat" with "dog" sed '/dog/d' file.txt # Delete all lines containing "dog" sed -n '/cat/p' file.txt # Print lines containing "cat"

6. File Compression

  • gzip: Compresses files using the GZIP algorithm (.gz).

bash gzip file.txt

  • zcat: Displays the contents of a gzipped file without decompressing it.

bash zcat file.txt.gz

  • sha256sum / md5sum: Generates checksums for files.

bash sha256sum file.txt # Calculates the SHA-256 checksum of file.txt md5sum file.txt # Calculates the MD5 checksum

7. File and Directory Operations

  • cp: Copies files or directories.

bash cp file1.txt file2.txt

  • mv: Moves or renames files or directories.

bash mv file1.txt newfile.txt

  • mkdir: Creates a new directory.

bash mkdir new_directory

  • rmdir: Removes an empty directory.

bash rmdir empty_directory

  • find: Searches for files in a directory hierarchy.

bash find /path/to/dir -name "*.txt" # Find all .txt files

  • tar: Used to create, extract, or list archive files.

bash tar -cvf archive.tar file1.txt file2.txt # Create tar archive tar -xvf archive.tar # Extract tar archive

  • dd: Low-level copying and conversion of data.

bash dd if=inputfile of=outputfile bs=64K conv=noerror,sync

8. More Redirection

  • 1>: Redirects standard output to a file (same as >).

bash echo "Hello" > file.txt

  • >&2: Redirects stdout to stderr.

bash echo "Error!" >&2

  • 2>/dev/null: Discards stderr output.

bash command 2>/dev/null # Suppress error output

These commands are essential tools for Unix/Linux users, especially for managing files, processes, environment variables, and system information.