Lecture 1 - Basic UNIX Commands

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Questions and Answers

What command structure is used to execute conditional commands in the given script?

for loops

case statements

if-else structures (correct)

while loops

Which file descriptor represents standard output (STDOUT)?

1 (correct)

In the command 'ls -l | wc -l', what does the output represent?

The total size of files

The number of files and directories (correct)

The longest file name

The list of hidden files

What is the significance of using the pipe character '|' in command pipelines?

It allows the output of one command to be the input of another. Signup and view all the answers

Which command would correctly count the number of words in 'food2.txt' using piping?

cat food2.txt | wc -w Signup and view all the answers

What does the permission '2' signify when applied to a file in UNIX?

The file can be modified/overwritten Signup and view all the answers

Which command is used to change the group ownership of a file in UNIX?

chgrp Signup and view all the answers

What role do the files ~/.bash_profile and ~/.bash_login serve in the UNIX shell?

They configure the shell environment upon login Signup and view all the answers

What does the 'x' permission mean for directories in UNIX?

You can move to that directory Signup and view all the answers

Which command would you use to view all currently set environment variables in the shell?

env Signup and view all the answers

What is meant by 'globbing' in a UNIX shell context?

Using pattern matching with special characters Signup and view all the answers

Which of the following statements regarding shell variables is correct?

Shell variables are unique to each shell session Signup and view all the answers

In a shell script, what is the purpose of the command 'echo -e'?

It enables interpretation of backslash escapes Signup and view all the answers

What happens when using the command 'ls -l > l.txt' if 'l.txt' already exists?

It overwrites the contents of 'l.txt'. Signup and view all the answers

Which command will read input from 'food2.txt' without knowing the filename as an argument?

wc -w < food2.txt Signup and view all the answers

What does the command 'ls -l 1> l.txt' achieve?

Redirects standard output to 'l.txt'. Signup and view all the answers

What is the function of the command 'cat non_existing.txt 2> l.txt'?

Redirects standard error to 'l.txt'. Signup and view all the answers

What wildcard character does '*' represent in Bash globbing?

Matches zero or more characters. Signup and view all the answers

What will happen if the command 'ls -l food2.txt massimo' is executed and 'massimo' does not exist?

An error message will be shown, but output will still go to the screen. Signup and view all the answers

Which command redirects both standard output and standard error to the same file?

ls non_existing.txt > l.txt 2>&1 Signup and view all the answers

What character is used to prevent Bash from interpreting output redirection as a file name?

& Signup and view all the answers

What does the command 'ls *.??' do?

Lists all files whose name ends with a dot followed by any two characters Signup and view all the answers

Which special parameter expands to the number of positional parameters?

$# Signup and view all the answers

What does the arithmetic expansion $((3 * 21 + 2)) evaluate to?

65 Signup and view all the answers

Which quoting mechanism allows for nested quotes?

Escape character Signup and view all the answers

Which command would display the contents of all files in the current directory?

cat * Signup and view all the answers

What is the function of the special parameter $? in a shell script?

Expands to the exit status of the last executed command Signup and view all the answers

Which symbols must be quoted to use them literally in the shell?

| & ; ( ) < > Signup and view all the answers

What does the command 'ls food[1-3].txt' list?

Files named food1.txt, food2.txt, and food3.txt Signup and view all the answers

What does the regex pattern p[aui]nt match?

Strings like 'pant', 'punt', or 'pint' Signup and view all the answers

Which grep option would you use to count the number of matching lines?

-c Signup and view all the answers

What is the result of the command 'grep -i "pattern" file.txt'?

It finds all occurrences of 'pattern' regardless of case. Signup and view all the answers

Which character in a regex indicates the end of a string?

$ Signup and view all the answers

How do you prevent the shell from altering a regex when using grep?

Enclose the regex in single quotes Signup and view all the answers

What will the command 'grep -l foo file1 file2 file3' output if only file1 and file3 contain 'foo'?

file1 file3 Signup and view all the answers

What does the regex \d\d match?

Any string of exactly two digits Signup and view all the answers

Which grep option inverts the match to show non-matching lines?

-v Signup and view all the answers

Study Notes

Basic UNIX Commands

General syntax: command [options] [arguments]
UNIX filesystem access categories: user, group, others
Permissions for files: readable (r), writable (w), executable (x)
Permissions expressed in octal:
- 4: readable
- 2: writable
- 1: executable
- 0: none, 5: readable + executable, 6: readable + writable, 7: all
Files:
- r: can be read
- w: can be modified
- x: can be executed
Directories:
- r: list files
- w: create/remove files
- x: navigate to the directory

Scripting

Shell reads configuration files at startup: ~/.profile, ~/.bash_profile, ~/.bash_login, ~/.bash_logout, ~/.bashrc
Bash features include:
- Shell variables and command substitution
- Arithmetic expression evaluation
- Globbing: pattern matching with wildcards (*, ?, [])
- Redirection, piping, functions, and control structures
Example script (display.sh):
- Displays date, time, username, and current directory
- Uses command substitution with whoami and iterates through a list of filenames

Piping and Redirection

Standard input, output, and error:
- STDIN: input from keyboard (file descriptor 0)
- STDOUT: output to screen (file descriptor 1)
- STDERR: error messages (file descriptor 2)
Piping connects commands using |, redirecting output of one to input of another.
Examples:
- ls -l | wc -l: counts files and directories
- cat food2.txt | wc -w: counts words in a file
Redirection to/from files:
- ls -l > l.txt: output to l.txt, overwriting existing content
- cat < l.txt: input redirected from l.txt
- ls -l >> l.txt: appends to l.txt
Standard error redirection: ls non_existing.txt 2> l.txt
Combined redirection of standard output and error: cat l.txt non_existing.txt > l.txt 2>&1

Globbing

Wildcards in commands for pattern matching:
- *: matches any string
- ?: matches any single character
- [...]: matches any enclosed character
Examples:
- ls *.txt: lists all .txt files
- cat *: displays content of all files

Special Parameters

Special parameters in the shell include:
- *: positional parameters as a single string
- $1 - $9: individual positional parameters
- @: positional parameters as separate strings
- #: number of positional parameters
- ?: exit status of last command
- $: process ID
- 0: script name

Arithmetic Expansion

Use $(( )) for evaluating arithmetic expressions
Example: echo $((3 * 21 + 2)) outputs 65

Quoting

Quoting mechanisms remove special meaning from characters:
- Escape character (\): preserves literal value
- Single quotes: preserves every character literally, cannot be nested
- Double quotes: similar, but $, `, and \ retain special meaning

Grep Command

grep searches files for lines matching a regular expression.
Common options:
- -c: count matching lines
- -l: list names of files with matches
- -i: ignore case
- -n: show line numbers
Regular expressions enclosed in single quotes to prevent shell interpretation: grep -E 'f?' f2 searches for lines matching expression f? in file f2

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Description

Explore the fundamentals of UNIX commands and their syntax in this quiz. Understand the UNIX filesystem's user, group, and others access categories. Test your knowledge of file permissions and their representations in octal form.