Shell & Friends

Class: CSCE-313

Notes:

I/O redirection in shell

When a program is executed in Linux, it usually has access to three special files: stdin (fd 0), stdout (fd 1), and stderr (fd 2). They correspond to objects cin, cout, and cerr that you're familiar with from C++ programming. The first is connected to the keyboard, while the second and third are attached to the screen or terminal. See Chapter 6, The Linux Command LineLinks to an external site. on IO Redirection. Answer the following questions.

Question 1

To redirect the contents of a file into a command such as md5sum, we can do? In other words you want md5sum to read from file when it's reading from its stdin.

$ md5sum ____ file

Options:

• |
• <
• <<

Overall explanation:

• < file redirects the contents of file into stdin of md5sum, so md5sum reads from standard input.
• << is a here-document operator, used to provide multiline input written directly in the script, not to read from an existing file.

Question 2

To redirect the output of a command such as date into a file, we can do?

$ date ____ file

Options:

• 2>&1
• |
• <

Overall explanation:

• > redirects stdout of date into file, overwriting the file (or creating it if it doesn’t exist).

Question 3

To append the output of a command such as uname to a file, we can do?

$ uname -a ____ file

Options:

• <<
• <
• >>

Overall explanation:

• >> appends the command’s stdout to the end of file instead of overwriting it.

Question 4

To redirect only the error messages of a command such as find to a file, we can do? That is, we want errors of the command to go to the file, but the output of the command to go to the terminal.

$ find /etc ____ file

Options:

• 2>
• <

Overall explanation:

• 2> redirects stderr (file descriptor 2) to file, while normal output (stdout) still goes to the terminal.

Question 5

To redirect both the output and error messages of a command such as du, we can do? Try various things in your Linux shell to see which one works.

$ du /etc ____ file

Answer:

$ du /etc > file 2>&1

• > file sends stdout to file.
• 2>&1 sends stderr to the same place as stdout, so both end up in file.

Question 6

To redirect the output of a command such as ls to the input of another command such as grep, we can do:

$ ls /etc ____ grep host

Options:

• |
• <

Overall explanation:

• | is a pipe that sends the stdout of ls into the stdin of grep, so grep filters the output of ls.

Date Formatting

Two useful features in a typical Unix shell are pathname expansion and brace expansion. Given the following output of ls, where all file names are in the format YEAR-MONTH.

2002-01  2002-07  2003-01  2003-07  2004-01  2004-07  2005-01  2005-07  2006-01  2006-07
2002-02  2002-08  2003-02  2003-08  2004-02  2004-08  2005-02  2005-08  2006-02  2006-08
2002-03  2002-09  2003-03  2003-09  2004-03  2004-09  2005-03  2005-09  2006-03  2006-09
2002-04  2002-10  2003-04  2003-10  2004-04  2004-10  2005-04  2005-10  2006-04  2006-10
2002-05  2002-11  2003-05  2003-11  2004-05  2004-11  2005-05  2005-11  2006-05  2006-11
2002-06  2002-12  2003-06  2003-12  2004-06  2004-12  2005-06  2005-12  2006-06  2006-12

Using bash's expansion syntax, complete the following prompts. See Chapter 7, The Linux Command LineLinks to an external site. for more information.

Question 7

To list all the files from 2002, we could use the following expressions (select all that work correctly):

$ ls ____

2002-01 2002-02 2002-03 2002-04 2002-05 2002-06 2002-07 2002-08 2002-09 2002-10 2002-11 2002-12

Options:

• 2002-{01..12}
• 2002-*
• 2002
• 2*2

Overall explanation:

• Correct:

  • 2002-{01..12}: Brace expansion explicitly generates 2002-01 through 2002-12.
  • 2002-*: Globbing matches any filename that starts with 2002-.

• Incorrect:

  • 2002: Matches only a file or directory named exactly 2002, not the monthly files.
  • 2*2*: Globbing matches any name with a 2, then anything, then 2, then anything — it will match all 2002-XX files but also other files with this requirement.

Question 8

To list all the files from the month of December, we could do:

$ ls ____

2002-12 2003-12 2004-12 2005-12 2006-12

Options:

• {2002..2006}-12
• *[12]
• *-12
• ????-12

Overall explanation:

• Correct:
  • {2002..2006}-12: Brace expansion generates 2002-12 through 2006-12 explicitly.
  • *-12: Matches any filename that ends with -12, which fits all December files.
  • ????-12: Matches exactly four characters, then -12 (e.g., any 4-digit year in December).
• Incorrect:
  • *[12]: This matches any filename that ends with either 1 or 2.
    • So *[12] matches names ending in …1 or …2, like:
      • 2002-01
      • 2003-12
      • file2

Question 9

To list all the files from the month of January to June, we could do:

$ ls ____

2002-01  2002-04  2003-01  2003-04  2004-01  2004-04  2005-01  2005-04  2006-01  2006-04
2002-02  2002-05  2003-02  2003-05  2004-02  2004-05  2005-02  2005-05  2006-02  2006-05
2002-03  2002-06  2003-03  2003-06  2004-03  2004-06  2005-03  2005-06  2006-03  2006-06

Options:

• 2?2[0]{1..6}
• *0[123456]
• *-{01..06}
• *-0[1-6]

Overall explanation:

• Correct:

  • *0[123456]: Matches any name ending in 01 to 06
  • *-{01..06}: Brace expansion generates -01 through -06, matching all months January to June.
  • *-0[1-6]: Matches any name ending in -01 to -06 using a character range.

• Incorrect:

  • 2?2[0]{1..6}: Mixed and invalid pattern: {1..6} is brace expansion but attached incorrectly after a glob.

Question 10

Are the following two pipelines equivalent?

$ du -h /etc 2> /dev/null | sort -h > output.txt
$ du -h /etc | sort -h > output.txt 2> /dev/null

Options:

• No, the first pipeline discards the output of du before sorting, while the second pipeline discards the output of the whole pipeline.
• No, the first pipeline redirects the output of du to a file and then sorts the file, while the second pipeline pipes the output of du into sort and then stores it into two files.
• Yes, the pipelines have the same behavior and produce the same output.
• No, the first pipeline discards the error messages of the du, while the second pipeline discards the error messages of sort.

Overall explanation:

• Option 1 is wrong: Redirection doesn't discard "output" (stdout), it discards "errors" (stderr).
• Option 2 is wrong: Neither pipeline involves temporary files or "two files."
• Option 3 is wrong: As explained, the error handling is scoped differently.
• Option 4 is the closest: It correctly identifies that the first pipeline silences du. While the second pipeline technically silences sort, in practice, it fails to silence the errors the user actually cares about (the ones from du).

Unix Shell Variables

Two other important features of a typical Unix shell are variables (i.e., parameter expansion) and command substitution. Using these features, complete the following prompts:

Question 11

To print the value of the HOME variable, we could do:

echo _____

Answer:

echo $HOME

• $HOME expands to the value of the HOME environment variable, and echo prints it to the terminal.

Question 12

To set the value of the SHELL variable to /bin/bash, we could do:

Answer:

SHELL=/bin/bash

• This assigns the value /bin/bash to the SHELL variable in the current shell environment.

Question 13

To compute the md5sum of the ls command while also looking up its location, we could do? See command substitution in the shell. Also look at the where built-in command in the shell.

Answer:

md5sum $(which ls)

• Command substitution allows the output of one command to replace the command name itself on the command line. There are two ways to write this:
  • The Modern Way (Recommended): $(command)
  • The Legacy Way: command (using backticks)
• If you run md5sum $(which ls), the shell performs these steps in order:
  1. Subshell Execution: It runs which ls.
  2. Resolution: which ls returns the full path, typically /usr/bin/ls.
  3. Substitution: The shell replaces the $(...) block with that path.
  4. Final Execution: The shell finally executes md5sum /usr/bin/ls.
• Using where vs which
  • which ls: Generally returns the first executable found in your PATH.
  • where ls: Specifically a zsh built-in (or an external utility in some distros) that shows all instances of ls, including aliases and functions.

Question 14

To print all the environment variables in the shell, we could do:

Answer:

printenv

• Both env and printenv display all environment variables currently set in the shell.
• set is even more talkative here

Question 15

What is the key difference between these two commands?

cmd > out.txt 2>&1
cmd 2>&1 > out.txt

Options:

• They are equivalent
• In (1), stderr is discarded
• In (2), stdout is discarded
• In (1), both stdout and stderr go to out.txt; in (2), stderr goes to the terminal (or original stdout) while stdout goes to out.txt

Overall explanation:

1. cmd > out.txt 2>&1
   • > sends stdout to out.txt
   • 2>&1 sends stderr to the same place as stdout → both go to out.txt
2. cmd 2>&1 > out.txt
   • 2>&1 first sends stderr to the current stdout (the terminal)
   • then > sends stdout to out.txt
   • → stderr still goes to the terminal, stdout goes to the file.

Question 16

Assume you're using the bash shell. If you're unsure, type echo $SHELL in your terminal. Suppose the directory only contains files a1.txt, a2.txt, b1.txt. What does this command print?

 echo "*.txt"

Options:

• prints all files in the directory
• error
• expands to matching .txt files
• prints *.txt literally

Overall explanation:

• The quotes "*.txt" prevent glob expansion, so the shell does not match filenames.
• echo receives the literal string *.txt and prints it exactly.

Question 17

What is the key difference between the following two commands?

$ ls *.txt
$ ls "*.txt"

Options:

• The first treats *.txt as a literal string, while the second expands it
• The first performs pathname expansion, while the second passes the literal string *.txt to ls
• Both commands result in an error if no .txt files exist
• Both commands perform pathname expansion and list all .txt files

Overall explanation:

• ls *.txt → the shell expands *.txt to matching filenames before running ls.
• ls "*.txt" → the quotes prevent expansion, so ls receives the literal string *.txt as an argument.

Question 18

Assume your current directory contains only the following two files: report_v1.txt, and report_v2.txt. Note that report_v3.txt does not exist in the directory. Which of the following commands will result in an error message (e.g., "No such file or directory") being displayed?

Options:

• $ ls report_v{1,2,3}.txt
• $ ls report_v?.txt
• $ ls report_v[123].txt
• $ ls report_v*.txt

Overall explanation:

• {1,2,3} expands to:
  • report_v1.txt
  • report_v2.txt
  • report_v3.txt
  • Since report_v3.txt does not exist, ls is given a non-existent filename and prints an error.
• Why the others do not error:
  • report_v?.txt → matches report_v1.txt and report_v2.txt only.
  • report_v[123].txt → matches 1 and 2; 3 is ignored since no such file exists.
  • report_v*.txt → matches both existing files.