- Suppose we have a file
directories.txt
where each line contains the name of a directory that we want to create. Our friend proposes that we runcat directories.txt | mkdir
into order to do this? Will this command work? If not, how can we change this command to get it to work/could we improve it in any way?
Solutions
One correct way to solve this is the command:
xargs mkdir < dir.txt
Many people might write this as the following. It’s fine, but it is a bit unnecessary to invoke the cat
command and using a |
when we already have input redirection (<
) available.
cat dir.txt | xargs mkdir
- Come up with a general statement about when we need to use the
xargs
command.
Solutions
We use xargs
whenever we want to take something that would be coming as input to use them as command-line arguments. So for the last problem, the list of files is coming from a file that we can only redirect as input, but mkdir
doesn’t read from stdin
, it only takes command-line arguments.
On top of this, people often think of using xargs
when you want to run one command over many possible input values.
- Suppose we want to run
Mystery.java
and save standard output to a file namedout1.txt
and have it print standard output to the console. How would we achieve this?
Solutions
java Mystery.java | tee out1.txt
- Again, suppose we are running
Mystery.java
. This time, however, we want standard output and standard error to print to the console, in addition to having ALL output printed toout1.txt
. How would we achieve this?
Solutions
java Mystery.java 2>&1 | tee out1.txt
- Suppose we have the
intro_survey.csv
from the previous homework assignment. Usingcut
, how many unique answers were there to the question “what is your favorite candy”?
Solutions
tail -n+2 intro_survey.csv | cut -d, -f1 | sort -f | uniq -i | wc -l
- Write a command that would delete all directories, recursively, starting from the current directory. Be careful about where you run this command!! Don’t run it from your home directory!
Solutions
find -type d | xargs rm
- Suppose we know that
Compile.java
compiles with no errors. If we were to runjavac Compile.java || java Compile
what would be the result? Would the program compile? Would it run?
Solutions
When we use the ||
operator, if the first statement is true, then we know the entire line is true, thus we wouldn’t run the second statement. In other words, Compile.java
will compile, but it won’t run.
- What would be some security issues with running some of the commands you learned from lecture (think of some of the practical implications of
rm
,find
, andxargs
in regards to malicious code injection)?
Solutions
An example of a security issue would be loss of data by running the command find -name *.java | xargs rm
which deletes all java files. Secondly, if attackers can create files, then xargs
is insecure unless the ‘-0’ flag is used (‘-0’ separates input items by null bytes and not necessarily ‘n‘). If a file with the name /home/someuser/foo/bar\n/etc/passwd exists
(assume that ānā stands for a newline character), then find ā¦ -print can be persuaded to print these three lines (including the blank line):
/home/someuser/foo/bar
/etc/passwd