Bash Redirection of Standard Output and Error

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Write the output of the ls /bin command to the ~/Desktop/some_executables file.

 
ls /bin > ~/Desktop/some_executables

Run the following command and then delete the contents of the greeting file (keep the file, just delete the contents).

echo "hi there" > ~/Desktop/greeting

> ~/Desktop/greeting

The > redirection operator clobbers files. In this example, no new text is written to the clobbered file, so it's empty.

Where are standard output and standard error redirected to by default?

Standard output and standard error are directed to the Terminal by default for commands typed into the Terminal.

Redirect the standard error of the following command to a text file called ~/Desktop/feed_me:

$ ls /fake_dir

ls /fake_dir 2> ~/Desktop/feed_me

The shell internally references standard input as 0, standard output as 1, and standard error as 2. The > operator redirects standard output by default, but it redirects standard error if 2 is prepended (e.g. 2>).

Write the standard output of the following command to the ~/Desktop/dunno files:

echo "does this work"

# one method with explicit syntax
echo "does this work" 1> ~/Desktop/dunno

# another method
echo "does this work" > ~/Desktop/dunno

Explain this code:

ls /not_here >> ~/Desktop/whatever 2>&1
cat ~/Desktop/whatever

2>&1 redirects standard error (2) to standard output (1). Standard output is appended to the ~/Desktop/whatever file, so 2>&1 appends standard error to the ~/Desktop/whatever file as well.

Explain this code:

echo $SHELL >> ~/Desktop/whatever 2>&1
cat ~/Desktop/whatever

The standard output is appended to the ~/Desktop/whatever file.

Explain the following code:

ls /weird 2> /dev/null

The standard error from the $ ls /weird command is redirected to the /dev/null file. /dev/null is a special type of file that accepts standard output and standard error and just discards it without doing anything.

Sort the executables in the /bin directory and view the sorted list with the less pager program.

 
ls /bin | sort | less

Return the number of unique executables in the PATH variable.

 
DIRS=$( echo $PATH | tr ":" " ")
ls $DIRS | sort | uniq | wc -l

List all the processes running on all terminals that contain the word bin.

 
ps -x | grep bin