Using the Shell, History, Variables, and Redirection

See what shells are available in the environment

cat /etc/shells

Determine what shell we're using:

set | grep SHELL

Find our user ID, group ID, and groups we're secondary members of:

id

Elevate privileges and run id:

sudo -i id

Find out who you're logged in as:

whoami

Elevate privileges and run whoami:

sudo -i whoami

View the history stack:

history

Write the history stack to a file:

history > shell-history.txt

Find the times you used whoami:

grep -i whoami shell-history.txt

Alternatively

whoami <up arrow>

run the last command

!!

run a history command by number

!<number> ex !48

Create a script:

vim uservar.sh

Contents

#/bin/bash

echo Hello, my name is $USER
echo My $PRODUCT costs 10 dollars

run it

bash uservar.sh

Create the $PRODUCT variable and rerun the script

PRODUCT=Snowblower

bash uservar.sh

Find out which shell level we're on:

echo $SHLVL

Edit the script: At the top of the file, right below #/bin/bash, enter:

echo Shell is at level $SHLVL

Run the script again:

bash uservar.sh

We need to export the variable to make it available to the current shell:

export PRODUCT

Run the script again:

bash uservar.sh

Edit the script:

vim uservar.sh

Replace 10 dollars with $COST dollars USD. Save and quit the file. Declare a COST variable:

export COST=100

View the file's contents:

cat uservar.sh

Run the script again:

bash uservar.sh

Edit the script:

vim uservar.sh

Change the PRODUCT variable line to:

echo "My $PRODUCT costs \$$COST dollars USD"

We have to include a \ to make the $ properly show up. Save and quit. View the file's contents:

cat uservar.sh

Run the script:

bash uservar.sh

Edit the script:

vim uservar.sh

At the bottom of the file, enter:

echo My $PRODUCT comes in $1 and I have $2 of them in stock

echo The executable name is $0
echo The arguments entered were $*
echo The number of arguments were $#
echo The PID of this shell is $$

Here, $1 is the color of the snowblower and $2 is the amount in stock. Save and quit. View the file's contents:

cat uservar.sh

Run the script:

bash uservar.sh

Enter the arguments:

bash uservar.sh Blue 10

List the files output so they're each on a separate line:

ls -1

Send the output to lsoutput.txt:

ls -1 > lsoutput.txt

Get its word count:

wc -l lsoutput.txt

View the file's contents, using it as an argument:

cat lsoutput.txt

View the file's contents, using it as an input:

cat < lsoutput.txt

The < is an input redirect.

Pipe the output to the nl (numbered lines) command:

cat lsoutput.txt | nl

This has each item on its own numbered line.

Pipe the output to the nl and tac (i.e., the opposite of cat) commands:

cat lsoutput.txt | nl | tac

This time, each item is numbered, but the list starts from the last number.

Run the following:

find /proc -iname "*.*"

We'll see a lot of stdrr (Permission Denied errors).

Put the good (or stdout) data in procdata.txt:

find /proc -iname "*.*" > procdata.txt

We'll still see all the error messages.

View the procdata.txt file's contents:

cat procdata.txt

We should see good/stdout data.

Send the stderr data to badprocdata.txt:

find /proc -iname "*.*" 2> badprocdata.txt

Put the stdout data in procdata.txt and get rid of the stderr data:

find /proc -iname "*.*" > procdata.txt 2> /dev/null

View the file's contents:

cat procdata.txt

Use the tee command to write the stdout data to a file and send the rest to the console:

find /proc -iname "*.*" | tee procdata.txt

Discard the stderr data and send the stdout data to the file:

find /proc -iname "*.*" 2> /dev/null | tee procdata.txt

Using a ; between commands makes them both run, no matter if one throws an error:

ls ; farg

Here, farg is not an actual command, but both commands still run.

Using double pipes means "if the first command fails, only then run the next command." Try it:

ls || farg

Because ls worked, farg did not run.

Switch them around:

farg || ls

Since farg failed, it had to failover to ls. Using double ampersands means "run the next command only if the first command is successful." Try it:

ls && farg

Since ls worked, it also ran farg. Reverse the order:

farg && ls

Because farg failed, ls did not run.