Lab 11 Configuring the bash Shell

Sequence 1: Configuring the bash Shell

Deliverable: A system with new aliases that clear the screen, and produce a useful timesorted ls listing.

Instructions:

  1. You have decided to create an alias so that when you type c, the system will run the clear command to clear the screen. Begin by logging in as user student on tty1, then input and test your alias.

[student@stationX ~]$ alias c=’clear’

[student@stationX ~]$ alias

[student@stationX ~]$ c

  1. This alias will be lost if you log out and log back in. To ensure that the new alias is available each time user student logs in, perform the following steps:

[student@stationX ~]$ cd

[student@stationX ~]$ vi .bashrc

Locate the line that contains the text:

# User specific aliases and functions

Add your alias to the line immediately below:

alias c=’clear’

Save the file and exit.

  1. Test your change by logging out, logging back in on tty1, and typing the following:

[student@stationX ~]$ alias

[student@stationX ~]$ c

  1. Create an alias called lr that invokes ls with the following features:
  • The alias displays a long listing format.
  • It lists all files, including those that beginning with a dot.
  • It classifies files by appending a file type indicator to filenames of certain types.
  • It sorts the listing by modification time.
  • It displays files in reverse order.

You probably will need to consult the man page for the ls command to figure out the appropriate options. After you have created and tested your alias, add the alias to your .bashrc file.

alias lr=’ls -laptr’

OR

alias lr=’ls -laFtr’

Sequence 2: Changing your bash prompt

Scenario: You have decided to customize your bash prompt to display the full path of the current working directory and the shell history number, and to make some other cosmetic changes.

Instructions:

  1. In a terminal window, display the current value of your primary prompt string. Note the presence of static ASCII characters and backslash-escaped special characters in the prompt.

[student@stationX ~]$ echo $PS1

  1. Change your prompt to print only a static string for testing purposes:

[student@stationX ~]$ PS1=’Red Hat -> ‘

Red Hat ->

Be sure to include a space after the -> so that commands have padding, that is, so that commands do not appear to touch the prompt (this is almost always desirable with prompts).

  1. That prompt is not very useful, so restore the traditional bash dollar-sign, along with the name of the host:

Red Hat -> PS1=’\h $ ‘

stationX $

  1. Insert the bash history prompt special character \! between the hostname and dollar-sign. Make sure you pad it on each side with a space character. This will insert the number of the current command in bash’s history. The actual number you see will probably be different than the one shown below.

stationX $ PS1=’\h \! $ ‘

stationX 21 $

  1. Now refer to the bash man page to find the special character for the current working directory. (Search the man page for PROMPTING, and beware: you want the special character for the full pathname, not the basename as in the default prompt.) Insert that special character into your PS1 prompt string preceded by a colon.
  2. Your customized prompt should appear as shown in the following examples. If not, continue to work on it. Your completed prompt setting should be something like: PS1=’\h:\w \! \$ ‘

station1:~ 21 $ cd /tmp

station1:/tmp 22 $

  1. Edit your new definition of PS1 into your .bashrc, then open a new terminal window to make sure your new prompt is in effect.

Sequence 3: Command line expansion

Scenario: You have decided to make a backup of the /etc/password file and giving the backup a name which will be useful as an indication of when the file was created.

Instructions:

  1. First, you want to find out the last time the file was modified. This will give you a rough idea of how often the file may be updated. The information can be found by making a long listing of the file

[student@stationX ~]$ ls -l /etc/passwd

Given the output of this command you can decide on what date information would be most relevant to include as part of the backup’s name. i.e., if you suspect the file is updated several times a week you will need to include more than just the month and year that a backup was made. One date format could be the following:

[student@stationX ~]$ date +%Y%m%d

Experiment with the date command to produce an alternative output which would produce a four figure representation of the year followed by the month number in two digits and day of the month in two digits. This would be more useful in that an alphanumeric listing of all backup files be in date order.

  1. Now make a backup of the file appending the output of the date command to the file name. In order to do this we will take advantage of two bash command line expansion tricks. The first substitutes the parameter given to the last command typed onto the current command line, while the second is an alternative syntax to the back tick command execution covered in this unit.

[student@stationx ~]$

cp /etc/passwd ~/passwd-$(date Alt-.)

  1. Lastly we demonstrate a bash trick which allows you to execute the last command from your history matching a given string.

[student@stationx ~]$ !cp

[student@stationx ~]$ !ls

An alternative to the above would be to type Ctrl-r and begin typing the command line you want to re-execute. This reverse search through your history shows the match as you type. If there are multiple matches you can delve further back in your history by repeatedly typing Ctrl-r until you reach the command you were looking for.

Sequence 4: Command substitution

Instructions:

  1. Determine the full pathname of the command executed when metacity is entered at the shell prompt. Use a shell shortcut to re-execute that command, appended with -message to run the same command on metacity-message, and then a second shell shortcut to run it on metacity-window-demo. Remember that Alt- in the instructions below means to press the Alt key.

[student@stationX ~]$ which metacity

[student@stationX ~]$ which Alt-.-message

[student@stationX ~]$ ^message^window-demo

  1. Repeat your last command containing ich.

[student@stationX ~]$ Ctrl-richEnter

  1. When a command contains another command enclosed in backquotes (“), bash evaluates the backquoted command first, and the result is substituted before the full command is executed.

Use this technique to perform an ls -l listing on the full pathname of the command executed when nautilus is entered at the shell prompt. Remember that which nautilus is evaluated first, its result is substituted on the command line, then ls -l is executed on the result.

[student@stationX ~]$ ls -l `which nautilus`

A dollar-sign followed by parentheses can be used to do the same thing as backquotes with the added advantage that parentheses can be nested. The following example uses echo and date to insert a datestamp before ls -l’s output. Note how the output of which nautilus is nested within the call to ls, which is in turn nested within the call to echo:

[student@stationX ~]$ echo “$(date): $(ls -l $(which nautilus))”

Challenge Sequence 5: Creating a more versatile backup script

Scenario: You have been asked to write a script called backup.sh. This script will be similar to the backup-sysconfig.sh script created for earlier labs, but instead of always backing up the same directory, it will take the directory to be backed up as an argument. The script will also be housed in /usr/local/bin instead of /root/bin so that anyone can execute it.

Instructions:

  1. Log in as student and use sudo to open a new file in your text editor:

[student@stationX ~]$ sudo vim /usr/local/bin/backup.sh

  1. 2. Remember to insert a comment line explaining what the script does and what arguments it takes. Do not forget the line containing the magic “shbang” sequence:

#!

#!/bin/bash

# A script for backing up any directory

# First argument: The directory to be backed up

# Second argument: The location to back-up to.

  1. Next, define some variables with a meaningful names, which will take their values from the script’s first two arguments. This is not strictly necessary. We could just use $1 and $2 the whole time, but it will make the rest of the script more readable.

ORIG=$1

BACK=$2

  1. Next, use a conditional to test whether the destination ($BACK) already exists. If it does, you can give the user a chance to bail out with a clever use of the read command:

if [ -e $BACK ]

then

  echo “WARNING: $BACK exists”

  read -p “Press Ctrl-c to exit or Enter to proceed:”

fi

If the user presses Ctrl-c, the whole script, not just read, will quit. If not, the script will continue.

  1. Finally, add a line to actually perform the backup and another to print the date, as in your previous scripts:

cp -av $ORIG $BACK

echo “Backup of $ORIG to $BACK completed at: $(date)”

  1. Save your script. It should look something like this:

#!/bin/bash

# A script for backing up any directory

# First argument: The directory to be backed-up

# Second argument: The location to back-up to.

ORIG=$1

BACK=$2

if [ -e $BACK ]

then

echo “WARNING: $BACK exists”

read -p “Press Ctrl-c to exit or Enter to proceed:”

fi

cp -av $ORIG $BACK

echo “Backup of $ORIG to $BACK completed at: $(date)”

  1. Create a backups directory in student’s home:

[student@stationX ~]$ mkdir ~/backups

  1. Make the script executable and test that it works the way you expect. Because the script was created as root and because /etc/sysconfig contains protected files, both of these will have to be done with sudo.

[student@stationX ~]$ sudo chmod 755 /usr/local/bin/backup.sh

Password:

[student@stationX ~]$ sudo backup.sh /etc/sysconfig

~/backups/sysconfig-$(date +%Y%m%d)

Challenge Sequence 6: Improving Your Backup Script

Scenario: Note that your new backup.sh script has one major disadvantage relative to backup-sysconfig.sh. Unlike the previous script, you have to manually specify the destination you wish to back up to. It would be more convenient if the script could automatically set $BACK to a reasonable location in ~/backups/.

Instructions:

  1. How might you change the BACK=$2 line of your script so that instead of taking a second argument, backup.sh automatically sets BACK to the original filename plus -yyyymmdd in ~/backups/? In other words, if the first argument is /etc/sysconfig, how could you make the script automatically back up to ~/backups/sysconfig-yyyymmdd?

HINT: Be careful to only use the file or directory name being backed up, not the entire path. You want your backup to end up in ~/backups/sysconfig-yyyymmdd, not ~/backups/etc/sysconfig-yyyymmdd! This can be accomplished with the basename command. Try running basename /etc/sysconfig to see how it works. Add this line to your script.

BACK=~/backups/$(basename $ORIG)-$(date ‘+%Y%m%d’)

  1. Save your changes and try running your new, simpler, backup.sh:

[student@stationX ~]$ sudo backup.sh /etc/sysconfig

By |2015-01-01T01:48:08+00:00January 1st, 2015|Linux and Unix, Linux/Unix Basics|0 Comments

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