Editing Linux 101 hacks (section)

='''Chapter 5: PS1, PS2, PS3, PS4 and'''=


'''PROMPT_COMMAND'''


=='''Hack 29. PS1 - Default Interaction Prompt'''==


The default interactive prompt on your Linux can be modified as shown below to something useful and informative. In the following example, the default PS1 was “\s-\v\$”, which displays the shell name and the version number. Let us change this default behavior to display the username, hostname and current working directory name as shown below.


-bash-3.2$ '''export PS1="\u@\h \w> "'''


ramesh@dev-db ~> cd /etc/mail


ramesh@dev-db /etc/mail>


<nowiki>[</nowiki>'''Note''': Prompt changed to "username@hostname current-dir>" format]


Following PS1 codes are used in this example:


o \u – Username


o \h – Hostname


# \w - Full pathname of current directory. Please note that when you are in the home directory, this will display only ~ as shown above 

Note that there is a space at the end in the value of PS1. Personally, I prefer a space at the end of the prompt for better readability.


Make this setting permanent by adding export PS1=”\u@\h \w> ” to either


.bash_profile (or) .bashrc as shown below.

ramesh@dev-db ~> vi ~/.bash_profile


ramesh@dev-db ~> vi ~/.bashrc


<nowiki>[</nowiki>'''Note''': Add export PS1="\u@\h \w> " to one of the above files]


Refer to the next chapter for several practical examples of PS1 usage in detail.


=='''Hack 30. PS2 - Continuation Interactive Prompt'''==


A very long command can be broken down to multiple lines by giving \ at the end of the line. The default interactive prompt for a multi-line command is “> “. Let us change this default behavior to display “continue->” by using PS2 environment variable as shown below.


'''ramesh@dev-db ~> '''myisamchk --silent --force --fast --''' '''update-state \


# --key_buffer_size=512M --sort_buffer_size=512M \ 

# --read_buffer_size=4M --write_buffer_size=4M \ 

# /var/lib/mysql/bugs/*.MYI 

<nowiki>[</nowiki>'''Note:''' This uses the default ">" for continuation prompt]


ramesh@dev-db ~> '''export PS2="continue-> "'''


'''ramesh@dev-db ~> '''myisamchk --silent --force --fast --''' '''update-state \


'''continue-> '''--key_buffer_size=512M --''' '''sort_buffer_size=512M \


'''continue-> '''--read_buffer_size=4M --write_buffer_size=4M


\


'''continue-> '''/var/lib/mysql/bugs/*.MYI


<nowiki>[</nowiki>'''Note:''' This uses the modified "continue-> " for


continuation prompt]


I found it very helpful and easy to read, when I break my long commands into multiple lines using \. I have also seen others who don’t like to break-up long commands.


=='''Hack 31. PS3 - Prompt used by “select” inside shell script'''==


You can define a custom prompt for the select loop inside a shell script, using the PS3 environment variable, as explained below.


'''Shell script and output WITHOUT PS3:'''


ramesh@dev-db ~> '''cat ps3.sh''' select i in mon tue wed exit do


case $i in


mon) echo "Monday";; tue) echo "Tuesday";; wed) echo "Wednesday";; exit) exit;;


esac done


ramesh@dev-db ~> '''./ps3.sh''' 1) mon

2) tue


3) wed


4) exit


'''<nowiki>#? </nowiki>1'''


Monday


'''<nowiki>#? </nowiki>4'''


<nowiki>[</nowiki>'''Note''': This displays the default "#?" for select command prompt]


'''Shell script and output WITH PS3:'''


ramesh@dev-db ~> '''cat ps3.sh''' PS3="Select a day (1-4): " select i in mon tue wed exit do


case $i in


mon) echo "Monday";; tue) echo "Tuesday";; wed) echo "Wednesday";; exit) exit;;


esac done


ramesh@dev-db ~> '''./ps3.sh''' 1) mon

2) tue


3) wed


4) exit


'''Select a day (1-4): 1'''


Monday


'''Select a day (1-4): 4'''


<nowiki>[</nowiki>'''Note''': This displays the modified "Select a day (1-4):" for select command prompt]


=='''Hack 32. PS4 - Used by “set -x” to prefix tracing output'''==


The PS4 shell variable defines the prompt that gets displayed, when you execute a shell script in debug mode as shown below.


'''Shell script and output WITHOUT PS4:'''

ramesh@dev-db ~> '''cat ps4.sh'''


set -x


echo "PS4 demo script" ls -l /etc/ | wc -l du -sh ~


ramesh@dev-db ~> '''./ps4.sh'''


++ echo 'PS4 demo script' PS4 demo script

# ls -l /etc/ 

# wc -l 

243


++ du -sh /home/ramesh 48K /home/ramesh


<nowiki>[</nowiki>'''Note''': This displays the default "++" while tracing the output using set -x]


'''Shell script and output WITH PS4:'''


The PS4 defined below in the ps4.sh has the following two codes: o $0 - indicates the name of script


o $LINENO - displays the current line number within the script


ramesh@dev-db ~> '''cat ps4.sh'''


export PS4='$0.$LINENO+ ' set -x


echo "PS4 demo script" ls -l /etc/ | wc -l du -sh ~


ramesh@dev-db ~> '''./ps4.sh'''


../ps4.sh.3+ echo 'PS4 demo script' PS4 demo script

../ps4.sh.4+ ls -l /etc/


../ps4.sh.4+ wc -l


243


../ps4.sh.5+ du -sh /home/ramesh


48K/home/ramesh


<nowiki>[</nowiki>'''Note''': This displays the modified "{script-name}.{line-number}+" while tracing the output using set -x]


=='''Hack 33. PROMPT_COMMAND'''==


Bash shell executes the content of the PROMPT_COMMAND just before displaying the PS1 variable.


ramesh@dev-db ~> '''export PROMPT_COMMAND="date +%k:%m:%S"'''


22:08:42 ramesh@dev-db ~>


<nowiki>[</nowiki>'''Note''': This displays the PROMPT_COMMAND and PS1 output on different lines]


If you want to display the value of PROMPT_COMMAND in the same line as the PS1, use the echo -n as shown below.


ramesh@dev-db ~> '''export PROMPT_COMMAND="echo -n <nowiki>[$(date</nowiki>'''


'''+%k:%m:%S)]"'''


<nowiki>[22:08:51]ramesh@dev-db </nowiki>~>


<nowiki>[</nowiki>'''Note''': This displays the PROMPT_COMMAND and PS1 output on the same line]