Operating systems 1 -- 2008-2009 -- info.uvt.ro/Laboratory 5

Quick links: front; laboratories agenda, 1, 2, 3, 4, 5, 6, projects, evaluation, tools, references.

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Command substitution

• syntax:

`<command> [argument] ...`
$( <command> [argument] ... )

• catching the current working directory in a variable:

WORKING=`pwd`

• or:

WORKING=$( pwd )

• or (better with quoting):

WORKING="$( pwd )"

• catching the current date in a variable:

DATE=`date`

• or:

DATE=$( date )

• or:

DATE="$( date )"

• catching the whole content of a file in a variable:

PASSWD="$( cat /etc/passwd )"
echo "${PASSWD}"

• links:
  • Advanced Bash-Scripting Guide:
    • Command Substitution;

Regular expressions and globbing

• links:
  • Advanced Bash-Scripting Guide:
    • A Brief Introduction to Regular Expressions;
    • Globbing;
  • Regular Expressions - User guide;

Mini projects

System users and groups querying

• write a simple bash script that allows us to query the (UNIX/Linux) system users and groups database;
• note: all the needed information is inside /etc/passwd and /etc/group files;
• it will list all users:

$ ./query.sh show-users
root
bin
fetchmail
ciprian
...

• it will list all users that are allowed to login (they have the shell set to /bin/bash, /bin/sh):

$ ./query.sh show-valid-users
root
ciprian

• it will list all groups:

$ ./query.sh show-groups
root
daemon
audio
...

• it will list all users that are in a group:

$ ./query.sh show-users-in-group audio
root
ciprian

• it will list all groups a user is in:

$ ./query.sh show-groups-for-user ciprian
ciprian
audio
cdrom
...

• it will print the home-dir of a given user:

$ ./query.sh show-home-for-user ciprian
/home/ciprian

• note: try to use only grep, sed, cat, head/tail, don't use existing commands that do the required operations;
• links:
  • passwd.5;
  • group.5;

Syslog querying

• write a script that allows us to query through the syslog messages (a file located at /var/log/syslog);
• to view all messages from a given host:

$ ./query.sh show-from-host router
May 10 20:20:08 router kernel: Enabling...
May 10 20:20:08 router sshd: Starting...
...

• to view all messages from a given component:

$ ./query.sh show-from-component kernel
May 10 20:20:08 router kernel: Enabling...
...

• to view all messages from a given host and component:

$ ./query.sh show-from-host-and-component router kernel
May 10 20:20:08 router sshd: Starting...
...

• to view only those entries containing a given string:

$ ./query.sh search ACPI
May 10 20:20:08 router kernel: Enabling ACPI...
...

Simple file versioning system

• implement something similar with SVN, but in Bash, and much simpler...
• links:
  • Subversion;
  • Revision control;

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• Ciprian Dorin Crăciun, ccraciun@info.uvt.ro