BASH shell en
Using shell scripting to automate Linux maintenance tasks
Author
Translated from Estonian by Steven Rugam
Short introduction
Bash (Bourne-again shell) is a well-known shell. Bash scripts are used to simplify and automate system work. When talking about bash in few words, then the structure of the language and possible usages are being checked. At length, however, focus lies on the certain administrating assignments which are solved with bash scripting language. It is a powerful shell, and features, among other things:
- Command line editing
- Command history
- A directory stack (pushd, popd)
- Command substitution
- Special variables, like $PPID
- Autocompletion
- In-process integer arithmetic: $((...))
- In-process regexes
- Aliases
- Functions
- Arrays
- Expansions: tilde, brace, variable
- Substring awesomeness
- Conditional expressions
- Security (restricted shell mode)
- Job Control
- Timing
- Prompt customization
Purpose
Providing skills for the bash scripting language to simplify and to improve efficiency towards our daily work.
Prerequisites/Assumptions
History with GNU/Linux operation system and acquired skills, which are being tested by automated self-test.
To test prerequisites, you should pass self-test.
Target group
Support engineers and system administrators.
Educational materials for learning BASH shell
Additional learning materials
https://bash.cyberciti.biz/guide/Main_Page
Using Bash for administrating
Useful commands
Command: find
Search for files in the given directory, hierarchically starting at the parent directory and moving to sub-directories.
root@user:~# find -name *.sh
Command: grep
root@user:~# grep user /etc/passwd
user:x:1000:1000:user,,,:/home/user:/bin/bash
Ignore word case and all other combination with ‘-i‘ option.
root@user:~# grep -i USER /etc/passwd
user:x:1000:1000:User,,,:/home/user:/bin/bash
Search recursively (-r) i.e. read all files under each directory for a string “127.0.0.1“.
root@user:~# grep -r "127.0.0.1" /etc/
Command: man
The ‘man‘ is the system’s manual pager. Man provides online documentation for all the possible options with a command and its usages. Almost all the command comes with their corresponding manual pages. For example,
root@user:~# man bash
Manual page for man page itself, similarly ‘man cat‘ (Manual page for cat command) and ‘man ls‘ (Manual page for command ls).
Command: ps
ps (Process) gives the status of running processes with a unique Id called PID.
root@user:~# ps
To list status of all the processes along with process id and PID, use option ‘-A‘.
root@user:~# ps -A
Note: This command is very useful when you want to know which processes are running or may need PID sometimes, for process to be killed. You can use it with ‘grep‘ command to find customised output. For example,
root@user:~# ps -A | grep -i ssh
Here ‘ps‘ is pipelined with ‘grep‘ command to find customised and relevant output of our need.
Command: kill
You need a process’s pid (ps) to kill it. Let suppose you want to kill program ‘apache2‘ that might not be responding. Run ‘ps -A‘ along with grep command.
root@user:~# ps -A | grep -i apache2
1285 ? 00:00:00 apache2
Find process ‘apache2‘, note its pid and kill it. For example, in that case ‘apache2‘ pid is ‘1285‘.
root@user:~# kill 1285 (to kill the process apache2)
Note: Every time you re-run a process or start a system, a new pid is generated for each process and you can know about the current running processes and its pid using command ‘ps‘. Another way to kill the same process is.
root@user:~# pkill apache2
Note: Kill requires job id / process id for sending signals, where as in pkill, you have an option of using pattern, specifying process owner, etc.
Command: whereis
The ‘whereis‘ command is used to locate the Binary, Sources and Manual Pages of the command. For example, to locate the Binary, Sources and Manual Pages of the command ‘ls‘ and ‘kill‘.
root@user:~# whereis ls
root@user:~# whereis kill
Note: This is useful to know where the binaries are installed for manual editing sometimes.
Command: service
The ‘service‘ command controls the Starting, Stopping or Restarting of a ‘service‘. This command make it possible to start, restart or stop a service without restarting the system, for the changes to be taken into effect.
Startting an apache2 server on Ubuntu
root@user:~# service apache2 start
Restarting a apache2 server on Ubuntu
root@user:~# service apache2 restart
Stopping a apache2 server on Ubuntu
Stopping a apache2 server on Ubuntu
Note: All the process script lies in ‘/etc/init.d‘, and the path might needs to be included on certain system, i.e., in spite of running “service apache2 start” you would be asked to run “/etc/init.d/apache2 start”.
Command: alias
alias is a built in shell command that lets you assign name for a long command or frequently used command. For example.
alias root='sudo -i'
Command: df
Report disk usages of file system. Useful for user as well as System Administrator to keep track of their disk usages. ‘df‘ works by examining directory entries, which generally are updated only when a file is closed.
root@user:~# df
Filesystem 1K-blocks Used Available Use% Mounted on
/dev/sda1 47929224 7811908 37675948 18% /
none 4 0 4 0% /sys/fs/cgroup
udev 1005916 4 1005912 1% /dev
tmpfs 202824 816 202008 1% /run
none 5120 0 5120 0% /run/lock
none 1014120 628 1013492 1% /run/shm
none 102400 44 102356 1% /run/user
/dev/sda5 184307 79852 94727 46% /boot
/dev/sda7 95989516 61104 91045676 1% /data
/dev/sda8 91953192 57032 87218528 1% /personal
Command: du
Estimate file space usage. Output the summary of disk usages by ever file hierarchically, i.e., in recursive manner.
root@user:~# du
308 ./.local/share/Trash/files/jdk1.8.0_121/jre/lib/desktop/icons
344 ./.local/share/Trash/files/jdk1.8.0_121/jre/lib/desktop
4 ./.local/share/Trash/files/jdk1.8.0_121/jre/lib/desktop/applet
548 ./.local/share/Trash/files/jdk1.8.0_121/jre/lib/desktop/oblique-fonts
2048 ./.local/share/Trash/files/jdk1.8.0_121/jre/lib/desktop/fonts
Note: ‘df‘ only reports usage statistics on file systems, while ‘du‘, on the other hand, measures directory contents.
Bash scripts
Cpu monitoring
#!/bin/bash
while [ true ] ;do
used=`free -m |awk 'NR==3 {print $4}'`
if [ $used -lt 1000 ] && [ $used -gt 800 ]; then
echo "Free memory is below 1000MB. Possible memory leak!!!" | /bin/mail -s "HIGH MEMORY ALERT!!!" user@itcollege.ee
fi
sleep 5
done
Adding new users to a Linux system
This script allows the root user or admin to add new users to the system in an easier way by just typing the user name and password (The password is entered in an encrypted manner).
#!/bin/bash
# Script to add a user to Linux system
if [ $(id -u) -eq 0 ]; then
read -p "Enter username : " username
read -s -p "Enter password : " password
egrep "^$username" /etc/passwd >/dev/null
if [ $? -eq 0 ]; then
echo "$username exists!"
exit 1
else
pass=$(perl -e 'print crypt($ARGV[0], "password")' $password)
useradd -m -p $pass $username
[ $? -eq 0 ] && echo "User has been added to system!" || echo "Failed to add a user!"
fi
else
echo "Only root may add a user to the system"
exit 2
fi
Login Script
When bash is invoked, it runs /etc/profile if that file exists. Next, it looks for these files (in this order) ~/.bash_profile, ~/.bash_login , and ~/.profile. The first one that is found gets executed (any others are ignored). In the case of Ubuntu, ~/.profile is found and executed. (All of ~/.bash_profile, ~/.bash_login , and ~/.profile are ignored if --noprofile is used as an option to the underlying bash call). So for example, if you login via a virtual console, or if you change to another user e.g:
sudo su student2
or if u run:
bash --login
or if you login to a machine via ssh e.g:
ssh user@machine
You'll be invoking ~/.profile Note that ~/.profile (or if not there, one of ~/.bash_profile or ~/.bash_login) normally contains relevant commands to run another script called ~/.bashrc (if said file exists):
if [ -f ~/.bashrc ]; then
. ~/.bashrc
fi
Checking Server Utilization
Checking the server utilization is one of the important task of an administrator, and a good administrator is one who knows how to automate his day to day task. Below is the script that will give many such information about your server
#!/bin/bash
date;
echo "uptime:"
uptime
echo "Currently connected:"
w
echo "--------------------"
echo "Last logins:"
last -a |head -3
echo "--------------------"
echo "Disk and memory usage:"
df -h | xargs | awk '{print "Free/total disk: " $11 " / " $9}'
free -m | xargs | awk '{print "Free/total memory: " $17 " / " $8 " MB"}'
echo "--------------------"
start_log=`head -1 /var/log/messages |cut -c 1-12`
oom=`grep -ci kill /var/log/messages`
echo -n "OOM errors since $start_log :" $oom
echo ""
echo "--------------------"
echo "Utilization and most expensive processes:"
top -b |head -3
echo
top -b |head -10 |tail -4
echo "--------------------"
echo "Open TCP ports:"
nmap -p- -T4 127.0.0.1
echo "--------------------"
echo "Current connections:"
ss -s
echo "--------------------"
echo "processes:"
ps auxf --width=200
echo "--------------------"
echo "vmstat:"
vmstat 1 5
Checking disk space + alert
MAX=95
EMAIL=user@itcollege.ee
PART=sda1
USE=`df -h |grep $PART | awk '{ print $5 }' | cut -d'%' -f1`
if [ $USE -gt $MAX ]; then
echo "Percent used: $USE" | mail -s "Running out of disk space" $EMAIL
fi
Links
http://gnome-look.org/content/show.php/Ultimate+Bashrc+File?content=129746 hacking .bashrc
