Bash

Bash How to do Math

You can use arithmetic expansion with parentheses or square brackets:

a=0
echo $((a+5))

Output: 5

echo $[a+2]

Output: 2

Another option is using the command “bc”:

echo "5+3" | bc

Output: 8

If you need to do math with a float number, use the variable “scale” to define how operations use decimal numbers

echo "scale=2; 1/4" | bc

Output: 0.25

Evaluate expression with only integer you can use the command “expr”

echo `expr 10 - 2`

Output: 8

Be careful with multiply operations. Always escape * (asterisk) char with \. For example:

expr 5 \* 3

Output: 15

Bash How to Basename

Command basename strip directory and suffix from filenames. Command syntax:

basename [option] name [suffix]

If the suffix is specified it will remove a trailing suffix. Example:

basename dir1/dir2/dir3/text_file.txt .txt

Output: text_file

Basename takes one argument (filename) and an optional suffix. If you want to give more file names use the option “a” which supports multiple arguments and threat each as “name”.

basename -a /dir/file.txt /dir2/picture.jpg

Output: file.txt picture.jpg

If you want to get the name of your home folder:

basename ~

Often used option is option -s which removes a trailing suffix. Here is an example:

basename -s .txt -a /dir/file.txt /dir2/picture.jpg

file picture.jpg

Bash Vs KSH

Linux and Unix have various shells. Two kinds of these numerous shells are KSH and BASH.

KSH (The Korn Shell) was developed many years before the BASH. Ksh has associative arrays and handles loop syntax better than bash. Also, ksh’s command print is better than bash’s echo command. In other way, ksh does not support history completion, process substitution, and rebindable command-line editing.

Bash has more added extension than ksh. Bash has tab completion and an easier method to set a prompt in order to display the current directory.

Compared to ksh, bash is newer and more popular.

Example of difference ksh and bash in condition test. First bash:

if [ $i -eq 3 ]

and condition test in ksh:

if (($i==3))

Bash can handle exit codes from pipes in a cleaner way. Bash and KSH are both Bourne=compatible shells, they share common functions and features and can be interchangeable to use.

Bash How to Measure Execution Time

The command “time” report an execution time information

time sleep 1

Output:
real 0m1.001s
user 0m0.000s
sys 0m0.001s

Description:

• Real-time from start to finish
• User – the amount of CPU time spent in use mode. It is the actual CPU time used in executing the process
• Sys – the amount of CPU time spent in kernel

You can measure execution time by subtraction start date and end date:

start=`date +%s`
commands...
end=`date +%s`
echo Execution time was `expr $end - $start` seconds.

If you need to have more accurate measures:

start=`date +%s%N`
commands...
end=`date +%s%N`
echo Execution time was `expr $end - $start` nanoseconds.

Adding %N to date +%s causes nanosecond accuracy.

Bash How to Sum Variables

Let’s define two variables:

FILES_ETC=1416
FILES_VAR=7928

If you want to get total value, you can do:

TOTAL_FILES=$((FILES_ETC+FILES_VAR))
echo $TOTAL_FILES

For 100% compatibility with old shells, you can use external command expr. In modern systems (ec. Centos 7 64bit), in fact, expr is not an external command. It is an internal shell function. So there is not a significant difference in speed between the above and this command:

TOTAL_FILES=`expr $FILES_ETC + $FILES_VAR`
echo $TOTAL_FILES

Funny thing is that there must be spaces between arguments. In fact, expr is a command, $FILES_ETC is the first argument, + is the second argument and $FILE_VAR is the third argument.

If you want to add numbers with decimal points, you can use bc. bc is a very useful external command for making arithmetical operations:

TOTAL_FILES=$(echo $FILES_ETC + $FILES_VAR | bc)
echo $TOTAL_FILES

Common mistakes are spaces before or after ‘=’ sign. However bc and expr commands expect spaces between arguments and operators, so in these cases don’t forget to spaces.

Bash How to Use Variables

Firstly, you have to declare a variable:

var="Hello"

To get value from variable use the symbol “$” before variable name like in the following example:

echo $var friends.

Output: Hello friends.

Curly brackets “{}” are used to expand variables in the string. Often use to append string after variable value:

echo ${var}ween

Output: Helloween

If we didn’t use “{}” around var, the echo would only write an empty line.

There are examples of special variables:

• $0 – Name of the bash script.
• $# – How many arguments were passed to the script.
• $? – Exit code of the last run process.
• $$ – Process ID of the actual script.
• $HOSTNAME – Hostname of the machine that script is running on.
• $LINENO -Current line number in the script.

Bash How to Trim String

You can use a bash parameter expansion, sed, cut and tr to trim a string.

Let’s use bash parameter expansion to remove all whitespace characters from the variable foo:

foo="Hello world."
echo "${foo//[["space:]]/}"

Output: Helloworld.

“${foo// /}” removes all space characters, “$foo/ /” removes the first space character.

To remove only space characters before and after string use sed:

foo=" Hello world. "
echo "${foo}" | sed -e 's/^[[:space:]]*//'

Output: Hello world.

Easy to use and remember the way how to remove whitespaces before and afterword:

echo " text. text2 " | xargs

xargs remove all whitespaces before “text.” and let one space between text. and text2.

If you want to delete only the last character from the variable:

foo="hello"
echo "${foo::-1}"

Output: hell

Bash How to know if a Process is Running

In this article, assume that we are looking for a process named “bash”. If you are not using a different shell, the bash process is running probably all of you.

If you want to see just PID (Process ID), you can use pgrep. If there is more than one process called bash, the output will be multi-line. If there is no process called bash, nothing is written to screen:

pgrep bash

What if you want to write some output if bash is running? In this example, pgrep finds a process called “bash”, redirects all output to /dev/null (nowhere). If pgrep return zero exit code, echo “The bash is running” will be executed:

pgrep bash 1>/dev/null 2>&1 && echo "The bash is running"

This example output “The bash is not running”, if bash is not running, you can use:

pgrep bash 1>/dev/null 2>&1 || echo "The bash is not running"

If you want to see, all processes that are running, there are also possible to do that in the traditional way. In the next example, ps aux is redirected to grep. Grep will filter lines that end ($) with bash:

ps aux | grep bash$

The second column of output is PID (Process ID). It is useful when you want to kill processes.

Command ps has ‘-o’ (overloaded) flag which shows you information about processes. One of the options is the stat option. Example:

ps -o comm, stat

Output: COMMAND STAT
bash Ss
ps R+

If the process is running it has the capital letter R or S.

Bash How to Echo Quotes

In this case, it is essential to put a backslash. Backslash will focus on character followed by a sign with a backslash. The following characters depose functions of the meta tag.

If you want to echo single quotes, you can to:

echo Single quote: \'

Output: Single quote: ‘

If you want to echo double quote, you can do:

echo Double quote: \"

Output: Double quote: ”

If you want to quote a single quote in double-quotes:

echo "Single quote in double-quotes: '"

Output: Single quote in double-quotes: ‘

If you want to quotes double quotes in double-quotes:

echo "Double quote in double-quotes \""

Output: Double quote in double quotes ”

There are 2 kinds of quoting: weak (“) and strong (‘). Using weak quoting there is no special meanings of:

• pathname expansion
• process substitution
• single-quotes
• characters for pattern matching

If we use strong quoting, nothing is interpreted, except a single quote. Examples:

echo "Path to your shell is: $SHELL"
echo 'Path to your shell is: $SHELL'

Output: Path to your shell is: /bin/bash
Path to your shell is: $SHELL

Bash How to Stop Script

If you want to stop the bash script, that is currently running, you can do the following sentence in bash:

kill $(ps aux | grep name_of_scirpt | grep -v grep | awk '{ print $2 }')

Command ps aux will give you all processes that are currently running. Grep will filter the name that you want to kill. You should be very specific, in another word, the whole name of the process name should be used here. Grep -v grep will filter grep process. AWK will filter the second column from the output, that is PID (process id).

If you want to stop script on a specific line, just add

exit 45

This line exits your script and gives 45 to the parent process. You can choose a number between 0 – 255. Number 0 is special, it means that your script exits without any problem. Any number between 1 – 255 means that something wrong happens in your script.

If you want to stop your script, if 1st error occurs, just add in hashpling line (1st line of the script) -e parameter:

#!/bin/bash -e
...
command1
command2
command3

This script will terminate immediately if some line fails. What means that line of the script fails? It means that line will exit with a non-zero exit code (1-255). For example, if command2 fails (return non-zero exit code), command3 will not be executed, at all.

I love this option in a test environment. It helps me to avoid unnecessary script execution after failure. I also like the bash -u parameter that exits your script if you use an uninitialized shell variable.

If you want to get the exit code of the last command enter:

echo $?

Bash will print integer values between 0 and 255.

Bash How to Assign Output to Variable

Backquotes (“) are used for command substitution.

var=`date`

Command “date” return date to variable “var”. The alternative method to using command substitution is “$()”:

var =$(date)

If we want to get value from “var” use “$var”:

echo "$var"

Output: Tue Jul 12 15:33:06 CEST 2016

It properly always uses double quotes around variable substitution. Without double quotes, the variable is expanded which may cause problems if the variable contains special characters.

Make sure there is no space between variable name before and after assign char (=). You can assign output from pipeline to a variable:

NUM_FILES=`ls | wc -l`