Bash

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 Echo Array

You can define three elements array (there is no space between the name of an array variable, equal symbol, and starting bracket):

FILES=(report.jpg status.txt scan.jpg)

This command will write each element in the array:

echo ${FILES[*]}

Index in shell arrays starts from 0. So, if you want to write just the first element, you can do this command:

echo ${FILES[0]}

Output: report.jpg

Do you want to process each element in the array in the loop? Here is an example:

for ELEMENT in ${FILES[@]}
do
echo File: $ELEMENT.
done

If you want to get only indexes of the array, try this example:

echo ${!FILES[@]}

“${!FILES[@]}” is a relatively new bash feature, it was not included in the original array implementation.

Bash Error Output

Bash provides I/O redirection. There are 3 standard files: STDIN (standard input) with descriptor 0, STDOUT (standard output) with descriptor 1, and STDERR (standard error) with descriptor 2.

If you want to redirect your messages to STDERR, you can use >&2 symbol. This symbol is abbreviation of 1>&2 symbol which means, that everything in STDOUT will go to STDERR.

So, if you want to put the message “Cannot delete directory” in STDERR, you can do it this way:

echo Cannot delete directory >&2

Even more, you might to create your own function for error messages:

recho() { echo "$*" >&2 ; }
recho "Cannot delete directory" > /dev/null

Output: Cannot delete the directory

On the first line, we define recho function (error echo). This function will print all its arguments ($*) to STDERR (>&2).

On the second line, we try to use the function. To prove, that output will be written to STDERR, we will redirect STDIN to nowhere (/dev/null). So, if you see some output, it should be in STDERR.

Bash Function How to Return Value

Bash functions have “return” statement, but it only indicates a return status (zero for success and a non-zero value for failure).

function myfunc() {
var='some text'
echo Hello..
return 10
}

myfunc
echo "Return value of previous function is $?"

Output: Hello.. Return value of the previous function is 10

If you want to return value you can use a global variable.

var=0
string () {
var="My return value."
}
string; echo $var

Output: My return value.

It is simple, but using global variables in complex scripts or programs causes harder methods to find and fix bugs.

We can use command substitution and assign an output from function:

string () {
local local_var="Value from function."
echo $local_var
}

var=$( string )
echo $var

Output: Value from function.

It’s good practice to use within-function local variables. Local variables are safer from being changed by another part of the script.

Bash How to Compare Numbers

The first option is to use the command test to binary compare numbers. For example:

if [ $a -eq $b ]; then
echo "a == b"
else
echo a!=b
fi

An operator”-eq” is equal to, -gt is greater than. For more operators type:

man test

If you are more familiar with symbols “<, >, <=, >=, ==” use double parentheses:

if (( $a < $b )); then
echo "a < b"
fi

For POSIX shells that don’t support double parentheses use test command.

Few examples of test options:

• -ge: greater or equal
• -le: less or equal
• -lt: less than
• -ne: not equal

Bash How to Return String from Function

You can return string from function in many ways, but you can not use the command “return” to return string:

return "Hello..."

Return statement can return only an integer value.

First option uses a passing argument to the function. To assign to the first argument use in function “$1”:

eval "$1='Hello...'"

Then call the function “my_function”:

my_var=''
my_function my_var
echo $my_var

Output: Hello…

Other way is to use a global variable which you modify within the function.

You can also use command echo to write string value and use command substitution to get it:

hello() {
var='Hello friend.'
echo "$var"
}

greeting=$(hello)
echo $greeting

Bash How to Get Exit Status

The answer is in this special shell variable “$?”. In this variable is saved exit status of the last command that ended in the background.

In the next example “paranormal_directory” doesn’t exists, at all. It is a paranormal directory 🙂 In this example, $? variable will be 2, because command ls fails:

ls paranormal_directory 1>/dev/null 2>&1
echo $?

Output: 2

Listing the home folder is always safe. Executing ls without any parameter lists home folder of the current user. In this example, ls will succeed, so variable “$?” will be 0:

ls 1>/dev/null 2>&1
echo $?

Output: 0

But be careful, if you read this variable two times (echo $?). In the next example, “echo $?” (on line 3) will show you the output of the first echo command (on line 2):

ls paranormal_directory 1>/dev/null 2>&1
echo $?
echo $?

Output: 2 0

What happens if you don’t specify the exit code in the script? When the exit code is not specified with the exit command, the exit code of the script will be the exit code of the last executed command.

Bash How to Run Command from Variable

You can run the command from variable using the command “eval”:

eval $foo

The command eval takes an argument, construct and execute the command of it.

Another option is the symbol “$”:

$foo

Let’s have the following example:

foo='date'
foo2='echo "Hello :)"'
eval $foo
$foo2

Output: Wed Jul 27 14:17:40 CEST 2016 Hello 🙂

It is also possible to use bash’s option -c which executes commands from variables in a separate script, that inherits file descriptors, environment variables.

bash -c "$foo"

eval “$foo” executes the command in the current script, not in a separate script. If you want to execute the eval command in a separate script, use brackets: (eval “$1”).

Bash Color Shell Prompt

You can customize 4 prompts: PS1 (Primary prompt, displayed before each command), PS2 (secondary prompt, displayed when a command needs more input), PS3 (rarely used, displayed for Bash’s select built-in which displays interactive menus).

Display current bash prompt (PS1) settings:

echo $PS1

Output: [\u@\h \W]\$

It is the default setting. The backslash-escaped characters means: \u (username), \h (hostname), \W (current working directory).

To modify colors to the prompt use following syntax:

\e[x;ym $PS1 \e[m

Meaning: \e[ (start color scheme), x;y (color pair to use), $PS1 (shell prompt variable), \e[m (stop color scheme)

To set red color enter:

export PS1="\e[0;36m[\u@\h \W]\$ \e[m "

Few examples of color codes:

• black(0;30)
• red (0;31)
• greed (0;32)
• brown (0;33)
• blue (0;34)
• purple (0:35)
• cyan (0:36)

If you replace digit 0 with 1 you get a lighter color version.
Setting variable PS1 is temporary, when you log out your settings will be lost. You have to append the following line to $HOME/.bash_profile file or $HOME/.bashrc file:

export PS1="\e[0;36m[\u@\h \W]\$ \e[m "

Now ur new prompt color is permanent.

Bash How to Pass Arguments

The special shell variable “$@” represents a list of all arguments that is passed to the script.

If you want to pass all arguments to your function, you can use this syntax:

function_name "$@"

If you want to pass all arguments to another script, you can use this syntax:

script_mame "$@"

Let’s take an example called passit.sh. In this script, we defined function print_argument, that print argument that comes from the command line:

#!/bin/bash

# function's definition
function PRINT_ARGUMENTS()
{
echo "Arguments of shell are: $@"
}

# in this place we want to call function
PRINT_ARGUMENTS "$@"

Let’s try to execute passit.sh in this way:

chmod u+x ./passit.sh
./passit.sh aa bb cc

Output: Arguments of the shell are: aa bb cc

You can see that function obtains all shell arguments, which were written in the bash command line.

To get a number of arguments use:

echo "Number of arguments of shell are: $#"

It is often used to check if a required number is equal to some value.

Bash How to List Environment Variables

If you want to print all environment variables, you can use printenv. Command printenv print all or just part of environment variables:

printenv

If you want to get names of variables, that was exported, you can use the export command:

export

You can use also the just env command. If you want to see all environment variables:

env

In fact, env run some program in a modified environment. In the next example, env will run only with a variable called “DIRECTORY”:

env -i DIRECTORY="/etc/mydir" bash
env

If you want to see functions, that you have declared, you can use “declare -f”. Maybe you have not defined any function. To overcome this, in the next example, we defined FUNKY_FUNCTION first:

FUNKY_FUNCTION ()
{
echo ":-)"
}
declare -f

The next command includes shell variables to output:

( set -o posix ; set ) | less

This command shows not only shell variables, but environment variables too.