Bash Examples: Basic Information and Control Structures

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Table of Contents

Shebang

add this to the start of every script.

#!/usr/bin/env bash

If then else

spaces before and after [[ and before ]] are important!

if [[ condition ]];then
    # do stuff
elif [[ other condition ]]; then
    # something else
else
    # or do this 
fi

If then else with grep matches

Does $PATH include "some_string"?

if echo $PATH | grep -q "some_string"; then
    echo "matches!"
else
    echo "no matches"
fi

If not

Spaces are important!

if [[ ! condition ]];then
    # do stuff
fi;

AND operator

Use &&:

a=true
b=false

if [[ $a && $b ]];then
    # will NOT print
    echo "a and b is true"
fi

OR operator

Use ||

a=true
b=false

if [[ $a || $b ]];then
    # will print
    echo "a or b is true"
fi

Access command-line arguments

$# is the number of arguments provided to the current script

$0 is the script name

$1, $2, $3 and so on until $9 are the first argument, second argument, third argument and so on.

Function definition

function do_something {
    echo "this gets printed to standard output"
}

# to call it, just use its name
do_something

Function with parameters

$1 is the first argument, $2 is the second argument, etc.

function say_something {
    echo "$1"
}

$ say_something "foobar"
# prints "foobar"

$ say_something
# prints nothing

$ say_something foo bar
# prints "foo" (other arguments weren't accessed)

Number of parameters passed to function

To get the number of arguments sent to the function use $#

myfunction(){
    if [[ $# -eq 0 ]]; then
      # no parameters passed
    elif [[ $# -eq 1 ]]; then
      # 1 parameter passed
    else
      # 2 or more parameters passed
    fi
}

Check if command exists

The program deactivate is enabled if you have an active python virtual environment.

If you wish to test whether the current user is in a virtualenv, one way is to test whether the command deactivate exists, you can use the type builtin and test its result;

Example: Test if command deactivate exists in this system

# piping the output of this command to dev null
# to avoid noise
ret=$(type deactivate &> /dev/null)

if [[ $? -eq 0 ]]; then
  echo "returned 0, we're in a virtual env"
else
  echo "nonzero return code, no virtualenv" 
fi

bash shell

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