Shell Scripting Back to Basics

Shell Scripting 1. Variables naming conventions recommended file="filename.txt" no spaces allowed between '=' house_color, source_file constants written in UpperCase Example: read only FILE="filename.txt" varialbe outside of scope to be used then use exports ### 2. Functions: functions can start with function keyword or without it. calculate_area() { } clone_repo() { } Expanding Variables The below both versions works well. #!/bin/bash var="value of var" echo ${var} #!/bin/bash var="value of var" echo $var The difference come when appending additional string in the end. Expanding variables with {} should be used based on need. #!/bin/bash height=70 echo "Your height is = $heightcm" # throws error echo "Your height is = ${height}cm" # works good Without the quotes the variable string will split by field seperators here space. #!/bin/bash string="One Two Three" for element in ${string}; do echo ${element} # or echo $element done O/P: One Two Three Now try enclosing with the double quotes. The string will not split out and will be act as entire string as one element. #!/bin/bash string="One Two Three" for element in "${string}"; do echo ${element} # or echo $element done O/P: One Two Three For the below scenario we want server names seperately. #!/bin/bash readonly SERVERS="server1 server2 server3" for server in ${SERVERS}; do echo "${server}.worker-node" done O/P: server1.worker-node server2.worker-node server3.worker-node Recommended to use "" expanding variables: Directory paths, filenames Assigning URLs to variables When in doubt, use double quotes :) Let revisit basic concepts of linux commands, loops, performing tasks. Shell scripting is much more powerfull. It follows the top down approach and only when we can it gets executed ( calling functions). # list items ls pwd cd mkdir Lets say there is some command will throws the errors to break the script over there we have declare "set -x" in the top of script. Usage of [[ ]] for conditional controls. #!/bin/bash if [[ 3 > 4 ]]; then echo " three greater than four"; fi Shell script has 4 loop types While for until similar to do-while loop While loop #!/bin/bash i=1 while [[ $i -le 3 ]]; do echo "Iteration $i" i=$((i+1)) done For loop for i in {1..3}; do # for i in $(seq 1 3); do using the sequence as sub-shell the output from it is treated input to i echo "Iteration $i" done Until Loop #!/bin/bash i=3 until [[ $i -eq 0 ]]; do echo "Iteration $i" i=$((i-1)) done To import the other script file There were sometimes we need to import variables from other files(.sh) # conf file name="Ubuntu" #!/bin/bas source .conf # sourcing the conf file echo "${name}" what if the conf file doesnt exists. We need to checkpoint in the script #!/bin/bash readonly CONF_FILE=".conf" if [[ -f ${CONF_FILE} ]]; then source "${CONF_FILE}" else name="Bob" fi echo "${name}" exit 0

Mar 7, 2025 - 01:37

Shell Scripting

1. Variables naming conventions recommended

file="filename.txt" no spaces allowed between '='
house_color, source_file
constants written in UpperCase Example: read only FILE="filename.txt"
varialbe outside of scope to be used then use exports ### 2. Functions: functions can start with function keyword or without it.

calculate_area() {
}
clone_repo() {
}

Expanding Variables

The below both versions works well.

#!/bin/bash
var="value of var"
echo ${var}

#!/bin/bash
var="value of var"
echo $var

The difference come when appending additional string in the end. Expanding variables with {} should be used based on need.

#!/bin/bash

height=70
echo "Your height is = $heightcm" # throws error
echo "Your height is = ${height}cm" # works good

Without the quotes the variable string will split by field seperators here space.

#!/bin/bash
string="One Two Three"
for element in ${string}; do
    echo ${element} # or echo $element
done

O/P:

One
Two
Three

Now try enclosing with the double quotes. The string will not split out and will be act as entire string as one element.

#!/bin/bash
string="One Two Three"
for element in "${string}"; do
    echo ${element} # or echo $element
done

O/P:

One Two Three

For the below scenario we want server names seperately.

#!/bin/bash
readonly SERVERS="server1 server2 server3"
for server in ${SERVERS}; do
    echo "${server}.worker-node"
done

O/P:

server1.worker-node
server2.worker-node
server3.worker-node

Recommended to use "" expanding variables:

Directory paths, filenames
Assigning URLs to variables
When in doubt, use double quotes :)

Let revisit basic concepts of linux commands, loops, performing tasks. Shell scripting is much more powerfull. It follows the top down approach and only when we can it gets executed ( calling functions).

# list items
ls
pwd
cd
mkdir

Lets say there is some command will throws the errors to break the script over there we have declare "set -x" in the top of script.

Usage of [[ ]] for conditional controls.

#!/bin/bash
if [[ 3 > 4 ]]; then
   echo " three greater than four";
fi

Shell script has 4 loop types

While
for
until similar to do-while loop

While loop

#!/bin/bash
i=1
while [[ $i -le 3 ]]; do
    echo "Iteration $i"
    i=$((i+1))
done

For loop

for i in {1..3}; do
# for i in $(seq 1 3); do using the sequence as sub-shell the output from it is treated input to i
   echo "Iteration $i"
done

Until Loop

#!/bin/bash

i=3
until [[ $i -eq 0 ]]; do
    echo "Iteration $i"
    i=$((i-1))
done

To import the other script file

There were sometimes we need to import variables from other files(.sh)

# conf file
name="Ubuntu"

#!/bin/bas
source .conf # sourcing the conf file
echo "${name}"

what if the conf file doesnt exists. We need to checkpoint in the script

#!/bin/bash
readonly CONF_FILE=".conf"
if [[ -f ${CONF_FILE} ]]; then
   source "${CONF_FILE}"
else
   name="Bob"
fi

echo "${name}"
exit 0