How-To:: bash scripting reminders of basics

Monday, 6 April 2020

bash scripting reminders of basics

Problem:

I want quick reminders for bash shell scripting because I don't write bash scripts often enough to memorize everything.

Notes:

The following reminders assume you're already familiar with the basics of bash shell scripting and simply need a reminder of more commonly used syntax, etc. These reminders are for myself but I hope you also find them useful.

Reminder shortcuts

As the number of reminders grow I hope this index will help you find what you need. (It certainly helps me.)

References

Top of script starts with #!/bin/bash
Simple variables
Concatenate strings
String quotes of bash scripting
Bash check if directory exists
Bash for-loop
Bash if-and, bash if-or
Bash if directory does not exist
Bash check if file exists
For items in bash array
Command line arguments in bash scripting
How to check if git repo needs update from remote upstream with bash scripting
How to check if script is on correct git branch for bash script

Reminders:

Top of script starts with #!/bin/bash


#!/bin/bash

See this tldp.org reference page for details.

Simple variables

Examples of simple variables

NAME="Delta"
echo "Hello $NAME!"

Concatenate strings

HELLO="Hello, "
NAME="Lily"

echo $HELLO$NAME
# output: Hello, Lily

HELLO="Hello, "
NAME="Alex"
GREET="${HELLO}${NAME}!"

echo "$GREET"
#output: Hello, Alex!

GREET="Hello, "
GREET+="Ninja"

echo "$GREET"
#output: Hello, Ninja

String quotes of bash scripting

DIRECTION="Right"
echo "Turn $DIRECTION"  # => Turn Right
echo 'Turn $DIRECTION'  # => Turn $DIRECTION

Bash for-loop

VAR=""
for NAME in 'alpha' 'bravo' 'charlie' 'delta'; do
  VAR+="${NAME} "
done

echo $VAR
# output: alpha bravo charlie delta

Bash if-and, bash if-or

if [ $FILENAME == 'important.txt' ] && [ -f $FILENAME ]; then
  echo "this is an important file"
fi

if [ $NAME == 'Hector' ] || [ $NAME == 'Miguel' ]; then
  echo "this person is on a hero's journey"
fi

Bash check if directory exists

DIR="/usr/mydir/"

if [ -d "$DIR" ]; then
  echo "directory exists"
else
  echo "directory not found"
  exit 1
fi

DIR="/usr/mydir/"

[ "$DIR" == "" ] && { echo "directory string is empty"; exit 1; }
[ -d "${DIR}" ] && echo "directory exists" || echo "directory not found"

Bash if directory does not exist


DIR="/usr/mydir/"

if ! [ -d "$DIR" ]; then
  echo "directory does not exist"
else
  echo "directory exists"
fi

Bash check if file exists

if [ -f "/opt/myfile.txt" ]; then
  echo "file found"
else
  echo "file not found
fi

For items in bash array

DIR3="/opt/mydir3"
DIRTOCHECK=("/opt/mydir1" "/opt/mydir2" $DIR3)

for dir in "${DIRTOCHECK[@]}"
do
  if [ -d "${dir}" ]; then
    echo "found directory ${dir}"
  else
    echo "did not find directory ${dir}"
  fi
done

Command line arguments in bash scripting

# example: yourscript.sh arg1 2nd-arg

echo "All arguments values:" $@
# output: All arguments values: arg1 2nd-arg

echo "First argument:" ${1}
# output: First argument: arg1

echo "Second argument:" ${2}
# output: Second argument: 2nd-arg

echo "Total arguments:" $#
# output: Total arguments: 2

For more detailed information, please refer to this tecadmin.net tutorial

How to check if git repo needs update from remote upstream with bash scripting

This how-to assumes you already have a git repository installed, a remote upstream setup, and have navigated into the local repository via your script.

git fetch
if [ $(git rev-parse HEAD) == $(git rev-parse @{u}) ]; then
  # add logic for when no update is needed, e.g.
  echo 'already up to date.'
else
  # add logic for when update is needed, e.g.
  echo 'updating to latest.'
  git pull
fi

How to check if script is on correct git branch for bash script

The following example assumes master branch is desired and the script has already navigated to the local repository.

if ! [ $(git rev-parse --abbrev-ref HEAD) == "master" ]; then
  echo "NOT in master branch."
else
  echo "in master branch."
fi

Why these reminders?

This page is intended to remind myself quickly without requiring too much reading. I hope it's helped you out, too.

How-To: