Interpreting a Bash Script

What it is

A Bash script is a text file of shell commands that run in order, with variables, conditions and loops added.

To interpret one, you read the commands in sequence and say what the script achieves. The usual jobs are file handling, installing an application, managing users and moving through directories.

The first line, #!/bin/bash, tells the system to run the file with Bash.

How it works

Look for the shell features first, then name the job that the commands do together.

Script building blocks

Useful tests

Example 1: file management and backup

#!/bin/bash
SRC=/home/user/project
DEST=/var/backups

if [ ! -d "$DEST" ]; then
    mkdir -p "$DEST"
fi

cp -r "$SRC" "$DEST/project_backup"
cd "$DEST"
ls -l

This script sets two paths. It creates the backup folder if it does not exist, copies the project into it, moves into the folder and lists it. The workflow is back up a project folder.

Example 2: application install and user creation

#!/bin/bash
APP=nginx

sudo apt update
sudo apt install -y "$APP"

sudo useradd -m deploy
sudo systemctl enable "$APP"
sudo systemctl start "$APP"

The commands refresh the package list, install nginx without asking, create a user with a home directory, and enable and start the service. The workflow is install a web server, add a deploy user and start the service.

Example 3: a loop and log handling

#!/bin/bash
for f in /var/log/app/*.log; do
    if [ -f "$f" ]; then
        gzip "$f"
        mv "$f.gz" /var/log/archive/
    fi
done
echo "Logs archived"

The loop goes through every log file, compresses it, moves the result into an archive folder and prints a message at the end. The workflow is compress and archive log files.

Example 4: an argument and an exit check

#!/bin/bash
if [ -z "$1" ]; then
    echo "Usage: deploy.sh VERSION"
    exit 1
fi

cd /opt/app || exit 1
git pull
echo "Deployed version $1"

The script needs an argument. It prints a usage message and stops with a non zero status if none is given.

It then changes to the application folder, pulls the latest code and reports. The workflow is update an application from Git.

Commands that give strong clues

Quoting and variables

Variables are expanded inside double quotes, and not inside single quotes. Wrap variables in double quotes to cope with spaces in names. There must be no spaces around the equals sign in an assignment.

Exit status and safe scripts

A command returns status 0 on success and a non zero value on failure. exit 1 stops the script with an error. Adding set -e near the top stops the script at the first failing command.