Shell scripting basics

Typing the same commands every day wastes time and invites mistakes. A shell script puts those commands in a file. Then you run them with one short word. Here you write your first scripts on Termux, using variables, tests and loops. You finish with a backup script that copies your files and prints a summary.

What you'll be able to do

• Write a script that starts with a shebang and runs as ./script.sh.
• Store values in variables and use them safely with quotes.
• Make decisions with if and tests, and repeat work with a for loop.
• Make a file executable and read its exit code with $?.
• Write a small backup script that copies files and reports what it did.

Step 1: Start with a shebang

The first line of a script is the shebang. It tells the system which program should run the file. For a portable script use /bin/sh. On Termux that is a POSIX shell.

#!/bin/sh

The command below writes a two-line script to a file called hello.sh. The %s\n format prints each argument on its own line. The > sends the result into the file.

printf '%s\n' '#!/bin/sh' 'echo "Hello from Termux"' > hello.sh

Check what landed in the file.

cat hello.sh

#!/bin/sh
echo "Hello from Termux"

Step 2: Variables and quoting

A variable holds a value. You set it without a dollar sign and read it back with one. The commands below set name and then print it two ways.

name="Study Cipher"
echo "Hello, $name"
echo 'Hello, $name'

Hello, Study Cipher
Hello, $name

Double quotes expand the variable. Single quotes print the text exactly as written. Use double quotes around variables almost everywhere, so spaces and special characters survive.

You can also store the output of a command in a variable.

today=$(date +%F)
echo "backup for $today"

backup for 2026-02-14

Step 3: Branch with if and loop with for

The if statement runs one block or another depending on a test. The test goes in square brackets. The block ends with fi.

#!/bin/sh
if [ -d "$HOME/storage" ]; then
  echo "shared storage is set up"
else
  echo "run termux-setup-storage first"
fi

run termux-setup-storage first

The -d test is true when the path is a directory. Other useful tests are -f for a file, -z for an empty string, and -eq for number equality.

A for loop repeats a block once for each item in a list.

#!/bin/sh
for name in alpha beta gamma; do
  echo "item: $name"
done

item: alpha
item: beta
item: gamma

Step 4: Make it executable and read the exit code

A script file is not runnable until you add the execute permission. This command prints nothing when it works.

chmod +x hello.sh

Confirm the new permission, then run the script.

ls -l hello.sh

-rwxr-xr-x 1 u0_a123 u0_a123 41 Feb 14 10:00 hello.sh

./hello.sh

Hello from Termux

Every command finishes with an exit code. Zero means success. Anything else means a problem. You read the last one with $?.

./hello.sh
echo $?

Hello from Termux
0

A command that fails sets a non-zero code.

cat /no/such/file
echo $?

cat: /no/such/file: No such file or directory
1

Step 5: A complete backup script

Here is a small script that copies files into a dated folder and prints a summary. It uses everything from the steps above.

#!/bin/sh
# backup.sh - copy notes into a dated folder

src="$HOME/notes"
dest="$HOME/backups/$(date +%F)"

if [ ! -d "$src" ]; then
  echo "source folder $src is missing" >&2
  exit 1
fi

mkdir -p "$dest"
count=0
for file in "$src"/*; do
  cp "$file" "$dest/"
  count=$((count + 1))
done

echo "copied $count file(s) to $dest"

Save it, make it executable, and run it.

chmod +x backup.sh

./backup.sh

copied 3 file(s) to /data/data/com.termux/files/home/backups/2026-02-14

Run it again the next day and it makes a fresh folder for that date. An earlier backup is never overwritten.

Quick reference

Command	What it does
#!/bin/sh	Shebang that names the interpreter for the script
name=value	Sets a variable, with no spaces around the equals sign
"$name"	Reads a variable, expanding it inside double quotes
$(command)	Runs a command and captures its output
if [ -f "$file" ]; then ... fi	Runs a block only when the test is true
for x in a b c; do ... done	Repeats a block once per item
chmod +x script.sh	Adds the execute permission
./script.sh	Runs a script in the current folder
$?	Holds the exit code of the last command

Next steps

• Processes and jobs to run scripts in the background.
• Shell config and aliases to load your tools on every shell.
• Buried Strings to practise the tools you used here.