Dev 101: Shell in a nutshell

In the Dev 101 series I cover some basic concepts of computer programming for a broad audience. I guess it’s the explanation I was looking for myself, when I first started out as a programmer…

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In a previous installment of the Dev 101 series, I explained the concept of the shell — a command language interpreter that executes commands read from standard input, a command line string, or a specified file.

Dev 101: Terminal velocity

This means that shell can be used either interactively via the terminal:

tdeneire@XPS-13-9370:~$ sh
$ echo "hello"
hello
$ exit
tdeneire@XPS-13-9370:~$

or as a CLI:

sh -c "echo $(date)"

or to execute Shell scripts, saved as .shfiles like this:

#!/bin/sh
echo "Hello world"

You can then execute them with sh myfile.sh

Especially the last option is a fantastic tool for writing scripts for automation, installation, configuration, and so on.

Because knowing your way around Shell scripts is a vital tool for any developer working with UNIX systems, I will try to summarize Shell syntax below.

I’ve taken care to limit this summary to “pure” UNIX shell syntax, i.e. the Bourne shell or sh . Most Unix-like systems contain the file /bin/sh that is either the Bourne shell, or a symbolic link (or hard link) to a compatible shell (e.g. dash). The reason for sticking to sh is that different shell versions (bash, dash, ksh, zsh, …) all come with different features, although they are mostly sh-compatible. By sticking to sh syntax you allow your scripts to be executed by a variety of shells, which makes them dependable and robust.

Hello world

echo "Hello World!"

Comments

# This is a comment

Variable assignment

NAME_1="Tom"    # set
readonly NAME_1 # read-only
unset NAME_1    # unset

Quoting (strong vs weak)

Quoting variables prevents word splitting and glob expansion, and prevents the script from breaking when input contains spaces, line feeds, glob characters and such.

$ echo '$HOME'
$HOME
$ echo "$HOME"
/home/tdeneire

Arrays

# POSIX shell does not support array variables!
# dash, bash, ksh and others do, however

User input

echo "Enter user name: "
read -r FIRST_NAME
echo "Current user name is $FIRST_NAME"

Operators (arithmetic)

VAL=$((2 + 2))
# +  (Addition) 
# -  (Subtraction) 
# *  (Multiplication)
# /  (Division) 
# %  (Modulus) 
# =  (Assignment) 
# == (Equality) 
# != (Not Equality)

Operators (relational)

[ "$VAL" -eq 4 ]
# -eq (equal)
# -ne (not equal)
# -gt (greater than)
# -lt (less than)
# -ge (greater or equal)
# -le (less or equal)

Operators (boolean)

# && (AND)
# || (OR)
# !  (NOT)

Operators (string)

NAME="Tom"
[ "$NAME" = "Tom" ]
# =   (equal)          
# !=  (not equal)
# -z  (zero length)    
# -n  (non-zero length)
# str (not empty)

Conditional statements (if…elif…else)

NAME="Tom"
if [ "$NAME" = "Tom" ];
then
    echo "me"
elif [ "$NAME" = "Peter" ];
then
    echo "my cousin"
else
    echo "someone else"
fi

Conditional statements (case)

NAME="Tom"
case $NAME in
    "Tom")
        echo "me"
    ;;
    "Peter")
        echo "my cousin"
    ;;
    *)
        echo "someone else"
    ;;
esac

Loops

For

for NUMBER in 0 1 2 3 4 5 6 7 8 9
do
    echo $NUMBER
done

While

I=0
while [ $I -lt 10 ]
do
    echo $I
    I=$(($I + 1))
done

Until

I=0
until [ ! $I -lt 10 ]
do
    echo $I
    I=$(($I + 1))
done

Break

for VAR1 in 1 2 3
do
    for VAR2 in 0 5
    do
        if [ $VAR1 -eq 2 ] && [ $VAR2 -eq 0 ]
        then
            break 2
        else
            echo "$VAR1 $VAR2"
        fi
    done
done

Continue

NUMS="1 2 3 4 5 6 7"
for NUM in $NUMS
do
    Q=$(($NUM % 2))
    if [ $Q -eq 0 ]
    then
        echo "$NUM is an even number"
        continue
    fi
    echo "Found odd number: $NUM"
done

Command substitution

echo "Date is $(date)"

Functions

Hello () {
    echo "Hello, $1"
    return 0  # Can only return 0-255. Other data should be written   
    to stdout
}
Hello "Tom"
RETURN=$?
echo "This command exited with code $RETURN"

Sources

POSIX Shell Tutorial

POSIX shell cheat sheet

GitHub - dylanaraps/pure-sh-bible: 📖 A collection of pure POSIX sh alternatives to external…

Shell Scripting Tutorial

Hi! 👋 I’m Tom. I’m a software engineer and a writer. My content is free, but if you want, you can support me at https://www.buymeacoffee.com/tomdeneire