Shell in a nutshell

Photo by Jonas Dücker on Unsplash

In another blog, 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.

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

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Hi! 👋 I’m Tom. I’m a software engineer, a technical writer and IT burnout coach. If you want to get in touch, check out https://tomdeneire.github.io