Shell

The shell is the command-line interface between you and the operating system. Each user account is assigned a default shell, stored in the last field of /etc/passwd. System accounts are typically assigned /bin/false or /usr/sbin/nologin to prevent interactive login:

cat /etc/passwd | cut -d: -f 1,7
...
dhcpcd:/bin/false
sshd:/usr/sbin/nologin
testuser:/bin/bash

General command efficiency

Common keyboard shortcuts for navigating and editing commands at the prompt:

History

The shell records every command you run in ~/.bash_history. To prevent a command from being saved, prefix it with a space. On some distributions, add HISTCONTROL=ignoreboth to ~/.bashrc to enable this behavior:

Aliases

An alias substitutes a short command for a longer one. Add aliases to ~/.bashrc to make them permanent across sessions.

Manage aliases in the current session:

For example, define aliases for common tasks:

alias cpu10='ps -L aux | sort -nr -k 3 | head -10'   # list top 10 CPU-consuming processes
alias mem10='ps -L aux | sort -nr -k 4 | head -10'   # list top 10 memory-consuming processes
alias lsmount='mount | column -t'                    # list mounted filesystems in columns

Scripts

A shell script is an executable text file containing commands that the shell runs in sequence. Scripts are useful for tasks you perform more than once. Two common shell constructs to know:

• (( )): Arithmetic evaluation
• $(): Run a command in a subshell and return its output to the current shell

Check for a file

Use [ ! -f <path> ] to test whether a file exists before running commands. For example, install Apache only if it is not already present:

#!/bin/bash
if [ ! -f /usr/sbin/apache2 ]; then
    sudo apt install -y apache2
    sudo apt install -y libapache2-mod-php8.1
    sudo a2enmod php8.1
    sudo systemctl restart apache2
fi

Check for a directory

Use [ ! -d <path> ] to test whether a directory exists:

#!/bin/bash
if [ ! -d /path/to/dir ]; then
    ...
fi

Variables

When bash sees a string followed by =, it assigns the value on the right to a variable. Variable names are case-sensitive and contain no spaces around =.

Looping

Two loop types are available in bash:

• while: Executes a block of commands until a condition is no longer true
• for: Executes a block of commands once for every item in a set

A while loop that counts from 1 to 15:

#!/bin/bash
myvar=1
while [ $myvar -le 15 ]; do
    echo $myvar
    ((myvar++))
done

A for loop that iterates over a list of names:

#!/bin/bash
turtles="Donatello Leonardo Michelangelo Raphael"
for t in $turtles; do
    echo $t
done

Backup script

This script uses rsync to perform an incremental backup. It installs rsync if it is not present, then syncs /src to /target. Deleted files are moved to a dated subdirectory rather than removed permanently. Key options:

#!/bin/bash
curdate=$(date +%m-%d-%Y)

if [ ! -f /usr/bin/rsync ]; then
    sudo apt install -y rsync
fi

rsync -avb --delete --backup-dir=/backup/incremental/$curdate /src /target