Trivial question Link to heading

title: Introduction to Linux Shell author: Dennis Ruiz date: 7/3/21 theme: Copenhagen Link to heading

Why does sudo id works and sudo cd does not?

Because cd is a builtin command not a program (executable).

$ compgen -b
jobs
exit
bg
pwd
source
eval
alias
cd
export
echo
wait
kill
fg
...

What is a Linux shell? Link to heading

It is a command-line interpreter software that can be interactive or scripted.

Some popular shells: sh, bash, zsh, csh, ksh.
Some popular terminals: xterm, iterm2, kitty, gnome-terminal

sh / bash:

sh was created by Ken Thompson
sh was created by Stephen Bourne
Bash (Bourne Again Shell) is based on Bourne Shell created by Brian Fox

Where are the programs or executables? Link to heading

Programs are located in $PATH

We can also see where exactly a program is located at:

~sh» whereis vim
vim: /usr/bin/vim /usr/share/vim

How to run an executable? Link to heading

$ man id
$ id --help

Executables always comes with a help menu or a manual page.

$ $COMMAND help | --help | -h

Standard input, output & error Link to heading

How data is being streamed:

Kernel reads input from: /dev/stdin
Kernel outputs a result: /dev/stdout
Kernel outputs an error: /dev/stderr

Background and Foreground commands Link to heading

Shell jobs can be managed in a way that background commands can be hidden from terminal and foreground occupies the terminal screen.

# List jobs
$ jobs -l

# Switch job to background or foreground
$ bg %n
$ fg %n

# Job stopped
$ CTRL-z or kill -tstp $PID

Completition Link to heading

A shell can autocomplete commands, options, files and directories by using .

Bash example of completion:

_foo()
{
    local cur=${COMP_WORDS[COMP_CWORD]}
    COMPREPLY=( $(compgen -W "bar baz" -- $cur) )
}
complete -F _foo foo

Wildcards and expansiosn Link to heading

If we don’t know the exact name of a file or directory we just use *:

$ ls a/**/*z*
a/b/c/z.txt

Brace expansions allow us to combine options:

$ echo {a,b,c}{d,e,f}
ad ae af bd be bf cd ce cf

~» echo {1..10}
1 2 3 4 5 6 7 8 9 10

History Link to heading

Random command:

~sh» ls a/b
a.txt  b.txt  c  c.txt  foo

Simple access to a previous command:

~sh» ls !$/c
ls a/b/c
z.txt

Accessing to a specific command in history:

~sh» ls -ltra !10096:$
ls -ltra a/b/c

Splitting outputs with cut & awk Link to heading

Cut example:

$ echo "abc def" | cut -f 2 -d ' '
def

AWK example:

$ echo "abc def" | awk '{print $2}'
def

Environment variables Link to heading

Environment variables help programs to be dynamic allowing support for custom configurations or values:

~sh» TZ=Japan date
vie 15 oct 2021 14:27:24 JST

Conditionals and loops Link to heading

Simple if conditional to create a file:

$ [ -f file.txt ] || touch file.txt

Simple loop conditional to iterate over a list of files & dirs:

$ for i in *; do echo $i; done
foo-completion-bash
intro-linux-shell.md
intro-linux-shell.pdf