Trivial question

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

Why does sudo id works and sudo cd does not?

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?

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?

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? 

$ man id
$ id --help

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

$ $COMMAND help | --help | -h

Standard input, output & error

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

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

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

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

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

Cut example:

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

AWK example:

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

Environment variables

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

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

More information