103.1 Work on the command line

Weight: 4

Description: Candidates should be able to interact with shells and commands using the command line. The objective assumes the Bash shell.

Objectives

• Use single shell commands and one-line command sequences to perform basic tasks on the command line.
• Use and modify the shell environment including defining, referencing, and exporting environment variables.
• Use and edit command history.
• Invoke commands inside and outside the defined path.

Terms

• bash
• echo
• env
• export
• pwd
• set
• unset
• type
• which
• man
• uname
• history
• .bash_history
• Quoting

Shells and Bash

You issue your commands in a shell; it's your command line interface and you have various options for it. To reach your shell you should login into the system in the text mode or run one of the various Terminal Emulators in your GUI. Some samples are gnome-terminal, konsole, xterm, etc.

After running the Terminal Emulator or logging into the text mode, you are in the shell and you can issue commands. Although bash (GNU Bourne Again shell) is the most common one, you might use zsh, dash, ksh, csh, and others.

You can check where your general sh command links to via

$ readlink /bin/sh

Or check your $SHELL variable using:

echo $SHELL

Your bash has some internal commands that it understands without any external dependency (say cd, break, exec, ...) but if it does not understand something internally, it will try to run it as an external executable.

You can use the type command to determine this:

[jadi@fedora ~]$ type cd
cd is a shell builtin
[jadi@fedora ~]$ type ls
ls is aliased to `ls --color=auto'
[jadi@fedora ~]$ type ping
ping is /usr/bin/ping

cd, pwd & uname

cd

You've already seen lots of cd commands :) it changes directory, including . (current directory) and .. (parent directory).

You can point to directories in two ways:

1. Absolute Paths: Like /home/jadi/lpic1/lesson3.1
2. Relative Paths: Like lpic1/lesson3.1. In this case, we are not adding the / in the beginning so the bash will try to find lpic1 directory where we are (local / relative)

The ~ characters means home directory of the user issuing the command

It is also possible to issue the cd without any parameters. It will move you to your home directory. So these 3 commands are all equal:

cd
cd ~
cd $HOME

pwd

Will show you your current directory:

[jadi@fedora lesson3.1]$ pwd
/home/jadi/lpic1/lesson3.1

uname

Gives you data about the system. Common switches are:

Example:

[jadi@fedora lesson3.1]$ uname -a
Linux fedora 5.14.0-60.fc35.aarch64 #1 SMP Mon Aug 30 16:30:42 UTC 2021 aarch64 aarch64 aarch64 GNU/Linux

Getting Help

Most of the commands we use do have a cool and complete manual, accessible via the man command. It uses the less pager by default and contains the documentation, switches, parameters, ... of commands, and utilities.

Make yourself familiar with the man by reading the manual of the yes command:

$ man yes

Please note that man pages are categorized in different sections (books). You can check these by reading the man's manual:

$ man man
$ man 5 passwd

`

Special characters and Quoting/Escaping

In the computer world, some characters do have special meanings. For example in bash, the * character will expand to all files. In these cases, if you want to use this character without this expansion, you have to Quote it or Escape it. In many cases this is done via adding a \ character before it:

$ echo 2 \* 3 = 6
2 * 3 = 6

These are the character with special meanings that you need to quote if you are using them in your commands:

* ?[]'"\$;&()|^<>

Please note that there is a space character in the character list above.

As you can see, the \ has a specific meaning so if you want to use the back-slash itself (without its escaping usage), you have to quote your back-slash with another back-slash \\.

In addition to escaping, you can use \ to create some special characters. For example, as you can not type a return character, you create it via \n (new line):

jadi@funlife:~$ echo -e "hello\nthere"
hello
there

Some other cases are:

On bash you can use \ to break a command into more lines:

$ echo You know slashes! But this \
is another \
usage
You know slashes! But this is another usage

Shell environment variables

Environment Variables contain some configs and information about the shell. For example, your default editor is set in the EDITOR variable. You can query the value of a shell variable like this:

[jadi@fedora ~]$ echo $EDITOR
/usr/bin/nano

It is possible to check all the env variables using the set or env command.

These are some of the most used bash environment variables:

When trying to access the value, you should add a $ to the beginning of the variable name.

$ echo $USER $UID
jadi 1000
$ echo $SHELL $HOME $PWD
/bin/bash /home/jadi /home/jadi/lpic

To define a new EV (Environment Variable) or change or delete one, we can do:

$ MYMOOD=happy
$ echo I am $MYMOOD
I am happy
$ MYMOOD="Even Happier" # space has a specific meaning
$ unset MM

If you want new programs starting from this shell to have access to the variable you defined, you have to set them with export or export them later.

$ export MYMOOD
$ export YOURMOOD="Not Confused"

Global bash configs are stored at /etc/profile and each user has her config at ~/.profile & ~/.bash\_profile & ~/.bash\_logout. If you need a permanent change, add your configs to these.

Path

When you issue a command, bash will run if it's an internal bash command. Otherwise, bash will go and check the PATH variables one by one and will try to find them there. If not, it will give you an error. If you want to run something on a specific path, you have to exclusively describe the location:

$ echo $PATH
/home/jadi/bin:/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/usr/games:/usr/local/games

But what happens if I try to run tar? Let's check with which, type, and whereis commands:

jadi@funlife:~$ which tar
/bin/tar
jadi@funlife:~$ type tar
tar is /bin/tar
jadi@funlife:~$ whereis tar
tar: /usr/lib/tar /bin/tar /usr/include/tar.h /usr/share/man/man1/tar.1.gz

A cooler example is ping on Fedora:

[jadi@fedora ~]$ whereis ping
ping: /usr/bin/ping /usr/sbin/ping /usr/share/man/man8/ping.8.gz
[jadi@fedora ~]$ which ping
/usr/bin/ping
[jadi@fedora ~]$ /usr/sbin/ping 4.2.2.4
PING 4.2.2.4 (4.2.2.4) 56(84) bytes of data.
64 bytes from 4.2.2.4: icmp_seq=1 ttl=50 time=160 ms

That's why when you want to say "run this_program in this directory" you issue "./this_program". You are exclusively telling bash where the file is. In Linux, the current directory (.) is not part of the PATH by default.

Command history

Bash saves its history in ~/.bash_history. You can cat it and see its contents or run the history command. You can also use the below keys (combinations) to access your previous commands:

If you want to clear your history, issue `HISTSIZE=0

Exiting the Shell

The exit command exits the shell. Same as CTRL+d.

If you run a command inside parentheses that command will be run inside a sub-shell and exec will run a command and closes the current shell.