Navigating the Linux Shell

The shell is command-line interface (CLI), a type of user interface that interprets commands typed at a prompt. The default shell on Pegasus is Bash.

Users send commands to the shell, which runs them and outputs results. Commands can include options (or flags) to modify output and arguments to specify command targets. In Bash, command history can be accessed from the prompt with up and down arrow keys - use this to repeat a previously issued command.

Below are some useful Linux shell commands with explanations and examples. Recall that Linux is case-sensitive– “name” is distinct from “NAME” and other capital and lower-case combinations (“Name”, “nAMe”, etc.). To cancel a process and return to the prompt in Linux, press CTRL-C in Windows or Command-C in Mac.

View your current shell with echo:

The echo command displays a line of text. In Linux, $ denotes a variable. The $SHELL environment variable contains your current shell. To view the contents of this variable, send it to the echo command. Variables will be interpreted by the command even when inside lines of text, as shown below.

[username@pegasus ~]$ echo $SHELL
/bin/bash
[username@pegasus ~]$ echo "My shell is $SHELL"
My shell is /bin/bash

View all environment variables with env:

To view all your environment variables, use the env command. To view this list alphanumerically, use env | sort.

[username@pegasus ~]$ env
MODULE_VERSION_STACK=3.2.10
LC_PAPER=en_US.utf8
HOSTNAME=login4
SHELL=/bin/bash
...
[username@pegasus ~]$ env | sort
...

View your current directory with pwd:

Directories in Linux are similar to folders in other operating systems. Your home directory is the default location after login. The shortcut for home in Bash is the tilde (~), shown below just before the prompt ($). pwd outputs the absolute path, the unique location starting from the topmost, or root, directory (/).

[username@pegasus ~]$ pwd
/nethome/username

View the contents of a directory with ls:

Entering the ls command without arguments (as shown below) lists the contents of the current directory. If this is your first connection to Pegasus, your home directory may be empty. Directories can be distinguished from files by the leading d in file permissions.

[username@pegasus ~]$ ls
example_file1  example_file2  testdir1

To view the contents of a specific directory, send the path as an argument to ls. In this example the current directory is home, which contains testdir1. As shown in the output, testdir1 contains one file: testdir1_file1

[username@pegasus ~]$ ls testdir1
testdir1_file1

Note that you can press the TAB key on your keyboard to auto-complete names. If there are multiple matches, a list of options will be shown. Type the next letter and press TAB again until tab-complete finishes.

Command details and flag information can be found in the Linux manual pages, accessible via the command line:

[username@pegasus ~]$ man topic or command

Press SPACE to see the next set of lines. To scroll, use the arrow keys or Page Up and Page Down. To exit, type q.

ls can be run with options, or flags, to customise output. For example, view more detailed information such as file permissions using the -lh flags.

[username@pegasus ~]$ ls -lh
total 0
-rw-r--r-- 1 username ccsuser  54  example_file1
-rw-r--r-- 1 username ccsuser 476  example_file2
drwxr-xr-x 2 username ccsuser 512  testdir1
...

The flags on this ls -lh command:

• -l long list format (includes permissions, owner, and more)
• -h human readable filesize format (useful for larger file sizes)

Other useful ls flags:

• -a include hidden files *
• -d list properties of a directory itself, not the contents
• -1 (number 1) one result per line
• -R recursively list subdirectory contents
• -S sort by file size
• -X sort alphanumerically by extension
• -m comma-separated list

* Hidden files include the . and .. directories, which represent the current and parent (respectively). These can be used as shortcuts in relative paths:

[username@pegasus testdir1]$ ls -a
.  ..  testdir1_file1
[username@pegasus testdir1]$ ls ..
example_file1  example_file2  testdir1

Navigate to directories with cd:

This command changes your current directory to the path specified, which can be absolute, starting with /, or relative, starting from the current directory.

[username@pegasus ~]$ cd testdir1
[username@pegasus testdir1]$

Some useful cd commands:

• cd or cd ~ move to user’s home directory
• cd .. move to parent directory
• cd - move to previous working directory

View directory contents with tree:

Pegasus has the tree package installed, which recursively outputs a depth-indented list of contents. This may be more helpful than ls for nested directories.

[username@pegasus ~]$ tree -vC
.
|-- example_file1
|-- example_file2
|-- testdir1
    `-- testdir1_file1

1 directory, 3 files

The flags on this tree -vC command:

• -v sort alphanumerically by type
• -C colorise output

Other useful tree flags:

• -a include hidden files
• -d list directories only
• -r sort reverse alphanumerically
• -L number descend only number levels deep

Check command availability and location with which:

The which command returns the full path of any shell commands registered in the current environment by searching locations in the $PATH environment variable. Use which to check command and software availability and location.

[username@pegasus ~]$ which bash
/bin/bash
[username@pegasus ~]$ which vim
/usr/bin/vim
[username@pegasus ~]$ which python
/share/opt/python/2.7.3/bin/python