Introduction to Unix CLI

What is Unix CLI?

Unix CLI is actually the shell, a text interface to the operating system. Shell interprets and executes your commands and their arguments by passing them to the operating system.
There are different shell variants, like bash, csh, ksh, sh, tcsh, etc., probably the most popular being bash. The name “bash” is an acronym for “Bourne Again SHell” and is an enhanced replacement for sh. In this short introductory we are going to cover bash.

Bash

Bash is an interactive and scriptable shell. Supports name expansion, wildcards and typing shortcuts.

Note: Interaction with bash is case-sensitive just like everything in Unix system.

Some useful shortcuts:

TAB, UP, DOWN, Ctrl+R, Esc+.

Popular commands

man

man is the system's manual pager. It is an interface to on-line manuals. man has it's own manual pager.

$ # Show default man page for top command
$ man sleep
$ # Show all manual pages for top command, successively
$ man -a sleep

Whenever you are uncertain what a command does, what options and argument can be passed to, man is your friend.

apropos

apropos searches the manual page names and descriptions for the specified keyword.

$ apropos shell
bash                 (1)  - GNU Bourne-Again SHell
bash [sh]            (1)  - GNU Bourne-Again SHell
capsh                (1)  - capability shell wrapper
chroot               (1)  - run command or interactive shell with special root directory
chsh                 (1)  - change your login shell
…

pwd

pwd prints the full name of the current working directory.

$ pwd
/tmp/test/course2015

cd

cd changes the current working directory to the specified directory name. If no directory is specified than cd changes current working directory to users home directory. cd supports relative and absolute paths. Additionally shortcuts like ~ (points to user's home directory).

$ pwd
/tmp/test/course2015
$ cd bin
$ pwd
/tmp/test/course2015/bin
$ cd ../../src
$ pwd
/tmp/test/src
$ cd ~/bin
$ pwd
/home/lorand/bin
$ # Change back to previous directory
$ cd -

mkdir

mkdir command creates directories if they do not exist.

$ mkdir build

To create a whole directory structure, use "-p" option.

$ mkdir -p ~/build/wrf/3.7

ls

ls command lists content of the current or the specified directory. One of the most used options for ls is "-l", which will generate a long list format.

$ ls
GConf                                   libegroupwise-1.2.so.13                     libospf.so.0.0.0
ImageMagick-6.4.3                       libegroupwise-1.2.so.13.0.1                 libossp-uuid.so.16
Mcrt1.o                                 libelf-0.152.so                             libossp-uuid.so.16.0.22
PackageKitDbusTest.py                   libelf.so.0                                 libp11.so.1
…

$ ls -l
total 806992
drwxr-xr-x  3 root root     4096 Mar 11  2012 GConf
drwxr-xr-x  4 root root     4096 Nov 18  2014 ImageMagick-6.4.3
-rw-r--r--  1 root root     1358 Oct  8 20:07 Mcrt1.o
-rw-r--r--  1 root root     3383 May 29  2013 PackageKitDbusTest.py
-rw-r--r--  1 root root     4267 Oct  8 20:07 Scrt1.o
…

"-a" will list all files and folders, even hidden ones.
NB: Files, directories starting with "." character are treated as hidden files.

$ pwd
/work/shared/course2015/intro/bash

$ ls -l
total 0

$ ls -la
total 33
drwxr-xr-x 2 lorand hpcadmin 24576 Oct 27 10:10 .
drwxr-xr-x 3 lorand hpcadmin  4096 Oct 27 10:07 ..
-rw-r--r-- 1 lorand hpcadmin  1177 Oct 27 10:10 .bashrc
-rw-r--r-- 1 lorand hpcadmin   849 Oct 27 10:10 .vimrc

Filename expansion is also supported with the usage of special characters, so called wildcards "?" or "*". "?" replaces one character, while "*" replaces any number of characters.

$ ls -a .*rc
.bashrc  .vimrc

"-t" option sorts listing by modification time, while "-r" can reverse sorting order.

$ ls -lt
total 8
drwxr-xr-x 3 lorand hpcadmin 4096 Oct 27 10:07 intro
drwxr-xr-x 4 floan  hpcadmin 4096 Oct 21 12:10 openMP

$ ls -ltr
total 8
drwxr-xr-x 4 floan  hpcadmin 4096 Oct 21 12:10 openMP
drwxr-xr-x 3 lorand hpcadmin 4096 Oct 27 10:07 intro

cp

cp command copies files and directories. Copies source (might be one or more files/directories) to destination. When multiple files/directories are specified as source, the destination has to be a directory.

$ # copy messages file to /backup folder
$ cp /var/log/messages /backup
$ # copy messages and maillog files to /backup folder
$ cp /var/log/messages /var/log/maillog /backup
$ # copy all C source to network mounted home folder
$ # preserving timestamps and permissions, be verbose
$ cp -a -v ~/development/my_project/*.c /mnt/nfs/projects/sources/
$ # copy all new or modified files to destination
$ cp -u -v ~/development/my_project/*.c /mnt/nfs/projects/sources/
$ # copy all new or modified files to destination
$ # be verbose and interactive - ask what to do
$ cp -u -i -v ~/development/my_project/*.c /mnt/nfs/projects/sources/

mv

mv moves source (might be one or more files/directories) to destination. Same as for cp command, when multiple files/directories are specified as source, the destination has to be a directory. mv command is also used for renaming a file. NB: Irreversible!

$ # interactively move python files to python_files/ subdirectory
$ mv -i *.py python_files/
$ # move python files and force overwrite if similar file exists
$ # be verbose
$ mv -f -v *.py python_files/

rm

rm command removes files and directories. rm will not remove directories by default unless recursive - -r - option is specified. NB: Irreversible! It is advisable to create an alias for rm command to "rm -i" (see alias command).

$ # interactively remove all object files in current directory
$ rm -i ./*.o
$ # force removal of ~/tmp/ directory and all of it's contents
$ rm -rf ~/tmp/

cat

cat command concatenates files and prints on the standard output.

$ # concatenate motd
$ cat /etc/motd
Rocks Compute Node
Rocks 6.1 (Emerald Boa)
Profile built 16:14 10-Oct-2015

less

less allows paging trough text, one screenful at a time. Allows forward and backward movements in the text, line by line, or page by page. Useful commands in less are:

• "/" searches forward
• "?" searches backward

• "-I" switching between case-sensitive/case-insensitive searches
• "q" quits less

tail

tail outputs the last part (by default last 10 lines) of the file.

$ # show last 25 lines from messages file
$ tail -25 /var/log/messages
$ # follow file output, output will continue until it is
$ # broke with Ctrl+C
$ tail -f /var/log/messages

alias

alias command defines or lists aliases. Command itself without any arguments will list existing aliases.

$ lc
-bash: lc: command not found
$ alias lc='ls -l --color'
$ lc
-rwxr-xr-x  1 root   root       5008747 Oct 13 13:35 helloworld_c
-rw-------  1 lorand hpcadmin         0 Oct 13 13:36 helloworld_c.err
-rw-------  1 lorand hpcadmin      2068 Oct 13 13:38 helloworld_c.out

$ alias
alias ..='cd ..'
alias ...='cd ../..'
alias build='cd /work/$USER/build'
alias cd..='cd ..'
alias dir='ls -l'
alias lc='ls -l --color'