Linux commands with output

BASH command output to the variable

Different types of bash commands need to be run from the terminal based on the user’s requirements. When the user runs any command from the terminal then it shows the output if no error exists otherwise it shows the error message. Sometimes, the output of the command needs to be stored in a variable for future use. Shell command substitution feature of bash can be used for this purpose. How you can store different types of shell commands into the variable using this feature is shown in this tutorial.

Command Substitution Syntax:

***Note: Don’t use any space before and after the equal sign when using the above commands.

Single command output to a variable

Bash commands can be used without any option and argument for those commands where these parts are optional. The following two examples show the uses of simple command substitution.

Example#1:

bash `date` command is used to show the current date and time. The following script will store the output of `date` command into $current_date variable by using command substitution.

Output:

Example#2:

`pwd` command shows the path of the current working directory. The following script stores the output of `pwd` command into the variable, $current_dir and the value of this variable is printed by using `echo` command.

Output:

Command with option and argument

The option and argument are mandatory for some bash commands. The following examples show how you can store the output of the command with option and argument into a variable.

Example#3:

Bash `wc` command is used to count the total number of lines, words, and characters of any file. This command uses -c, -w and -l as option and filename as the argument to generate the output. Create a text file named fruits.txt with the following data to test the next script.
fruits.txt

Run the following commands to count and store the total number of words in the fruits.txt file into a variable, $count_words and print the value by using `echo` command.

Output:

Example#4:

`cut` is another bash command that uses option and argument to generate the output. Create a text file named weekday.txt with seven-weekday names to run the next script.

weekday.txt

Create a bash file named cmdsub1.sh with the following script. In this script, while loop is used to read the content of weekday.txt file line by line and read the first three characters of each line by using `cut` command. After cutting, the string value is stored in the variable $day. Next, If the statement is used to check the value of $day is ‘Sun’ or not. The output will print ‘Sunday is the holiday‘ when if the condition is true otherwise it will print the value of $day.

cmdsub1.sh

Output:

Using command substitution in loop

You can store the output of command substitution into any loop variable which is shown in the next example.

Example#5:

Create a file named cmdsub2.sh with the following code. Here, `ls -d */` command is used to retrieve all directory list from the current directory. For loop is used here to read each directory from the output and store it in the variable $dirname which is printed later.

cmdsub2.sh

Output:

Using nested commands

How you can use multiple commands using pipe(|) is shown in the previous example. But you can use nested commands in command substitution where the output of the first command depends on the output of the second command and it works opposite of the pipe(|) command.

Nested command syntax:

Example#6:

Two commands, `echo` and `who` are used in this example as the nested command. Here, `who` command will execute first that print the user’s information of the currently logged in user. The output of the `who` command will execute by `echo` command and the output of `echo` will store into the variable $var. Here, the output of `echo` command depends on the output of `who` command.

Output:

Using Command path

If you know the path of the command then you can run the command by specifying the command path when using command substitution. The following example shows the use of command path.

Example#7:

`whoami` command shows the username of the currently logged in user. By default, this command is stored in /usr/bin/ folder. Run the following script to run `whoami` command using path and store in the variable, $output, and print the value of $output.

Output:

Using Command Line argument

You can use the command line argument with the command as the argument in the command substitution.

Example#8:

Create a bash file named cmdsub3.sh with the following script. `basename` command is used here to retrieve the filename from the 2 nd command line argument and stored in the variable, $filename. We know the 1 st command line argument is the name of the executing script which is denoted by $0.

Run the script with the following argument value.

Here, the basename of the path, Desktop/temp/hello.txt is ‘hello.txt’. So, the value of the $filename will be hello.txt.

Output:

Conclusion:

Various uses of command substitutions are shown in this tutorial. If you need to work with multiple commands or depended commands and store the result temporary to do some other tasks later then you can use this feature in your script to get the output.

More info in the video:

About the author

Fahmida Yesmin

I am a trainer of web programming courses. I like to write article or tutorial on various IT topics. I have a YouTube channel where many types of tutorials based on Ubuntu, Windows, Word, Excel, WordPress, Magento, Laravel etc. are published: Tutorials4u Help.

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Linux Shell Commands

The Shell is the command interpreter on Linux systems. This document intoduces some of the basic features of the Shell and lists many of the commands or programs available on the Linux computers in Cardiff School of Computer Science & Informatics.

The Shell

The Linux command interpreter or shell is the program users interact with in a terminal emulation window. The terminal emulation window can be one in the workstation’s Graphical User Interface mate-terminal on Linux. Alternatively, it can be an application such as SSH secure shell client or PuTTY on a Windows PC that’s logged into Linux over the network.

The shell used in the School of Computer Science & Informatics is bash Bourne Again Shell. There are other shells available such as the Bourne Shell, the C-Shell and the TC-Shell, and you can choose to use a different shell if you prefer. They all have similar characteristics but each has its own particular features. This document assumes you are using bash .

Bash has the following features:

A command prompt which may be configured by the user. The default prompt is a dollar symbol preceded by «bash» and the bash program’s version number.

/bin containing the following.

/bin/countc
bash-2.05$ countc
45

516 ls -lrt
517 ps -ef
518 pdflatex myfile.tex

See the manual page on bash for more details (type man bash ).

Bash has an additional mechanism which allows you to recall and edit previous commands using the keyboard up-arrow key. If you press up-arrow, the last command re-appears on the terminal. Press up-arrow again to get earlier commands. To rerun the command, press RETURN . To amend the command before rerunning it, use the delete key to remove characters from the end or use the back-arrow key to reposition the cursor to delete or insert characters within the command.

Shell Commands

Here is a summary of some of the commands available. For more details refer to the manual page of each command. You can see these on-line by using the man command. Just type man followed by the name of the command you want to see.

Logging out

Command	Description
logout	log out of a Linux terminal

Note, on a Linux workstation you will need to exit the Desktop Environment instead.

Files and Directories

These commands allow you to create directories and handle files.

Command	Description
cat	concatenate and print data
lpr	spool file for line printing
cd	change current directory
lprm, cancel	remove jobs from line printer queue
chgrp	change file group
ls	list and generate statistics for files
chmod	change file mode
mkdir	make a new directory
cp	copy file data
more, page	display file data at your terminal

Command	Description
file	determine file type
mv	move or rename files
find	find files
pwd	print working directory
grep	search file for regular expression
rm, rmdir	remove (unlink) files or directories
head	give first few lines
tail	print last lines from file
just	text justification program
touch	update access and modification times of a file
lpq	spool queue examination program

File Editors

Editors are used to create and amend files.

Command	Description
emacs	GNU project Emacs
xemacs	emacs with mouse action
ex, edit	line editor

Command	Description
pico	easy text editor for vdus
pluma	Mate GUI text editor
gedit	GNOME text editor
vi, vim	standard text editor

Vi , pico and emacs are screen-based editors which run on a vdu or in a workstations terminal emulation window; pluma , gedit and xemacs are graphical user interface (GUI) based editors with cut and paste and mouse-controlled cursor positioning.

Manipulating data

The contents of files can be compared and altered with the following commands.

Command	Description
awk	pattern scanning and processing language
perl	data manipulation language
cmp	compare the contents of two files
paste	merge file data
comm	compare sorted data
sed	stream text editor
cut	cut out selected fields of each line of a file
sort	sort file data
diff	differential file comparator

Command	Description
split	split file into smaller files
expand, unexpand	expand tabs to spaces, and vice versa
tr	translate characters
gawk	pattern scanning and processing language
uniq	report repeated lines in a file
join	join files on some common field
look	find lines in sorted data
wc	count words, lines, and characters

Compressed files

Files may be compressed to save space. Compressed files can be created and examined.

Command	Description
gzip	compress files
zmore	file perusal filter for crt viewing of compressed text
uncompress	uncompress files

Command	Description
zcat	cat a compressed file
gunzip	uncompress gzipped files
zcmp, zdiff	compare compressed files

Information

Manuals and documentation are available on-line. Go to our web site www.cs.cf.ac.uk/systems for web-based documentation. The following Shell commands give information.

Command	Description
apropos	locate commands by keyword lookup
man	displays manual pages online

Command	Description
info	displays command information pages online
yelp	GNOME help viewer

Status

These commands list or alter information about the system.

Command	Description
ps	print process status statistics
date	print the date
quota -v	display disk usage and limits
reset	reset terminal mode
du	print amount of disk usage
script	keep script of terminal session
stty	set terminal options
groups	show group memberships
time	time a command
homequota	show quota and file usage
iostat	report I/O statistics
tty	print current terminal name

Command	Description
kill	send a signal to a process
uptime	display system status
last	show last logins of users
users	print names of logged in users
lun	list user names or login ID
vmstat	report virtual memory statistics
netstat	show network status
w	show what logged in users are doing
who	list logged in users
printenv	display value of a shell variable

Printing

Files can be printed using shell commands, using the GUI print manager, or direct from some applications.

You must specify a printer by name. Printers are called

Printer Name	Location
tl1_lw	Teaching Lab 1 (C/2.04) laser printer
tl3_lw	Teaching Lab 3 (C/2.08) laser printer

Printer Name	Location
tl2_lw	Teaching Lab 2 (C/2.05) laser printer
tl4_lw	Teaching Lab 4 (C/2.10) laser printer

Most commands which can be used to print files, expect the printer name to be given following a -P argument.

Files may be sent to the printers as simple text files or they may be processed in various ways for the laser printers.

Command	Description
lpr -P printer	send a file to a printer
dvips -P printer	postprocess TeX file into Postscript and print on laser printer
a2ps -P printer	format text file in PostScript and print on laser printer

Messages between Users

The Linux systems support on-screen messages to other users and world-wide electronic mail.

Command	Description
write	send a message to another local user
wall	send a message to all local users

Command	Description
pine	vdu-based mail utility
mail	simple send or read mail program
thunderbird	GUI mail handling tool on Linux

Networking

The School of Computer Science & Informatics is connected to the JANET Internet Protocol Service (JIPS), the UK Universities’ network.

These commands are used to send and receive files from Campus Linux hosts and from other hosts on JIPS and the Internet, that permit such connections, around the world.

Command	Description
ftp	file transfer program
tftp	trivial file transfer program
sftp	secure shell file transfer program
rcp	remote file copy
scp	secure shell remote file copy
wget	non-interactive network downloader

Command	Description
telnet	make terminal connection to another host
ssh	secure shell terminal or command connection
rlogin	remote login to a Linux host
rsh	remote shell
curl	transfer data from a url
firefox	web browser
google-chrome	web browser

These commands work only where the remote host permits such connections.

Programming

The following programming tools and languages are available.

General

Command	Description
make	maintain groups of programs
size	print program’s sizes

Command	Description
nm	print program’s name list
strip	remove symbol table and relocation bits

Command	Description
cb	C program beautifier
gcc	GNU ANSI C Compiler

Command	Description
ctrace	C program debugger
indent	indent and format C program source
cxref	generate C program cross reference

Command	Description
g++	GNU C++ Compiler

Command	Description
appletviewer	JAVA applet viewer
javac	JAVA compiler
eclipse	Java integrated development environment on Linux

FORTRAN

Command	Description
f95	GNU Fortran 95 compiler

Other Languages

(Not available on all systems).

Command	Description
bc	interactive arithmetic language processor
matlab	maths package
gcl	GNU Common Lisp
perl	general purpose language

Command	Description
python	object-oriented programming language
squeak	smalltalk
php	web page embedded language
mathematica	symbolic maths package
asp	web page embedded language

Text Processing

TeX is a typesetting language used extensively in Linux and other operating systems for producing high-quality printed documents. Another set of programs based on Troff is the standard Linux text formatting family used, for example to format manual pages.

General Commands

Command	Description
fmt	simple text formatter
evince	GNOME PostScript previewer

Command	Description
acroread	PDF viewer
spell	check text for spelling error
aspell	interactive spelling checker

Troff

Command	Description
eqn	mathematical preprocessor for troff
tbl	prepare tables for nroff or troff
grap	pic preprocessor for drawing graphs
troff	text formatting and typesetting language

Command	Description
nroff	text formatting language
groff	GNU troff interface for laserprinting
pic	troff preprocessor for drawing pictures

Command	Description
tex	text formatting and typesetting
latex	latex formatter

Command	Description
pdflatex	latex formatter with PDF output
xdvi	dvi previewer
dvips	convert a DVI file to POSTSCRIPT

Word Processing

LibreOffice is available on the School’s Linux systems and attempts compatibilty with Microsoft Office.

Command	Description
libreoffice	start LibreOffice applications

Database Management

MySQL and Oracle are available.

Command	Description
sqlplus	run the Oracle SQL interpreter
mysql	run the mysql SQL interpreter
sqldeveloper	Oracle SQL Developer GUI interface
mysql-workbench	GUI interface for MySQL

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