Building Shell Commands

Building Shell Commands

First, we need wild cards

Many functions in Unix deal with text strings like file names. You’ll need at least two file name wild cards (or shell wild cards) to make life easier.

The first wild card is “ * “, which means “match any character or string of characters.” It’s handy, for instance, for finding a file:

ls -l chap*

will get you a listing of all the files (in your current directory) with names beginning with “chap”.

It will also attempt to return listings from any subdirectory with a name beginning with “chap”.

A one-letter wild card

The second wild card character to know is “?”. It matches only a single letter.

ls chapter?

will return any file names with only a single letter after chapter.

Variables: symbolic names used to represent values stored in memory

There are three types of variables:

Configuration variables

Environment variables

Shell variables

Configuration variables

Store information about the setup of the operating system

Cannot be changed

HOME

LOGNAME

TZ (Time Zone)

 

Environment variables

Determine the characteristics of your session

Can be changed by you or your sys admin

PATH

PS1 (Shell prompt)

PWD (Current Working Directory)

 

Shell Variables

Created at the command line or in a shell script

Useful in shell scripts to store temporary information

dog

memo

my_var

today

Assigning values to variables

~$: NAME=Becky

The string Becky is assigned to the shell variable NAME.

 

To display the value of NAME, it must be preceded with a dollar sign ($), for example

~$: echo NAME

displays NAME.

 

~$: echo $NAME

Displays Becky.

Assigning commands to variables

The backquote (`) operator can be used to store the output of a command in a variable:

~$: TODAY=`date`

~$: echo $TODAY

Displays today’s date.

 

~$: TODAY=date

~$: echo $TODAY

Displays the word “date.”

The pipe ( | )

Recall the pipe special character, and its use in feeding the stdout of one command into the stdin of another command:

ls –la /bin/ | less

Any command that can accept Standard Input and produce Standard Output is called a filter command.

The redirection character ( > )

Also recall the redirection character, which sends the output of a command to a text file:

(users) > /var/users.log

The append character ( >> )

If you want to append your command output to a file, rather than “clobbering” it, use the append character:

(users)>>/var/users.log

Sequential commands

Sometimes you need more than one command to do the job. Use the semicolon:

(date;users)>>/var/users.log

Running a command in the background ( & )

To execute a command in the background, follow the command with an &. When you run a command that you know is going to take a while to complete, running it in the background lets you continue working in the mean time. On many Linux distros, backgrounding a command gives it a nice value of 4 (see the nice section). Compare:

ls –laR /

with

ls –laR / >/files.txt &

The latter executes ls in the background and immediately gives you a new prompt to continue working.

Use the command:

jobs

to view background job IDs.

Add a ” % ” to kill a background job:

kill %1

Move a backgrounded job to the foreground:

fg %1

Move a foreground job to the background:

Ctrl-z #to assign a background ID to the job
bg %1 #to background that job; Use the ID supplied by the previous command

A word about arguments

In Unix, an argument isn’t something you get into with your sibling.

An argument is a parameter, like specifying which directory to list when you give an ls command:

ls –la /home/glenn

In this case, /home/glenn is the argument.

Expanding command output as an argument

Consider this chain of actions.

I want to “touch” every file in my home directory, so I could issue the commands:

touch /home/glenn/* ; touch home/glenn/ *.*; touch /home/glenn/.*

This will refresh the last-modified date of every file in my home. (Now, why would I want to do this?)

 

But, I decide I don’t want to touch all my files; only the .txt files. So instead I command:

find /home/glenn | grep \.txt

to find my .txt files.

 

Now I have the list of files I want, I can use that list (which is really the output of my last command) as the input for the touch command:

touch $(find /home/glenn | grep \.txt)

 

In this case, I’m indicating that I want to run the find command first by bracketing it with:

$( my_command )

But I could also use the format:

$‘my_command’

 

Expanding arithmetic

You can also bracket and expand math expressions, using slightly different notation:

echo “2 plus 2 equals $[2+2].”

Finding Files

locate

To use the locate command, you will need to have an slocate database set up on your system. On many systems it is updated periodically by the cron daemon. Try the locate command to see if it will work on your system:

locate hello

This should list every filename on your system that contains the string “hello.” If this doesn’t work, you can use the commands:

slocate -u

or

updatedb

to build the slocate database. This may take a few minutes. On most systems with slocate installed, slocate is run periodically by the cron facility.

find

The find command lets you search for files by criteria, for instance by user, size or date. The basic syntax is:

find directory_to_start_from [options_if_any] string_to_find

By default, the string_to_find is the file name, or some part of the file name. But you can search by criteria too, such as the owner of the file:

find / –user studenth

In a similar way, the -group option restricts searches to files owned by a group. Or search by the size of the file:

find / -size 100k

Another truly helpful option, if you have remote mounts on the workstation, is -mount:

find / -mount file_name

This restricts find to local disks only.

Note that a bare “find” command lists every file on the workstation.

whereis

This command only works for executables, and it will find their man pages and (particularly useful) their configuration files as well. For instance:

whereis bash

will return the full path to bash, bashrc and the man page bash.1.gz. This is handy for finding the true paths to programs that aren’t in your PATH, for instance.

which

which finds an executable file. Unlike whereis, it does not find man pages as well.

which bash

This command will show the full path to the bash executable.

Getting On-Line Help

Get a manual page for a command:

man command

man tracert

 

What? There’s no output? You need to build the man page database:

makewhatis

 

Search by keyword:

man –k keyword

man –k columns

apropos columns
(apropos gives the exact same output as man -k.)

 

Info pages:

info command

info coreutils utility_name

where utility_name is a utility like ls.

 

And finally, yes, there is a help command, not available for all commands but sometimes useful:

help echo

About those numbers following the command in man page output:

For instance WHOAMI(1).

Resources

LinuxCommand.org: http://linuxcommand.org/

A good intro to commands and scripting.

SS64: Unix, Oracle, Windows and Mac commands: http://ss64.com/

A great site devoted solely to the ins and outs of the command line in various environments.