• Unit 7.1 Shell basics
• Unit 7.2 Unix utilities
• Unit 7.3 Shell scripting

Unit 7.1 Shell Basics

After completing this unit, you should be able to:

• Explain the function of the shell
• Discuss metacharacters and reserved words
• Use wildcards to access files with similar names
• Use redirection and pipes
• Use command substitution
• Describe and use the most common filters
• Group commands to control their execution
• Work with shell variables
• Use aliases
• Apply quoting

• When the user types a command, various things are done by the shell before the command is actually executed.

  • Wildcard expansion　
  • Input/output redirection
  • Command grouping
  • Line continuation
  • Shell variable expansion
  • Alias expansion　
  • Shell scripting

- For example, the ls *.doc command could be expanded to /bin/ls --color=tty mydoc.doc user.doc before execution (depending on settings and files present).

• Metacharacters are characters that the shell interprets as having a special meaning.

  • Examples: < > | ; ! ? * $ \ ` ' " ~ [ ] ( ) { }

• Reserved words are words that the shell interprets as special commands.

  • Examples: case, do, done, elif, else, esac, for, fi, function, if, in, select, then, until, while

• When the shell encounters a word that contains a wildcard, it tries to expand this to all matching file names in the given directory. Hidden files are ignored.

  Example:
  . .. .et .w few myfile ne nest net new test1 test1.2 test1.3

  ? matches a single character
  $ ls ne? ==> net new
$ ls ?e? ==> few net new

  * matches any string, including the null string
  $ ls n* ==> ne nest net new
$ ls *w ==> few new

• The wildcards [, ], -, and ! match a sigle character in inclusive lists.

  Examples:

  . .. .et .w few myfile ne nest net new test1 test1.2 test1.3

  $ ls ne[stw] ==> net new

  $ ls *[1-5] ==> test1 test1.2 test1.3

  $ ls [!tn]* ==> few myfile

  $ ls ?[!y]*[2-5] ==> test1.2 test1.3

• Answer the following question :

  0 1 2 9 10 11 99 100 101

  $ ls [100]* ===> ???

• Every program has a number of file descriptors associated with it.
• Three descriptors are assigned by the shell when the program starts (STDIN, STDOUT, and STDERR).
• Other descriptors are assigned by the program when it opens files.

• Default standard output: /dev/tty
  $ ls ==> display file1 file2 file3 on the screen

• Redirect output to a file (override if file exist) :
  $ ls > /tmp/ls.out ==> store file1 file2 file3 in ls.out

• Redirect and append output to a file:
  $ ls >> /tmp/ls.out

• write a message to all logged in users.

  $ wall
  Let's go to lunch!
  <ctrl-d>

• now store the message in a file , then

  $ wall < message

• send a email

  $ mail -s subject boss < mail_body.txt

Create a file with redirection:

$ cat > newfile  
save me in the file...  
<ctrl-d>

STDIN redirected from file:

$ cat < cities  
Atlanta   
Chicago

Default standard input:

$ cat
Atlanta
Atlanta
Chicago
Chicago
<ctrl-d>

• Default standard error: /dev/tty
  $ cat fileA
  cat: fileA: No such file or directory
• Redirect error output to a file:
  $ cat fileA 2> error.file
  $ cat error.file
  cat: fileA: No such file or directory
• Redirect and append errors to a file:
  $ cat fileA 2>> error.file
• Discard error output:
  $ cat fileA 2> /dev/null

• Combined redirects
  $ cat < cities > cities.copy 2> error.file
  $ cat >> cities.copy 2>> error.file < morecities
• Association
  • This redirects STDERR to where STDOUT is redirected.
    $ cat cities > cities.copy 2>&1
    ...writes both stderr and stdout to cities.copy
  • be careful about this:
    $ cat cities 2>&1 > cities.copy
    ...writes stderr to /dev/tty and stdout to cities.copy

• A sequence of two or more commands separated by a vertical bar (|) is called a pipe or pipeline.

  $ ls -l | wc -l
  $ ps aux | wc -l
  $ who | wc -l

• The standard output of command1 becomes the standard input of command2.

• A filter is a command that reads from standard in, transforms the input in some way, and writes to standard out. They can, therefore, be used at intermediate points in a pipeline.

  $ ls | grep doc | wc -l
  $ echo hello | tr 'a-z' 'A-Z'
  $ ps aux | tee out | wc -l

• Command substitution allows you to use the output of a command as arguments for another command.

• Use backticks -- ` -- or $() notation.

  $ rm -i `ls *.doc | grep tmp`

  $ echo There are $(ps ax | wc -l) processes running.

Multiple commands can be entered on the same line, separated by a semicolon (;).

$ date ; pwd

Commands can be grouped into one input/output stream by putting curly braces ({ }) around them.

$  { echo Print date: ; date ; cat cities; }  | lpr

Commands can be executed in a subshell by putting round braces () around them.

$ ( echo Print date: ; date ; cat cities ) | lpr

Variables are part of the shell you are running. A variable has a unique name. To assign a value to a variable use variable=value .

The first character must not be a digit. No blank spaces are allowed around the "=" sign.

Examples:

$ var1="Hello class"
$ var2=2

To reference the value of a variable, use $variable.

$ echo $var1
Hello class
$ echo $var2
2

The export command is used to pass variables from a parent to a child process by putting it in the environment of the child process.

Changes made to variables in a child process do not affect the variables in its parent.

$ export x=123
$ y=456
$ bash
$ echo $x $y
123
$ x=hello
$ y=nihao
$ echo $x $y
hello nihao
$ exit
$ echo $x $y
123 456

The shell uses several shell variables internally.
These variables are always written in uppercase.
Examples include the following:

$: PID of current shell
PATH: Path which is searched for executables
PS1: Primary shell prompt
PS2: Secondary shell prompt
PWD: Current working directory
HOME: Home directory of user
LANG: Language of user

Overwriting these and other system variables by accident can cause unexpected results. Use lowercase variables in your shell scripts to avoid conflicts.

A command returns a value to the parent process. By convention, zero means success and a non-zero value means an error occurred.
A pipeline returns a single value to its parent. The environment variable question mark (?) contains the return code of the previous command.

$ whoami
tux1
$ echo $?
0
$ cat fileA
cat: fileA: No such file or directory
$ echo $?
1

When you want a metacharacter not to be interpreted by the shell, you need to quote it. Quoting a single character is done with the backslash (\).

$ echo The amount is US\$5
The amount is US$5

Quoting a string is done with single (') or double (") quotes. Double quotes allow interpretation of the dollar sign ($), backtick (`), and backslash (\).

$ amount=5
$ echo 'The amount is $amount'
The amount is $amount
$ echo "The amount is $amount"
The amount is 5

The backslash can also be used to give a special meaning to a non-metacharacter (typically used in regular expressions).

\n: New line
\t: Tab
\b: Bell

A backslash followed directly by Enter is used for line continuation. The continued line is identified with the $PS2 prompt (default: >).

$ cat/home/tux1/mydir/myprogs/data/information/letter\
> /pictures/logo.jpg

The alias command allows you to set up aliases for often-used commands. For example:

$ alias ll='ls -l'
$ alias rm='rm -i'
To show all currently defined aliases:
$ alias
To delete an alias:
$ unalias ll
$ ll
bash: ll: command not found

• The shell is the command interpreter.
• A shell has a number of additional features, such as wildcard expansion, alias expansion, redirection, command grouping, and variable expansion.
• Metacharacters are a number of characters that have a special meaning to the shell. Reserved words are words that have a special meaning to the shell.

• Unit 8: Shell basics, Linux Basics and Installation , ERC 7.2, IBM