Linux Free Course Module 2 Chapter 4: Meta Characters

Group Category

SYMBOL

MEANING

Reads standard input from a file.

Writes standard output to a file.

Appends standard output to a file.

Sends the output of one process to the input of another.

ls  -l

 ls  -l  >  out.txt

What would have been emitted to the shell window will now be present in the output file out.txt

ls  -l  >> out.txt

The output of the above command is appended to the file out.txt

cat  out.txt

cat < out.txt

cat out.txt |  wc

• Descriptors stdout and stderr are the terminal or window attached to the shell (stdout for program results, stderr for program errors,       warnings, and status).

• While stdout and stderr share the same default output device, they can be split as desired by the developer.

prog > out.txt

•  The output of prog would be redirected to the file out.txt. Note that if we wanted to append our

output to out.txt rather than replace the file altogether, we would use the double redirect, as:

prog >> out.txt

prog 2> error-out.txt

•  If instead, we wanted to redirect both the stdout to a file (out.txt) and stderr to a file (err.txt) :

prog 1> out.txt 2>err.txt

Descriptor

Description

• Redirect stdout to a file.
• Redirect stderr to a file.
• Redirect stdout to a stderr.
• Redirect stderr to a stdout.
• Redirect stderr and stdout to a file.
• Redirect stderr and stdout to stdout.
• Redirect stderr and stdout to stderr.

Wildcard

Meaning

ls -FR

a.c         b.c        cc.c           dir1/               dir2/

dir1:

d.c       e.e

dir2:

f.d       g.c

recursively list the current directory.

ls   *.c

a.c       b.c       cc.c

list any text ending in “.c”

ls   ?.c

a.c       b.c

list text for which one character is followed by “.c”

ls [ac]*

a.c         cc.c

list any string beginning with “a” or “c”

ls [A-Za-z]*

a.c         b.c          cc.c

list any string beginning with a letter.

ls   dir*/*.c

dir1/d.c     dir2/g.c

list all files ending in “.c” files in “dir*”

directories ( that is, in any directories beginning with “dir” ).

ls */*.c

dir1/d.c     dir2/g.c

 list all files ending in “.c” in any subdirectory.

ls *2/?.? ?.?

a.c      b.c     dir2/f.d        dir2/g.c

 list all files with extensions in “2*” directories and current directory.

Symbol

Meaning

Usage :    `ls  -l`

                  `ls   -l  | wc`

They are also used as substituting the command.

echo $SHELL

/bin/bash

 VARIABLE –  NO

WILDCARDS – NO

COMMAND SUBSTITUTION – NO

echo  ‘$SHELL’

$SHELL

Symbol

Meaning

echo  “My Shell is \$SHELL” 

My Shell is  $SHELL

 VARIABLE –  YES

WILDCARDS – NO

COMMAND SUBSTITUTION – YES

echo  “$SHELL”

/bin/bash

echo  “  My working directory is `pwd` ”

/home/cse

echo  “\$SHELL”

$SHELL

echo '$USER is $USER'

$USER is $USER

echo 'today is `date`'

today is `date`

echo 'No background&'

No background&

echo 'I said, "radio!"'

I said, "radio"

echo "My name is $USER "

My name is pythonbaba

echo "\$2.00 says `date`"

$2.00 says Sun Jan 8 01:43:32 EST 200

echo “I am using bash \$SHELL "

I am using bash $SHELL

echo “ My “name is $USER and “ I am using $SHELL “

My names is pythonbaba and I am using /bin/bash

• Allows exception of backquote, backslash, dollar sign, and double quote.
• Inhibits the special meaning of all other metacharacters.

• This may be because it is just easier to run a command than to type the output produced from the command.

The original Bourne(can also be used with BASH shell):

`command`

The Bash’s own extension:

$(command)‏

ls   -l    `pwd`

-rwxr-xr-x   1  root   root  139456  Jan 8 20:14 /usr/bin/less

ls    -l    $(pwd)

-rwxr-xr-x   1  root   root  139456  Jan 8 20:14 /usr/bin/less

This facility is useful for type-ahead (and think-ahead) addicts who like to specify an entire sequence of actions at once.

A sequence of commands can be entered on one line. Each command must be separated from its predecessor by semicolon.

There is no direct relationship between the commands.

command1 ; command 2 ; command3

echo $SHELL ;  pwd;   ls

/bin/bash

/home/pythonbaba

a.c    b.c    cc.c       dir1      dir2

echo $SHELL > shell.txt ;  ls ;   pwd  > pwd.txt

a.c        b.c        cc.c         date.txt          dir1          dir2

cat  shell.txt

/bin/bash

cat  pwd.txt

/home/pythonbaba

Commands are grouped by placing them into parentheses, which causes them to be executed by a child shell (subshell)

Note: we will refer to subshell in the coming slides.

We can also redirect the output to a file

( command 1 ; command2; ….. ; ….; command n)  > anyfile

( echo $SHELL ;  ls  ; pwd )

/bin/bash

 a.c        b.c        cc.c         date.txt          dir1          dir2

/home/pythonbaba

( echo $SHELL ;  ls  ; pwd )  > file.txt

cat file.txt

/bin/bash

 a.c        b.c        cc.c         date.txt          dir1          dir2

/home/pythonbaba

     AND (&&)

     OR (||).

• If we AND two commands, the second is executed only if the first is successful.

• If we OR two commands, the second is executed only if the first fails.

cp file1 file2 && echo “Copy successful”

cp file1 file2 || echo “Copy failed” 

• If the cp commands fail then echo command will be executed.

• The background process runs concurrently with the parent shell and does not take control of the keyboard.

• It means you immediately get a new prompt and can continue your work while the command is running.

• The background process runs in a subshell, created by the parent shell in which you typed the command.

• Background processing is therefore very useful for performing several tasks simultaneously, as long as the background tasks do not         require input from the keyboard.

• Since /usr directory contains a lot of files and subdirectories, it will take some time to

copy it to /tmp directory

 cp    /usr    /tmp

cp    /usr    /tmp&

Concatenate & operator at the end of your command.

 cp    /usr    /tmp   >  out.txt&