Monday, July 2, 2012

Understanding UNIX Shell Part - 3


We can join commands on a command line in such a way that the second command executes only if the first command has executed successfully.
For eg, we can use a first command to check whether a file exists and a second command to perform an operation on it if it exists.

1.    To make one command conditional on another, we can join the commands using a double ampersand (&&). The command after the && symbol executes only if the command before the && symbols produces a zero exit status – in other words, if its executes successfully.

For eg:
$ ls userlist && sort userlist
In this eg, the ‘ls’ command checks whether the ‘userlist’ file exists. Because it does exist, the ‘ls’ command executes without errors – so its exist state is zero.  This causes the ‘sort’ command to execute.

Also, if we delete the ‘userlist’ file and run the command again, the ‘ls’ command encounters an error – so its exit state is non-zero.  Because the ‘sort’ command is conditional, the shell doesn’t attempt to execute it.

2.    We can use double pipe (||) to make a command conditional on the unsuccessful execution of the previous command.
In such case, the second command executes only if the first command has non-zero exit state.

For eg:
$ ls userlist || touch userlist
In this example, the ‘ls’ command looks for a file called ‘userlist’. If it fails to find the file, the ‘touch’ command creates it.

Also, if the ‘ls’ command executes successfully, this means that the file already exists. In this case, the ‘touch’ command doesn’t execute.

3.    Can group commands using braces ({}). The shell treats any command block enclosed in braces as if it were a single command.
This allows us to redirect input and output to and from a group of commands.

For eg:
$ {sort | grep ‘che’}
In this example, the braces group the ‘sort’ and ‘grep’ commands into a code block so that the shell sorts input and then extracts any lines containing the word ‘che’.

4.    Can redirect input and output to a command block as if it were a single command.
For eg:
$ {sort | grep ‘Mexico’} mex_flights
In this example, the code specifies
 Flight ---> file as input
Mex_flights ---> file as output

5.    Can group commands using round brackets – Often called parentheses – instead of braces.  This causes the shell to spawn a subshell and execute the command block in the subshell.

6.    Commands that execute in a subshell do not affect what’s happening in the main shell.
This allows you to define variables that exist only for the lifetime of the subshell, and to change the working directory within the subshell without affecting the parent shell.

For eg:
$ (sort | grep ‘Mexico’) < massfile > mex_info

Want to create a file named hostname and containing the text However, you don’t want to overwrite any existing file by that name.

Ans: $ cat hostname || echo > hostname

Justification: The use of the || ensures that the code that writes the output from the echo command to the hostname file will only execute if the attempt to list the hostname file fails.