Bash: String Manipulations

Bash supports a surprising number of string manipulation operations. If you use bash in your daily work, I’m pretty sure you will find these operations many useful and handy.

1. Counting Arguments: ${#}
You need to know with how many parameters the script was invoked. Use the shell built-in variable ${#}.

2. String Length: ${#parameter}
The length in characters of the value of parameter is substituted. If parameter is * or @, the value substituted is the number of positional parameters.

3.1 Shortest Substring Match: ${parameter#word}
Deletes the shortest match of word from front of parameter

3.2 Shortest Substring Match: ${parameter%word}
Deletes the shortest match of word from back of parameter

4.1 Longest Substring Match: ${parameter##word}
Deletes the longest match of word from front of parameter

4.2 Longest Substring Match: ${parameter%%word}
Deletes the Longest match of word from back of parameter

5.1 Substring Replacement: ${parameter/pattern/word}
Replace first match of pattern with string.

5.2 Substring Replacement: ${parameter//pattern/word}
Replace All matches of pattern with word.

6.1 Replace beginning and end: ${parameter/#pattern/word}
If pattern matches front end of parameter, substitute word for pattern.

6.2 Replace end and beginning: ${parameter/%pattern/word}
If pattern matches end end of parameter, substitute word for pattern.

You can find more Information and Examples regarding mentioned operators in following links:
http://tldp.org
http://www.linuxtopia.org
http://linuxgazette.net
http://www.thegeekstuff.com

Bash: Saving or Grouping Output from Several Commands

You want to capture the output with a redirect, but you’re typing several commands
on one line.
$ pwd; ls; cd ../elsewhere; pwd; ls > /tmp/all.out
The final redirect applies only to the last command, the last ls on that line. All the
other output appears on the screen (i.e., does not get redirected).

Solution:

Use braces { } to group these commands together, then redirection applies to the
output from all commands in the group. For example:

$ { pwd; ls; cd ../elsewhere; pwd; ls; } > /tmp/all.out

There are two very subtle catches here. The braces are actually
reserved words, so they must be surrounded by white space. Also, the
trailing semicolon is required before the closing space.
Alternately, you could use parentheses ( ) to tell bash to run the commands in a subshell,
then redirect the output of the entire subshell’s execution. For example:

$ (pwd; ls; cd ../elsewhere; pwd; ls) > /tmp/all.out

lsof: A Unix Utility You Should Know About

LSOF lists information about files opened by processes. An open file may be a regular file, a directory, a NFS file, a block special file, a character special file, a shared library, a regular pipe, a named pipe, a symbolic link, a socket stream, an Internet socket, a UNIX domain socket, and many others. Since almost everything in Unix is a file, you can imagine how incredibly useful lsof is!

lsof in action:

– Find who’s using a file:
lsof /path/to/file

– List of Open files Per Process:
lsof -p

– List of open Files Per User:
lsof -u

– List of Open File Descriptors:
lsof -d

– List of Open Internet protocols & ports:
lsof -i

– Directory Search :
lsof +D

– Find all open files by program’s name:
lsof -c

Examples:
# lsof -u admin,nasser
This will list all the files that are open by users admin and nasser.

# lsof -c httpd
It the list open files for processes whose name begins with httpd.

# lsof -a -u nasser -c tcsh
-a means AND
The output will be list of files opened by tcsh, which is run under nasser user privilege.

# lsof -u ^root
The ^ character before root username will negates the match and causes lsof print all open files by all users who are not root.

# lsof -p ^1010
List all open files by all the processes EXCEPT process with PID 1010.

# lsof -i tcp
List all TCP network connections.

# lsof -i udp
List all UDP network connections.

# lsof -i :22
The :22 option to -i makes lsof find processes using TCP or UDP port 25.

# lsof -i tcp:80
Finds who’s using a TCP port 80.

# lsof -a -u nasser -i
Will Find all network activity by user nasser.

# lsof -U
List all Unix domain socket files.

# lsof -g 1234
List all files for processes with a specific group id.

# lsof -r 5 -i tcp:22
The -r option makes lsof repeatedly list files until interrupted.

Have Fun.