In earlier systems, passwords were stored in the file /etc/passwd and they were not encrypted. After the user is created, an entry gets recorded in /etc/passwd with 'x' in the second column instead of the acutal password. $ useradd timmy $ cat /etc/passwd timmy:x:1002:1003::/home/timmy:/bin/sh For security reasons, passwords are now stored in the file /etc/shadow and they are encrypted. Password was not set initially, when the user was created. This is indicated by !! mark (in RedHat, ! - Debian) $ cat /etc/shadow timmy:!!:15870:0:99999:7::: (more…)

Tar (tape archive) does not compress files. They are used for grouping all files in a folder so that it can be transferred across locations. In order to compress the files, you need to use zip, gzip or bzip2 utilities. gzip, bzip2 are file compression utilities and gunzip, bunzip2 are file uncompression utilities. bzip2 compresses files to a smaller size compared to gzip, but it takes longer to compress the files. Bzip2 uses Burrows-Wheeler block sorting text compression algorithm (with Huffman coding) and gzip uses Lempel-Ziv coding. so, a file compressed with one utility cannot be uncompressed with the other utility. (more…)

Linux variables have the prefix $. Variables that will be useful in shell scripting are listed below. $$ Process ID number of the shell in execution $? Variable indicating Exit status (from the last command that got executed) $* Entire argument string in the command line (excluding script name) $# Number of arguments in the command line (not counting the shell script name) To access arguments passed to a script, following variables are used. $0 Name of the program (with entire path) $1 First argument passed in the command line $2 Second argument passed in the command line $n Nth argument passed in the command line To move or shift to a specified argument in the list of arguments, "shift" command is used. For example, to shift 1 argument just type "shift" or "shift 1" and to shift to the third argument in the list type "shift 2". (more…)

$PATH is the environment variable related to Linux Path settings. To add a directory to the path, type $ export PATH=$PATH:/tmp/myshells The above change is in effect only in the shell from where it is being called To set the PATH for a specific user, type the below 2 lines in the user's .bash_profile file ($HOME/.bash_profile). PATH=$PATH:/tmp/myshells export PATH To set the PATH for ALL users except the root user, type the following line in /etc/profile export PATH=$PATH:/tmp/myshells Root user is not affected by any setting changes under /etc/profile. To set the path for the root user, we need to modify the root users' .bash_profile (/root/.bash_profile)

For example, if we have shell script named "myshellscript.sh" (with execute permission) we can make it run in the current working directory with the commands $ sh myshellscript.sh (or) $ ./myshellscript.sh If the shell scripts are stored in an executable path, it will save us from typing extra keystrokes. To find the paths that are setup, type $ echo $PATH /usr/bin: /usr/local/bin (more…)

Shell scripts are written to execute a set of commands and to group them. The scripts will have the following special line #!/bin/bash This is an indication that the shell script is used in bash shell and that it should be called for command execution. To know the path of the bash shell, type $ cat /etc/shells (more…)