Remove first word using sed

cat filename | sed -e 's/^ *[^ ]* //'

Rating: 4.5

Output redirection: > /dev/null 2>&1

One would frequently notice this training redirect construct in many Unix commands – all it means is, ALL output from this command should be pushed into /dev/null.

> /dev/null 2>&1

/dev/null is a place where we can dump anything we don’t want to use. Since, 1 is stdout and 2 is stderr: 2>&1 can be interpreted as redirect stderr to stdout.  Thus, Error messages which go to stderr are redirected to stdout which is being redirected to dev/null. Therefore, no output at all.

 

Number of occurrences of a string in a file using vi editor

:%s/string/&/gn

It counts the number of substitutions without actually performing the substitutions.

remove front spaces from a string

a simple sed command to do the same is as:

sed -e 's/^ *//' data

Unix mtime, ctime, atime

ctime: is the inode change time. When does the inode change, when you of course update a file, but also when you do things like changing the permissions of the file but not necessarily its contents. We can view the ctime with the ls -lc command.

atime: stands for access time which is when the file was last read. Displaying the contents of a file or executing a shell script will update a file’s atime. We can view the atime with the ls -lu command.

mtime: is the modification time, so if you change the contents of the file, this time is updated. Keep in mind when you change the contents of a file and and save it, the inode gets updated as well for that file, so ctime always changes too. We can view this with ls -l.

Therefore, when mtime changes, ctime will always change.

some arbit workaround with Unix Shell

>> finding if a directory exists or not:

to check if a directory ‘tempDir’ exists in your home directory, the shell command would be,

$ [ ! -d ~/tempDir ] && echo 'Directory tempDir not found'
OR
$ [ -d ~/tempDir ] && echo 'Directory found' || echo 'Directory tempDir not found'

Similarly, to check if a file exists, we use -f parameter in the above command.

Parameters like -r, -w, -x are also available to check if the file exists and is readable, writable or executable resp.

>> Adding a directory to the path of user or all users:

PATH=$PATH:/path/of/my/scripts

"export PATH" must be added to ensure that any programs called 
by bash have the altered $PATH variable.

- To add a directory to the path of a single user, 
place the lines in that user's .bash_profile file.

- For all users globally, set a path in /etc/profile.

Special parameters in UNIX Shell

$# Number of positional parameters

$? Exit value of last executed command

$$ Process number of current process

$! Process number of background process

$* All arguments on command line

“$@” All arguments on command line ,  individually quoted “$1” “$2” …

$-    Options currently in effect

What exactly does this command does: echo $?

It displays the exit status of previous unix command.

Every Linux command executed by the shell script or user, has an exit status. The exit status is an integer number. 0 exit status  means the command was successful without any errors. A non-zero (1-255 values) exit status means command was failure.