Linux Linux Tips 2 - ctrl a, crl r, tail -f, umask, and eval

When we find a typo error in our command, and we want to go back the the beginning of the command, we should use ctrl-a instead of scrolling via left arrow. To go to the end of a command, we should use ctrl e:

To display the first 10 lines:

$ head foo.txt

The last 10 lines:

$ tail foo.txt

Use -n option to specify the number of lines to display:

$ tail -n5 foo.txt  # the last 5 lines
$ head -n5 foo.txt  # the first 5 lines 

'+' in tail and '-' in head:

$ tail -n+10 foo.txt  # start from the 10th line to the last
$ head -n-10 foo.txt  # print all, except the last 10 lines 

Keep watching tail:

$ tail -f foo.log

Instead of using up or down arrow to view the command issued, we can use ctrl r + command key stroke to retrieve the previous commands. For example, suppose the following is the list of the recent commands:

$ command 1
$ command 2
$ command 3
$ command 4
$ command 5

If we hit cntr r, and type in "c", we get:

(reverse-i-search)`c': command 5

If that's what we want, just hit 'Return'. Then command 5" will be the current command:

If now, we can search in reverse by keep typing in cntr r.

Default permissions are assigned by the system whenever we create a new file or directory, and these are governed by the umask setting. The 'mask' serves to remove permissions as opposed to granting them.

That is, the digits in the umask number are 'subtracted' from 777 for directories or 666 for files when we are creating their initial permissions. Why 666 is the maximum value for a file? This is because only scripts and binaries should have execute permissions, normal and regular files should have just read and write permissions. Directories require execute permissions for viewing the contents in it, so they can have 777 as permissions.

Suppose we type:

% umask 022

Then when we create new files their default permissions will be 644 (666 minus 022, i.e. -rw-r--r--). When we create new directories their default permissions will be 755 (drwxr-xr-x). If the umask value were instead set to 077, our default permissions would be 600 (-rw-------) and our default directory permissions would be 700 (drwx------).

We can check what umask we're using:

$ umask
0002

Note: If USERGROUPS_ENAB is set to "yes", that will modify this UMASK default value for private user groups, i. e. the uid is the same as gid, and username is the same as the primary group name: for these, the user permissions will be used as group permissions, e. g. 022 will become 002 - /etc/login.defs

The '>' is used for redirection from the left to right side of '>'. The format looks like this:

input > output

Here is an example:

$ echo "pushing text" > myfile.txt
$ cat myfile.txt
pushing text

File descriptors are the places a program sends its output to and gets its input from. They are shared with user space, and are used directly by user programs to access files. When we execute a program, Linux opens three file descriptors for the program:

0 standard input (stdin)
1 standard output (stdout)
2 standard error (stderr)

The redirect output symbol ( >) is shorthand for 1>, which tells the shell to redirect standard output. Similarly, < is short for 0<, which redirects standard input. The symbol 2> redirects standard error.

So, the previous example can be like this:

$ echo "pushing text" 1> myfile.txt
$ cat myfile.txt
pushing text

When we redirect output using '>', since the '1' in '1>' is a default output file descriptor, we can drop it.

How about the error redirection?

$ cat nonExistingFile > error.out
cat: nonExistingFile: No such file or directory
$ cat error.out
$ 

As we can see, nothing's been written to the file 'error.out'. Though the message printed out looks similar to stdout, it's different kind. It's an error. So, if we want to save the stderr message, we should use file descriptor for error which is '2':

$ cat nonExistingFile 2> error.out
$ cat error.out
cat: nonExistingFile: No such file or directory

Now the error message has been written to 'error.out' file.

How about this one?

$ telnet dummy.com
Trying 5.22.149.135...
telnet: Unable to connect to remote host: Connection refused

$ telnet dummy.com 2> error.out
Trying 69.43.160.154...
$ cat error.out
telnet: Unable to connect to remote host: Connection refused

We captured the stderr successfully. What if we want to get both? In other words, stderr and stdout together. We use &>:

$ telnet dummy.com &> error.out
$ cat error.out
telnet: Unable to connect to remote host: Connection refused
Trying 69.43.160.154...

There is another way to redirect both the stdout and stderr though this is an old way of doing that. We use '2>&1'

$ telnet dummy.com > error.out 2>&1
$ cat errot.out
telnet: Unable to connect to remote host: Connection refused
Trying 69.43.160.154...

We hate errors, so let's try throw them away:

$ telnet dummy.com > error.out 2> /dev/null

The '<' is used for std input redirection. In the following example, the 'grep' is used to find a string from the given input:

$ grep a < input.txt
abc
abba

where the 'input.txt' looks like this:

$ cat input.txt
abc
def
abba
1234

eval command is used to execute arguments like a shell command. Arguments are joined in a string and taken as input for the shell command to execute the command.

$ d = "date"
-bash: d: command not found
$ d="date"
$ echo $d
d
$ eval $d
Fri Aug 14 16:03:08 PDT 2020

$ d2="date +%Y-%h-%d"
$ echo $d2
date +%Y-%h-%d
$ eval $d2
2020-Aug-14

$ minikube docker-env
export DOCKER_TLS_VERIFY="1"
export DOCKER_HOST="tcp://192.168.64.7:2376"
export DOCKER_CERT_PATH="/Users/kihyuckhong/.minikube/certs"
export MINIKUBE_ACTIVE_DOCKERD="minikube"

# To point your shell to minikube's docker-daemon, run:
# eval $(minikube -p minikube docker-env)

$ eval $(minikube docker-env)
$ echo $DOCKER_HOST
tcp://192.168.64.7:2376