15 Examples To Master Linux Command Line History

When you are using Linux command line frequently, using the history effectively can be a major productivity boost. In fact, once you have mastered the 15 examples that I’ve provided here, you’ll find using command line more enjoyable and fun.

1. Display timestamp using HISTTIMEFORMAT

Typically when you type history from command line, it displays the command# and the command. For auditing purpose, it may be beneficial to display the timepstamp along with the command as shown below.

# export HISTTIMEFORMAT='%F %T '
# history | more
1  2008-08-05 19:02:39 service network restart
2  2008-08-05 19:02:39 exit
3  2008-08-05 19:02:39 id
4  2008-08-05 19:02:39 cat /etc/redhat-release

2. Search the history using Control+R

I strongly believe, this may be your most frequently used feature of history. When you’ve already executed a very long command, you can simply search history using a keyword and re-execute the same command without having to type it fully. Press Control+R and type the keyword. In the following example, I searched for red, which displayed the previous command “cat /etc/redhat-release” in the history that contained the word red.

# [Press Ctrl+R from the command prompt,
which will display the reverse-i-search prompt]
(reverse-i-search)`red': cat /etc/redhat-release
[Note: Press enter when you see your command,
which will execute the command from the history]
# cat /etc/redhat-release
Fedora release 9 (Sulphur)

Sometimes you want to edit a command from history before executing it. For e.g. you can search for httpd, which will display service httpd stop from the command history, select this command and change the stop to start and re-execute it again as shown below.

# [Press Ctrl+R from the command prompt,
which will display the reverse-i-search prompt]
(reverse-i-search)`httpd': service httpd stop
[Note: Press either left arrow or right arrow key when you see your
command, which will display the command for you to edit, before executing it]
# service httpd start

3. Repeat previous command quickly using 4 different methods

Sometime you may end up repeating the previous commands for various reasons. Following are the 4 different ways to repeat the last executed command.

1. Use the up arrow to view the previous command and press enter to execute it.
2. Type !! and press enter from the command line
3. Type !-1 and press enter from the command line.
4. Press Control+P will display the previous command, press enter to execute it

4. Execute a specific command from history

In the following example, If you want to repeat the command #4, you can do !4 as shown below.

# history | more
1  service network restart
2  exit
3  id
4  cat /etc/redhat-release

# !4
cat /etc/redhat-release

# !ps
ps aux | grep yp
root     16947  0.0  0.1  36516  1264 ?        Sl   13:10   0:00 ypbind


root     17503  0.0  0.0   4124   740 pts/0    S+   19:19   0:00 grep yp

6. Control the total number of lines in the history using HISTSIZE

Append the following two lines to the .bash_profile and relogin to the bash shell again to see the change. In this example, only 450 command will be stored in the bash history.

# vi ~/.bash_profile HISTSIZE=450 HISTFILESIZE=450

7. Change the history file name using HISTFILE

By default, history is stored in ~/.bash_history file. Add the following line to the .bash_profile and relogin to the bash shell, to store the history command in .commandline_warrior file instead of .bash_history file. I’m yet to figure out a practical use for this. I can see this getting used when you want to track commands executed from different terminals using different history file name.

# vi ~/.bash_profile HISTFILE=/root/.commandline_warrior

If you have a good reason to change the name of the history file, please share it with me, as I’m interested in finding out how you are using this feature.

8. Eliminate the continuous repeated entry from history using HISTCONTROL

In the following example pwd was typed three times, when you do history, you can see all the 3 continuous occurrences of it. To eliminate duplicates, set HISTCONTROL to ignoredups as shown below.

# pwd # pwd # pwd # history | tail -4 44 pwd 45 pwd 46 pwd [Note that there are three pwd commands in history, after executing pwd 3 times as shown above] 47 history | tail -4 # export HISTCONTROL=ignoredups # pwd # pwd # pwd # history | tail -3 56 export HISTCONTROL=ignoredups 57 pwd [Note that there is only one pwd command in the history, even after executing pwd 3 times as shown above] 58 history | tail -4

9. Erase duplicates across the whole history using HISTCONTROL

The ignoredups shown above removes duplicates only if they are consecutive commands. To eliminate duplicates across the whole history, set the HISTCONTROL to erasedups as shown below.

# export HISTCONTROL=erasedups # pwd # service httpd stop # history | tail -3 38 pwd 39 service httpd stop 40 history | tail -3 # ls -ltr # service httpd stop # history | tail -6 35 export HISTCONTROL=erasedups 36 pwd 37 history | tail -3 38 ls -ltr 39 service httpd stop [Note that the previous service httpd stop after pwd got erased] 40 history | tail -6

10. Force history not to remember a particular command using HISTCONTROL

When you execute a command, you can instruct history to ignore the command by setting HISTCONTROL to ignorespace AND typing a space in front of the command as shown below. I can see lot of junior sysadmins getting excited about this, as they can hide a command from the history. It is good to understand how ignorespace works. But, as a best practice, don’t hide purposefully anything from history.

# export HISTCONTROL=ignorespace # ls -ltr # pwd # service httpd stop [Note that there is a space at the beginning of service, to ignore this command from history] # history | tail -3 67 ls -ltr 68 pwd 69 history | tail -3

11. Clear all the previous history using option -c

Sometime you may want to clear all the previous history, but want to keep the history moving forward.

# history -c

12. Subtitute words from history commands

When you are searching through history, you may want to execute a different command but use the same parameter from the command that you’ve just searched.

In the example below, the !!:$ next to the vi command gets the argument from the previous command to the current command.

# ls anaconda-ks.cfg anaconda-ks.cfg # vi !!:$ vi anaconda-ks.cfg

In the example below, the !^ next to the vi command gets the first argument from the previous command (i.e cp command) to the current command (i.e vi command).

# cp anaconda-ks.cfg anaconda-ks.cfg.bak anaconda-ks.cfg # vi !^ vi anaconda-ks.cfg

13. Substitute a specific argument for a specific command.

In the example below, !cp:2 searches for the previous command in history that starts with cp and takes the second argument of cp and substitutes it for the ls -l command as shown below.

# cp ~/longname.txt /really/a/very/long/path/long-filename.txt # ls -l !cp:2 ls -l /really/a/very/long/path/long-filename.txt

In the example below, !cp:$ searches for the previous command in history that starts with cp and takes the last argument (in this case, which is also the second argument as shown above) of cp and substitutes it for the ls -l command as shown below.

# ls -l !cp:$ ls -l /really/a/very/long/path/long-filename.txt

14. Disable the usage of history using HISTSIZE

If you want to disable history all together and don’t want bash shell to remember the commands you’ve typed, set the HISTSIZE to 0 as shown below.

# export HISTSIZE=0 # history # [Note that history did not display anything]

15. Ignore specific commands from the history using HISTIGNORE

Sometimes you may not want to clutter your history with basic commands such as pwd and ls. Use HISTIGNORE to specify all the commands that you want to ignore from the history. Please note that adding ls to the HISTIGNORE ignores only ls and not ls -l. So, you have to provide the exact command that you would like to ignore from the history.

# export HISTIGNORE="pwd:ls:ls -ltr:" # pwd # ls # ls -ltr # service httpd stop # history | tail -3 79 export HISTIGNORE="pwd:ls:ls -ltr:" 80 service httpd stop 81 history [Note that history did not record pwd, ls and ls -ltr]

Fedora release 9 (Sulphur)

5. Execute previous command that starts with a specific word

Type ! followed by the starting few letters of the command that you would like to re-execute. In the following example, typing !ps and enter, executed the previous command starting with ps, which is ‘ps aux | grep yp’.

Get a Grip on the Grep! – 15 Practical Grep Command Examples

This is part of the on-going 15 Examples series, where 15 detailed examples will be provided for a specific command or functionality. Earlier we discussed 15 practical examples for Linux find command, Linux command line history and mysql admin commands.

In this article let us review 15 practical examples of Linux grep command that will be very useful to both newbies and experts.

First create the following demo_file that will be used in the examples below to demonstrate grep command.

$ cat demo_file
THIS LINE IS THE 1ST UPPER CASE LINE IN THIS FILE.
this line is the 1st lower case line in this file.
This Line Has All Its First Character Of The Word With Upper Case.

Two lines above this line is empty.
And this is the last line.

1. Search for the given string in a single file

The basic usage of grep command is to search for a specific string in the specified file as shown below.

Syntax:
grep "literal_string" filename

$ grep "this" demo_file
this line is the 1st lower case line in this file.
Two lines above this line is empty.

2. Checking for the given string in multiple files.

Syntax:
grep "string" FILE_PATTERN

This is also a basic usage of grep command. For this example, let us copy the demo_file to demo_file1. The grep output will also include the file name in front of the line that matched the specific pattern as shown below. When the Linux shell sees the meta character, it does the expansion and gives all the files as input to grep.

$ cp demo_file demo_file1

$ grep "this" demo_*
demo_file:this line is the 1st lower case line in this file.
demo_file:Two lines above this line is empty.
demo_file:And this is the last line.
demo_file1:this line is the 1st lower case line in this file.
demo_file1:Two lines above this line is empty.
demo_file1:And this is the last line.

3. Case insensitive search using grep -i

Syntax:
grep -i "string" FILE

This is also a basic usage of the grep. This searches for the given string/pattern case insensitively. So it matches all the words such as “the”, “THE” and “The” case insensitively as shown below.

$ grep -i "the" demo_file
THIS LINE IS THE 1ST UPPER CASE LINE IN THIS FILE.
this line is the 1st lower case line in this file.
This Line Has All Its First Character Of The Word With Upper Case.
And this is the last line.

4. Match regular expression in files

Syntax:

grep "REGEX" filename

This is a very powerful feature, if you can use use regular expression effectively. In the following example, it searches for all the pattern that starts with “lines” and ends with “empty” with anything in-between. i.e To search “lines[anything in-between]empty” in the demo_file.

$ grep "lines.*empty" demo_file
Two lines above this line is empty.

From documentation of grep: A regular expression may be followed by one of several repetition operators:

• ? The preceding item is optional and matched at most once.
• * The preceding item will be matched zero or more times.
• + The preceding item will be matched one or more times.
• {n} The preceding item is matched exactly n times.
• {n,} The preceding item is matched n or more times.
• {,m} The preceding item is matched at most m times.
• {n,m} The preceding item is matched at least n times, but not more than m times.

5. Checking for full words, not for sub-strings using grep -w

If you want to search for a word, and to avoid it to match the substrings use -w option. Just doing out a normal search will show out all the lines.

The following example is the regular grep where it is searching for “is”. When you search for “is”, without any option it will show out “is”, “his”, “this” and everything which has the substring “is”.

$ grep -i "is" demo_file
THIS LINE IS THE 1ST UPPER CASE LINE IN THIS FILE.
this line is the 1st lower case line in this file.
This Line Has All Its First Character Of The Word With Upper Case.
Two lines above this line is empty.
And this is the last line.

The following example is the WORD grep where it is searching only for the word “is”. Please note that this output does not contain the line “This Line Has All Its First Character Of The Word With Upper Case”, even though “is” is there in the “This”, as the following is looking only for the word “is” and not for “this”.

$ grep -iw "is" demo_file
THIS LINE IS THE 1ST UPPER CASE LINE IN THIS FILE.
this line is the 1st lower case line in this file.
Two lines above this line is empty.
And this is the last line.

6. Displaying lines before/after/around the match using grep -A, -B and -C

When doing a grep on a huge file, it may be useful to see some lines after the match. You might feel handy if grep can show you not only the matching lines but also the lines after/before/around the match.

Please create the following demo_text file for this example.

$ cat demo_text
4. Vim Word Navigation

You may want to do several navigation in relation to the words, such as:

* e - go to the end of the current word.
* E - go to the end of the current WORD.
* b - go to the previous (before) word.
* B - go to the previous (before) WORD.
* w - go to the next word.
* W - go to the next WORD.

WORD - WORD consists of a sequence of non-blank characters, separated with white space.
word - word consists of a sequence of letters, digits and underscores.

Example to show the difference between WORD and word

* 192.168.1.1 - single WORD
* 192.168.1.1 - seven words.

6.1 Display N lines after match

-A is the option which prints the specified N lines after the match as shown below.

Syntax:
grep -A  "string" FILENAME

The following example prints the matched line, along with the 3 lines after it.

$ grep -A 3 -i "example" demo_text
Example to show the difference between WORD and word

* 192.168.1.1 - single WORD
* 192.168.1.1 - seven words.

6.2 Display N lines before match

-B is the option which prints the specified N lines before the match.

Syntax:
grep -B  "string" FILENAME

When you had option to show the N lines after match, you have the -B option for the opposite.

$ grep -B 2 "single WORD" demo_text
Example to show the difference between WORD and word

* 192.168.1.1 - single WORD

6.3 Display N lines around match

-C is the option which prints the specified N lines before the match. In some occasion you might want the match to be appeared with the lines from both the side. This options shows N lines in both the side(before & after) of match.

$ grep -C 2 "Example" demo_text
word - word consists of a sequence of letters, digits and underscores.

Example to show the difference between WORD and word

* 192.168.1.1 - single WORD

7. Highlighting the search using GREP_OPTIONS

As grep prints out lines from the file by the pattern / string you had given, if you wanted it to highlight which part matches the line, then you need to follow the following way.

When you do the following export you will get the highlighting of the matched searches. In the following example, it will highlight all the this when you set the GREP_OPTIONS environment variable as shown below.

$ export GREP_OPTIONS='--color=auto' GREP_COLOR='100;8'

$ grep this demo_file
this line is the 1st lower case line in this file.
Two lines above this line is empty.
And this is the last line.

8. Searching in all files recursively using grep -r

When you want to search in all the files under the current directory and its sub directory. -r option is the one which you need to use. The following example will look for the string “ramesh” in all the files in the current directory and all it’s subdirectory.

$ grep -r "ramesh" *

9. Invert match using grep -v

You had different options to show the lines matched, to show the lines before match, and to show the lines after match, and to highlight match. So definitely You’d also want the option -v to do invert match.

When you want to display the lines which does not matches the given string/pattern, use the option -v as shown below. This example will display all the lines that did not match the word “go”.

$ grep -v "go" demo_text
4. Vim Word Navigation

You may want to do several navigation in relation to the words, such as:

WORD - WORD consists of a sequence of non-blank characters, separated with white space.
word - word consists of a sequence of letters, digits and underscores.

Example to show the difference between WORD and word

* 192.168.1.1 - single WORD
* 192.168.1.1 - seven words.

10. display the lines which does not matches all the given pattern.

Syntax:
grep -v -e "pattern" -e "pattern"

$ cat test-file.txt
a
b
c
d

$ grep -v -e "a" -e "b" -e "c" test-file.txt
d

11. Counting the number of matches using grep -c

When you want to count that how many lines matches the given pattern/string, then use the option -c.

Syntax:
grep -c "pattern" filename

$ grep -c "go" demo_text
6

When you want do find out how many lines matches the pattern

$ grep -c this demo_file
3

When you want do find out how many lines that does not match the pattern

$ grep -v -c this demo_file
4

12. Display only the file names which matches the given pattern using grep -l

If you want the grep to show out only the file names which matched the given pattern, use the -l (lower-case L) option.

When you give multiple files to the grep as input, it displays the names of file which contains the text that matches the pattern, will be very handy when you try to find some notes in your whole directory structure.

$ grep -l this demo_*
demo_file
demo_file1

13. Show only the matched string

By default grep will show the line which matches the given pattern/string, but if you want the grep to show out only the matched string of the pattern then use the -o option.

It might not be that much useful when you give the string straight forward. But it becomes very useful when you give a regex pattern and trying to see what it matches as

$ grep -o "is.*line" demo_file
is line is the 1st lower case line
is line
is is the last line

14. Show the position of match in the line

When you want grep to show the position where it matches the pattern in the file, use the following options as

Syntax:
grep -o -b "pattern" file

$ cat temp-file.txt
12345
12345

$ grep -o -b "3" temp-file.txt
2:3
8:3

Note: The output of the grep command above is not the position in the line, it is byte offset of the whole file.

15. Show line number while displaying the output using grep -n

To show the line number of file with the line matched. It does 1-based line numbering for each file. Use -n option to utilize this feature.

$ grep -n "go" demo_text
5: * e - go to the end of the current word.
6: * E - go to the end of the current WORD.
7: * b - go to the previous (before) word.
8: * B - go to the previous (before) WORD.
9: * w - go to the next word.

10: * W - go to the next WORD.

The Ultimate Tar Command Tutorial with 10 Practical Examples

On Unix platform, tar command is the primary archiving utility. Understanding various tar command options will help you master the archive file manipulation.

In this article, let us review various tar examples including how to create tar archives (with gzip and bzip compression), extract a single file or directory, view tar archive contents, validate the integrity of tar archives, finding out the difference between tar archive and file system, estimate the size of the tar archives before creating it etc.,

1. tar command examples

Create a new tar archive.

$ tar cvf archive_name.tar dirname/

Extract from an existing tar archive.

$ tar xvf archive_name.tar

View an existing tar archive.

$ tar tvf archive_name.tar

. Creating an archive using tar command

Creating an uncompressed tar archive using option cvf

This is the basic command to create a tar archive.

$ tar cvf archive_name.tar dirname/

In the above command:

• c – create a new archive
• v – verbosely list files which are processed.
• f – following is the archive file name

Creating a tar gzipped archive using option cvzf

The above tar cvf option, does not provide any compression. To use a gzip compression on the tar archive, use the z option as shown below.

$ tar cvzf archive_name.tar.gz dirname/

• z – filter the archive through gzip

Note: .tgz is same as .tar.gz

Note: I like to keep the ‘cvf’ (or tvf, or xvf) option unchanged for all archive creation (or view, or extract) and add additional option at the end, which is easier to remember. i.e cvf for archive creation, cvfz for compressed gzip archive creation, cvfj for compressed bzip2 archive creation etc., For this method to work properly, don’t give – in front of the options.

Creating a bzipped tar archive using option cvjf

Create a bzip2 tar archive as shown below:

$ tar cvfj archive_name.tar.bz2 dirname/

• j – filter the archive through bzip2

gzip vs bzip2: bzip2 takes more time to compress and decompress than gzip. bzip2 archival size is less than gzip.

Note: .tbz and .tb2 is same as .tar.bz2

2. Extracting (untar) an archive using tar command

Extract a *.tar file using option xvf

Extract a tar file using option x as shown below:

$ tar xvf archive_name.tar

• x – extract files from archive

Extract a gzipped tar archive ( *.tar.gz ) using option xvzf

Use the option z for uncompressing a gzip tar archive.

$ tar xvfz archive_name.tar.gz

Extracting a bzipped tar archive ( *.tar.bz2 ) using option xvjf

Use the option j for uncompressing a bzip2 tar archive.

$ tar xvfj archive_name.tar.bz2

Note: In all the above commands v is optional, which lists the file being processed.

3. Listing an archive using tar command

View the tar archive file content without extracting using option tvf

You can view the *.tar file content before extracting as shown below.

$ tar tvf archive_name.tar

View the *.tar.gz file content without extracting using option tvzf

You can view the *.tar.gz file content before extracting as shown below.

$ tar tvfz archive_name.tar.gz

View the *.tar.bz2 file content without extracting using option tvjf

You can view the *.tar.bz2 file content before extracting as shown below.

$ tar tvfj archive_name.tar.bz2

4. Listing out the tar file content with less command

When the number of files in an archive is more, you may pipe the output of tar to less. But, you can also use less command directly to view the tar archive output, as explained

5. Extract a single file from tar, tar.gz, tar.bz2 file

To extract a specific file from a tar archive, specify the file name at the end of the tar xvf command as shown below. The following command extracts only a specific file from a large tar file.

$ tar xvf archive_file.tar /path/to/file

Use the relevant option z or j according to the compression method gzip or bzip2 respectively as shown below.

$ tar xvfz archive_file.tar.gz /path/to/file

$ tar xvfj archive_file.tar.bz2 /path/to/file

6. Extract a single directory from tar, tar.gz, tar.bz2 file

To extract a single directory (along with it’s subdirectory and files) from a tar archive, specify the directory name at the end of the tar xvf command as shown below. The following extracts only a specific directory from a large tar file.

$ tar xvf archive_file.tar /path/to/dir/

To extract multiple directories from a tar archive, specify those individual directory names at the end of the tar xvf command as shown below.

$ tar xvf archive_file.tar /path/to/dir1/ /path/to/dir2/

Use the relevant option z or j according to the compression method gzip or bzip2 respectively as shown below.

$ tar xvfz archive_file.tar.gz /path/to/dir/

$ tar xvfj archive_file.tar.bz2 /path/to/dir/

7. Extract group of files from tar, tar.gz, tar.bz2 archives using regular expression

You can specify a regex, to extract files matching a specified pattern. For example, following tar command extracts all the files with pl extension.

$ tar xvf archive_file.tar --wildcards '*.pl'

Options explanation:

• –wildcards *.pl – files with pl extension

8. Adding a file or directory to an existing archive using option -r

You can add additional files to an existing tar archive as shown below. For example, to append a file to *.tar file do the following:

$ tar rvf archive_name.tar newfile

This newfile will be added to the existing archive_name.tar. Adding a directory to the tar is also similar,

$ tar rvf archive_name.tar newdir/

Note: You cannot add file or directory to a compressed archive. If you try to do so, you will get “tar: Cannot update compressed archives” error as shown below.

$ tar rvfz archive_name.tgz newfile
tar: Cannot update compressed archives
Try `tar --help' or `tar --usage' for more information.

9. Verify files available in tar using option -W

As part of creating a tar file, you can verify the archive file that got created using the option W as shown below.

$ tar cvfW file_name.tar dir/

If you are planning to remove a directory/file from an archive file or from the file system, you might want to verify the archive file before doing it as shown below.

$ tar tvfW file_name.tar
Verify 1/file1
1/file1: Mod time differs
1/file1: Size differs
Verify 1/file2
Verify 1/file3

If an output line starts with Verify, and there is no differs line then the file/directory is Ok. If not, you should investigate the issue.

Note: for a compressed archive file ( *.tar.gz, *.tar.bz2 ) you cannot do the verification.

Finding the difference between an archive and file system can be done even for a compressed archive. It also shows the same output as above excluding the lines with Verify.

Finding the difference between gzip archive file and file system

$ tar dfz file_name.tgz

Finding the difference between bzip2 archive file and file system

$ tar dfj file_name.tar.bz2

10. Estimate the tar archive size

The following command, estimates the tar file size ( in KB ) before you create the tar file.

$ tar -cf - /directory/to/archive/ | wc -c
20480

The following command, estimates the compressed tar file size ( in KB ) before you create the tar.gz, tar.bz2 files.

$ tar -czf - /directory/to/archive/ | wc -c
508

$ tar -cjf - /directory/to/archive/ | wc -c
428

2. grep command examples

Search for a given string in a file (case in-sensitive search).

$ grep -i "the" demo_file

Print the matched line, along with the 3 lines after it.

$ grep -A 3 -i "example" demo_text

Search for a given string in all files recursively

$ grep -r "ramesh" *

Unix LS Command: 15 Practical Examples

ls – Unix users and sysadmins cannot live without this two letter command. Whether you use it 10 times a day or 100 times a day, knowing the power of ls command can make your command line journey enjoyable.

In this article, let us review 15 practical examples of the mighty ls command.

1. Open Last Edited File Using ls -t

To open the last edited file in the current directory use the combination of ls, head and vi commands as shown below.

ls -t sorts the file by modification time, showing the last edited file first. head -1 picks up this first file.

$ vi first-long-file.txt
$ vi second-long-file.txt

$ vi `ls -t | head -1`
[Note: This will open the last file you edited (i.e second-long-file.txt)]

2. Display One File Per Line Using ls -1

To show single entry per line, use -1 option as shown below.

$ ls -1
bin
boot
cdrom
dev
etc
home
initrd
initrd.img
lib

3. Display All Information About Files/Directories Using ls -l

To show long listing information about the file/directory.

$ ls -l
-rw-r----- 1 ramesh team-dev 9275204 Jun 13 15:27 mthesaur.txt.gz

• 1st Character – File Type: First character specifies the type of the file.
  In the example above the hyphen (-) in the 1st character indicates that this is a normal file. Following are the possible file type options in the 1st character of the ls -l output.
  • Field Explanation
  • - normal file
  • d directory
  • s socket file
  • l link file
• Field 1 – File Permissions: Next 9 character specifies the files permission. Each 3 characters refers to the read, write, execute permissions for user, group and world In this example, -rw-r—– indicates read-write permission for user, read permission for group, and no permission for others.
• Field 2 – Number of links: Second field specifies the number of links for that file. In this example, 1 indicates only one link to this file.
• Field 3 – Owner: Third field specifies owner of the file. In this example, this file is owned by username ‘ramesh’.
• Field 4 – Group: Fourth field specifies the group of the file. In this example, this file belongs to ”team-dev’ group.
• Field 5 – Size: Fifth field specifies the size of file. In this example, ’9275204′ indicates the file size.
• Field 6 – Last modified date & time: Sixth field specifies the date and time of the last modification of the file. In this example, ‘Jun 13 15:27′ specifies the last modification time of the file.
• Field 7 – File name: The last field is the name of the file. In this example, the file name is mthesaur.txt.gz.

4. Display File Size in Human Readable Format Using ls -lh

Use ls -lh (h stands for human readable form), to display file size in easy to read format. i.e M for MB, K for KB, G for GB.

$ ls -l
-rw-r----- 1 ramesh team-dev 9275204 Jun 12 15:27 arch-linux.txt.gz*

$ ls -lh
-rw-r----- 1 ramesh team-dev 8.9M Jun 12 15:27 arch-linux.txt.gz

5. Display Directory Information Using ls -ld

When you use “ls -l” you will get the details of directories content. But if you want the details of directory then you can use -d option as., For example, if you use ls -l /etc will display all the files under etc directory. But, if you want to display the information about the /etc/ directory, use -ld option as shown below.

$ ls -l /etc
total 3344
-rw-r--r--   1 root root   15276 Oct  5  2004 a2ps.cfg
-rw-r--r--   1 root root    2562 Oct  5  2004 a2ps-site.cfg
drwxr-xr-x   4 root root    4096 Feb  2  2007 acpi
-rw-r--r--   1 root root      48 Feb  8  2008 adjtime
drwxr-xr-x   4 root root    4096 Feb  2  2007 alchemist

$ ls -ld /etc
drwxr-xr-x 21 root root 4096 Jun 15 07:02 /etc

6. Order Files Based on Last Modified Time Using ls -lt

To sort the file names displayed in the order of last modification time use the -t option. You will be finding it handy to use it in combination with -l option.

$ ls -lt
total 76
drwxrwxrwt  14 root root  4096 Jun 22 07:36 tmp
drwxr-xr-x 121 root root  4096 Jun 22 07:05 etc
drwxr-xr-x  13 root root 13780 Jun 22 07:04 dev
drwxr-xr-x  13 root root  4096 Jun 20 23:12 root
drwxr-xr-x  12 root root  4096 Jun 18 08:31 home
drwxr-xr-x   2 root root  4096 May 17 21:21 sbin
lrwxrwxrwx   1 root root    11 May 17 20:29 cdrom -> media/cdrom
drwx------   2 root root 16384 May 17 20:29 lost+found
drwxr-xr-x  15 root root  4096 Jul  2  2008 var

7. Order Files Based on Last Modified Time (In Reverse Order) Using ls -ltr

To sort the file names in the last modification time in reverse order. This will be showing the last edited file in the last line which will be handy when the listing goes beyond a page. This is my default ls usage. Anytime I do ls, I always use ls -ltr as I find this very convenient.

$ ls -ltr

total 76
drwxr-xr-x  15 root root  4096 Jul  2  2008 var
drwx------   2 root root 16384 May 17 20:29 lost+found
lrwxrwxrwx   1 root root    11 May 17 20:29 cdrom -> media/cdrom
drwxr-xr-x   2 root root  4096 May 17 21:21 sbin
drwxr-xr-x  12 root root  4096 Jun 18 08:31 home
drwxr-xr-x  13 root root  4096 Jun 20 23:12 root
drwxr-xr-x  13 root root 13780 Jun 22 07:04 dev
drwxr-xr-x 121 root root  4096 Jun 22 07:05 etc
drwxrwxrwt  14 root root  4096 Jun 22 07:36 tmp

8. Display Hidden Files Using ls -a (or) ls -A

To show all the hidden files in the directory, use ‘-a option’. Hidden files in Unix starts with ‘.’ in its file name.

$ ls -a
[rnatarajan@asp-dev ~]$ ls -a
.                             Debian-Info.txt
..                            CentOS-Info.txt
.bash_history                 Fedora-Info.txt
.bash_logout                  .lftp
.bash_profile                 libiconv-1.11.tar.tar
.bashrc                       libssh2-0.12-1.2.el4.rf.i386.rpm

It will show all the files including the ‘.’ (current directory) and ‘..’ (parent directory). To show the hidden files, but not the ‘.’ (current directory) and ‘..’ (parent directory), use option -A.

$ ls -A
Debian-Info.txt               Fedora-Info.txt
CentOS-Info.txt               Red-Hat-Info.txt
.bash_history                 SUSE-Info.txt
.bash_logout                  .lftp
.bash_profile                 libiconv-1.11.tar.tar
.bashrc                       libssh2-0.12-1.2.el4.rf.i386.rpm
[Note: . and .. are not displayed here]

9. Display Files Recursively Using ls -R

$ ls  /etc/sysconfig/networking
devices  profiles

$ ls  -R /etc/sysconfig/networking
/etc/sysconfig/networking:
devices  profiles

/etc/sysconfig/networking/devices:

/etc/sysconfig/networking/profiles:
default

/etc/sysconfig/networking/profiles/default:

To show all the files recursively, use -R option. When you do this from /, it shows all the unhidden files in the whole file system recursively.

10. Display File Inode Number Using ls -i

Sometimes you may want to know the inone number of a file for internal maintenance. Use -i option as shown below to display inone number. Using inode number you can remove files that has special characters in it’s name as explained in the example #6 of the find command article.

$ ls -i /etc/xinetd.d/
279694 chargen      279724 cups-lpd  279697 daytime-udp
279695 chargen-udp  279696 daytime   279698 echo

11. Hide Control Characters Using ls -q

To print question mark instead of the non graphics control characters use the -q option.

ls -q

12. Display File UID and GID Using ls -n

Lists the output like -l, but shows the uid and gid in numeric format instead of names.

$ ls -l ~/.bash_profile
-rw-r--r--  1 ramesh ramesh 909 Feb  8 11:48 /home/ramesh/.bash_profile
$ ls -n ~/.bash_profile
-rw-r--r--  1 511 511 909 Feb  8 11:48 /home/ramesh/.bash_profile

[Note: This display 511 for uid and 511 for gid]

13. Visual Classification of Files With Special Characters Using ls -F

Instead of doing the ‘ls -l’ and then the checking for the first character to determine the type of file. You can use -F which classifies the file with different special character for different kind of files.

$ ls -F
Desktop/  Documents/  Ubuntu-App@  firstfile  Music/  Public/  Templates/

Thus in the above output,

• / – directory.
• nothing – normal file.
• @ – link file.
• * – Executable file

14. Visual Classification of Files With Colors Using ls -F

Recognizing the file type by the color in which it gets displayed is an another kind in classification of file. In the above output directories get displayed in blue, soft links get displayed in green, and ordinary files gets displayed in default color.

$ ls --color=auto
Desktop  Documents Examples firstfile Music  Pictures  Public  Templates  Videos

15. Useful ls Command Aliases

You can take some required ls options in the above, and make it as aliases. We suggest the following.

• Long list the file with size in human understandable form.

  alias ll="ls -lh"

• Classify the file type by appending special characters.

  alias lv="ls -F"

• Classify the file type by both color and special character.

  alias ls="ls -F --color=auto"

Understanding /etc/passwd File Format

/etc/passwd file stores essential information, which is required during login i.e. user account information. /etc/passwd is a text file, that contains a list of the system's accounts, giving for each account some useful information like user ID, group ID, home directory, shell, etc. It should have general read permission as many utilities, like ls use it to map user IDs to user names, but write access only for the superuser (root).

Understanding fields in /etc/passwd

The /etc/passwd contains one entry per line for each user (or user account) of the system. All fields are separated by a colon (:) symbol. Total seven fields as follows.

Generally, passwd file entry looks as follows (click to enlarge image):

(Fig.01: /etc/passwd file format - click to enlarge)

1. Username: It is used when user logs in. It should be between 1 and 32 characters in length.
2. Password: An x character indicates that encrypted password is stored in /etc/shadow file.
3. User ID (UID): Each user must be assigned a user ID (UID). UID 0 (zero) is reserved for root and UIDs 1-99 are reserved for other predefined accounts. Further UID 100-999 are reserved by system for administrative and system accounts/groups.
4. Group ID (GID): The primary group ID (stored in /etc/group file)
5. User ID Info: The comment field. It allow you to add extra information about the users such as user's full name, phone number etc. This field use by finger command.
6. Home directory: The absolute path to the directory the user will be in when they log in. If this directory does not exists then users directory becomes /
7. Command/shell: The absolute path of a command or shell (/bin/bash). Typically, this is a shell. Please note that it does not have to be a shell.

Task: See User List

/etc/passwd is only used for local users only. To see list of all users, enter:
$ cat /etc/passwd
To search for a username called tom, enter:
$ grep tom /etc/passwd

/etc/passwd file permission

The permission on the /etc/passwd file should be read only to users (-rw-r--r--) and the owner must be root:
$ ls -l /etc/passwd
Output:

-rw-r--r-- 1 root root 2659 Sep 17 01:46 /etc/passwd

Reading /etc/passwd file

#!/bin/bash
# seven fields from /etc/passwd stored in $f1,f2...,$f7
#
while IFS=: read -r f1 f2 f3 f4 f5 f6 f7
do
echo "User $f1 use $f7 shell and stores files in $f6 directory."
done < /etc/passwd

Your password is stored in /etc/shadow file

Your encrpted password is not stored in /etc/passwd file. It is stored in /etc/shadow file. In the good old days there was no great problem with this general read permission. Everybody could read the encrypted passwords, but the hardware was too slow to crack a well-chosen password, and moreover, the basic assumption used to be that of a friendly user-community.

Almost, all modern Linux / UNIX line operating systems use some sort of the shadow password suite, where /etc/passwd has asterisks (*) instead of encrypted passwords, and the encrypted passwords are in /etc/shadow which is readable by the superuser only.