Linux Commands

# Create a new tar archive.
# the folder dirname/ is compressed into archive_name.tar
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


# 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" *
# Match regular expression in files
grep "lines.*empty" demo_file
# Match the string as a whole word
grep -iw "is" demo_file
# When you want to count that how many lines matches the given pattern/
string, then use the option -c
grep -c "go" demo_text
# To show the line number of file with the line matched
grep -n "go" demo_text


# Find the passwd file under root and two levels down.
#(i.e root — level 1, and two sub-directories — level 2 and 3 )
find / -maxdepth 3 -name passwd
# Find files using file-name ( case in-sensitve find)
find -iname "MyCProgram.c"
# Execute commands on files found by the find command
# it is to calculate the file's signature which is a hashcode
find -iname "MyCProgram.c" -exec md5sum {} ;
# Find all empty files in home directory
find ~ -empty
# Shows the files or directories whose name are not MyCProgram.c
find -maxdepth 1 -not -iname "MyCProgram.c"
# Find files which has read permission only to group
find . -perm 040 -type f -exec ls -l {} ;
# The following command will display the top 5 largest file in the current
directory and its subdirectory.
find . -type f -exec ls -s {} ; | sort -n -r | head -5
# Top 5 smallest: Technique is same as finding the bigger files, but the
only difference the sort is ascending order.
find . -type f -exec ls -s {} ; | sort -n  | head -5
# Find files bigger than the given size
find ~ -size +100M
# Find files smaller than the given size
find ~ -size -100M


# When you substitute a path name which has ‘/’, you can use @ as a
delimiter instead of ‘/’.
# In the sed example below, in the last line of the input file, /opt/omni/
lbin was changed to /opt/tools/bin
sed 's@/opt/omni/lbin@/opt/tools/bin@g' path.txt
# sed & Usage: Substitute /usr/bin/ to /usr/bin/local
sed 's@/usr/bin@&/local@g' path.txt
# sed & Usage: Match the whole line
sed 's@^.*$@<<<&>>>@g' path.txt
# Grouping using ()
# Get only the first path in each line, the number "1" is for the first
matched group
sed 's/(/[^:]*).*/1/g' path.txt
# In the above command $ specifies substitution to happen only for the last
line.
# Output shows that the order of the path values in the last line has been
reversed.
sed '$s@([^:]*):([^:]*):([^:]*)@3:2:1@g' path.txt
# Get the list of usernames in /etc/passwd file
# This sed example displays only the first field from the /etc/passwd file.
sed 's/([^:]*).*/1/' /etc/passwd
# Parenthesize first character of each word
echo "Welcome To The Geek Stuff" | sed 's/(b[A-Z])/(1)/g'
# Convert DOS format to UNIX format
sed 's/$/r/' input.txt > output.txt
# Commify the simple number
sed 's/(^|[^0-9.])([0-9]+)([0-9]{3})/12,3/g' numbers


# Remove duplicate lines using awk
awk '!($0 in array) { array[$0]; print }' temp
# Print all lines from /etc/passwd that has the same uid and gid
awk -F ':' '$3==$4' passwd.txt
# Print only specific field from a file
awk '{print $2,$5;}' employee.txt
# Awk reads and parses each line from input based on whitespace character by
default and set the variables $1,$2
# Awk FS variable is used to set the field separator for each record
# Awk FS can be set to any single character or regular expression
# 1 Using -F command line option: awk -F 'FS' 'commands' inputfilename as
following
awk -F':' '{print $3,$4;}' /etc/passwd
# 2 Awk FS can be set like normal variable: awk 'BEGIN{FS="FS";}' as
following
BEGIN{
FS=":";
print "NametUserIDtGroupIDtHomeDirectory";
}
{
    print $1"t"$3"t"$4"t"$6;
}
END {
    print NR,"Records Processed";
}
# And the execution of the awk script
awk -f etc_passwd.awk /etc/passwd
# Output field separator: field 3 and 4 are connected with "="
awk -F':' 'BEGIN{OFS="=";} {print $3,$4;}' /etc/passwd
# Awk RS Example: Record Separator variable and its execution
BEGIN {
    RS="nn";
    FS="n";

}
{
    print $1,$2;
}
awk -f student.awk  student.txt
# Awk ORS Example: Output Record Separator Variable
awk 'BEGIN{ORS="=";} {print;}' student-marks
# Awk NR Example: Number of Records Variable
awk '{print "Processing Record - ",NR;}END {print NR, "Students Records are
processed";}' student-marks
# Awk NF Example: Number of Fields in a record
awk '{print NR,"->",NF}' student-marks
# Awk FILENAME Example: Name of the current input file
awk '{print FILENAME}' student-marks
# Awk FNR Example: Number of Records relative to the current input file
awk '{print FILENAME, FNR;}' student-marks bookdetails
#  diff command examples :Ignore white space while comparing.
diff -w name_list.txt name_list_new.txt


# sort command examples Sort a file in ascending order
sort names.txt
# Sort a file in descending order
sort -r names.txt
# Sort passwd file by 3rd field. Filed separator is : (-t)  by a key (-k)
sort -t: -k 3n /etc/passwd | more


# To view oracle related environment variables
export | grep ORACLE
# To export an environment variable:
export ORACLE_HOME=/u01/app/oracle/product/10.2.0


# To create a *.gz compressed file:
gzip test.txt
# To uncompress a *.gz file:
gzip -d test.txt.gz
# To extract a *.zip compressed file:
unzip test.zip


# To view current running processes
ps -ef | more
# To view current running processes in a tree structure. H option stands for
process hierarchy.
ps -efH | more
# for kill
ps -ef | grep vim
kill -9 7243


# To displays only the processes that belong to a particular user use -u
option.
# The following will show only the top processes that belongs to oracle user.
top -u oracle


# Miscellaneous
# Displays the file system disk space usage. By default df -k displays
output in bytes
df -k
# Get confirmation before removing the file.
rm -i filename.txt
# Following example recursively removes all files and directories under the
example directory
rm -r example
# Copy file1 to file2 preserving the mode, ownership and timestamp.
cp -p file1 file2
# Rename file1 to file2. if file2 exists prompt for confirmation before
overwritting it.
mv -i file1 file2
# Give full access to user and group (i.e read, write and execute ) on a
specific file
chmod ug+rwx file.txt
# Revoke all access for the group (i.e read, write and execute ) on a
specific file.
chmod g-rwx file.txt
# Apply the file permissions recursively to all the files in the sub-
directories. o is for others
chmod -R ugo+rwx file.txt
# One more way to change
chmod 777 file.txt
# create a directory
mkdir ~/temp
# Use ifconfig command to view or configure a network interface on the Linux
system
ifconfig -a
# Print N number of lines from the file named filename.txt
tail -n N filename.txt
# To install apache using yum
yum install httpd
# To upgrade apache using yum
yum update httpd
# To uninstall/remove apache using yum.
yum remove httpd
# Can do the samethings using rpm
# The quick and effective method to download software, music, video from
internet is using wget command.
wget http://prdownloads.sourceforge.net/sourceforge/nagios/nagios-3.2.1.tar.gz