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Home > Linux
 
Linux
 

Please go through your Unix basics and refresh your Unix commands

UNIX / Linux Command Summary

access()
Used to check the accessibility of files

int
Access(pathname, access_mode)
Char* pathname;
int access-mode;
The access modes are.
04 read
02 write
01 execute (search)
00 checks existence of a file

& operator
execute a command as a background process.

banner
prints the specified string in large letters. Each argument may be upto 10 characters long.

break
is used to break out of a loop. It does not exit from the program.

Cal
Produces a calender of the current month as standard output. The month (1-12) and year (1-9999) must be specified in full numeric format.

Cal [[ month] year]

Calendar
Displays contents of the calendar file

case operator
The case operator is used to validate multiple conditions.

Case $string in

Pattern 1)
Command list;;
Command list;;

Pattern 3)
Command list;;
easc

cat
(for concatenate) command is used to display the contents of a file. Used without arguments it takes input from standard input <Dtrl d> is used to terminate input.

cat [filename(s)]
cat > [filename]
Data can be appended to a file using >>

Some of the available options are :
Cat [-options] filename(S)
-s silent about files that
cannot be accessed
-v enables display of non printinging characters (except tabs, new lines, form-

feeds)
-t when used with –v, it causes tabs to be printed as ^I’s
-e when used with –v, it causes $ to be printed at the end of each line
The –t and –e options are ignored if the –v options is not specified.

cd
Used to change directories

chgrp
Changes the group that owns a file.
Chgrp [grou –id] [filename]

chmod
Allows file permissions to be changed for each user. File permissions can be changed only by the owner (s).
Chmod [+/-][rwx] [ugo] [filename]

chown
Used to change the owner of a file.
The command takes a file(s) as source files and the login id of another user as the target.
Chown [user-id] [filename]

cmp
The cmp command compares two files (text or binary) byte-by-byte and displays the first occurrence where the files differ.
Cmp [filename1] [filename2] -1 gives a long listing

comm.
The comm command compares two sorted files and displays the instances that are common. The display is separated into 3 columns.
Comm. filename1 filename2
first displays what occurs in first files but not in the second
second displays what occurs in second file but not in first
third displays what is common in both files

continue statement
The rest of the commands in the loop are ignored. It moves out of the loop and moves on the next cycle.

cp
The cp (copy) command is used to copy a file.
Cp [filename1] [filename2]

cpio(copy input/output)
Utility program used to take backups.
Cpio operates in three modes:
-o output
-i input
-p pass

creat()
the system call creates a new file or prepares to rewrite an existing file. The file pointer is set to the beginning of file.
#include<sys/tyes.h>
#include<sys/stat.h>
int creat(path, mode)

char *path;
int mode;

cut
used to cut out parts of a file. It takes filenames as command line arguments or input from standard input. The command can cut columns as well as fields in a file. It however does not delete the selected parts of the file.
Cut [-ef] [column/fie,d] filename
Cut-d “:” –f1,2,3 filename
Where –d indicates a delimiter specified within “:”

df
used to find the number of free blocks available for all the mounted file systems.
#/etc/df [filesystem]

diff
the diff command compares text files. It gives an index of all the lines that differ in the two files along with the line numbers. It also displays what needs to be changed.
Diff filename1 filename2

echo
The echo command echoes arguments on the command line.
echo [arguments]

env
Displays the permanent environment variables associated with a user’s login id

exit command
Used to stop the execution of a shell script.

expr command
Expr (command) command is used for numeric computation.
The operators + (add), -(subtract), *(multiplu), /(divide), (remainder) are allowed. Calculation are performed in order of normal numeric precedence.

find
The find command searches through directories for files that match the specified criteria. It can take full pathnames and relative pathnames on the command line.
To display the output on screen the –print option must be specified

for operator
The for operator may be used in looping constructs where there is repetitive execution of a section of the shell program.
For var in vall val2 val3 val4;

Do commnds; done

fsck
Used to check the file system and repair damaged files. The command takes a device name as an argument
# /etc/fsck /dev/file-system-to-be-checked.

grave operator
Used to store the standard the output of a command in an enviroment variable. (‘)

grep
The grep (global regular expression and print) command can be used as a filter to search for strings in files. The pattern may be either a fixed character string or a regular expression.
Grep “string” filename(s)

HOME
User’s home directory

if operator
The if operator allows conditional operator

If expression; then commands; fi
if … then…else… fi
$ if; then

commands
efile; then

commands
fi

kill
used to stop background processes

In
used to link files. A duplicate of a file is created with another name

LOGNAME
displays user’s login name

ls
Lists the files in the current directory

Some of the available options are:
-l gives a long listing
-a displays all file{including hidden files

lp
used to print data on the line printer.
Lp [options] filename(s)

mesg
The mesg command controls messages received on a terminal.
-n does not allow messages to be displayed on screen
-y allows messages to be displayed on screen

mkdir
used to create directories

more
The more command is used to dispay data one screenful at a time.
More [filename]

mv
Mv (move) moves a file from one directory to another or simply changes filenames. The command takes filename and pathnames as source names and a filename or exiting directory as target names.
mv [source-file] [target-file]

news
The news command allows a user to read news items published by the system administrator.

ni
Displays the contents of a file with line numbers

passwd
Changes the password

paste
The paste command joins lines from two files and displays the output. It can take a number of filenames as command line arguments.
paste file1 file2

PATH
The directories that the system searches to find commands

pg
Used to display data one page (screenful) at a time. The command can take a number of filenames as arguments.
Pg [option] [filename] [filename2]…..

pipe
Operator (1) takes the output of one commands as input of another command.



ps
Gives information about all the active processes.

PS1
The system prompt

pwd
(print working directory) displays the current directory.

rm
The rm (remove) command is used to delete files from a directory. A number of files may be deleted simultaneously. A file(s) once deleted cannot be retrieved.
rm [filename 1] [filename 2]…

sift command
Using shift $1becomes the source string and other arguments are shifted. $2 is shifted to $1,$3to $2 and so on.

Sleep
The sleep command is used to suspend the execution of a shell script for the specified time. This is usually in seconds.

sort
Sort is a utility program that can be used to sort text files in numeric or alphabetical order
Sort [filename]

split
Used to split large file into smaller files
Split-n filename
Split can take a second filename on the command line.

su
Used to switch to superuser or any other user.

sync
Used to copy data in buffers to files

system0
Used to run a UNIX command from within a C program

tail
The tail command may be used to view the end of a file.
Tail [filename]

tar
Used to save and restore files to tapes or other removable media.
Tar [function[modifier]] [filename(s)]

tee
output that is being redirected to a file can also be viewed on standard output.

test command
It compares strings and numeric values.
The test command has two forms : test command itself If test ${variable} = value then
Do commands else do commands

File
The test commands also uses special operators [ ]. These are operators following the of are interpreted by the shell as different from wildcard characters.
Of [ -f ${variable} ]

Then
Do commands
Elif
[ -d ${variable} ]

then
do commands

else
do commands

fi
many different tests are possible for files. Comparing numbers, character strings, values of environment variables.

time
Used to display the execution time of a program or a command. Time is reported in seconds.
Time filename values

tr
The tr command is used to translate characters.
tr [-option] [string1 [string2]]

tty
Displays the terminal pathname

umask
Used to specify default permissions while creating files.

uniq
The uniq command is used to display the uniq(ue) lines in a sorted file.
Sort filename uniq

until
The operator executes the commands within a loop as long as the test condition is false.

wall
Used to send a message to all users logged in.
# /etc/wall message

wait
the command halts the execution of a script until all child processes, executed as background processes, are completed.

wc
The wc command can be used to count the number of lines, words and characters in a fine.
wc [filename(s)]
The available options are:
wc –[options] [filename]
-1
-w
-c
while operator
the while operator repeatedly performs an operation until the test condition proves false.

$ while
Ø do

commands
Ø done

who
displays information about all the users currently logged onto the system. The user name, terminal number and the date and time that each user logged onto the system.
The syntax of the who command is who [options]

write
The write command allows inter-user communication. A user can send messages by addressing the other user’s terminal or login id.
write user-name [terminal number]

Also Read:

Comprehensive Linux FAQ with Answers

 
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