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]