Chapter2 Standard Output

1.1 Writing Output to the Terminal/Window

Use the echo built-in command.All the parameters on the command line are printed to the screen.

The echo command is one of the most simple of all bash commands .

keep in mind:

   (1) the shell is parsing the arguments on the echo command line.

   (2) the spacing between arguments is ignored

[[email protected] shelllearning]$ echo Please wait
Please wait
[[email protected] shelllearning]$ echo this     was     very      widely        spaced
this was very widely spaced
[[email protected] shelllearning]$ 

1.2 Writing Output but Preserving Spacing

Enclose the string in quotes.

[[email protected] shelllearning]$ echo "this    was    very   widely   spaced"
this    was    very   widely   spaced
[[email protected] shelllearning]$ echo 'this    was    very   widely   spaced'
this    was    very   widely   spaced
[[email protected] shelllearning]$ 

1.3 Writing Output with More Formatting Control

The numbers between the % and the format type (s or f) provide additional formatting details.

For a string , the first digit is the maximum field width, and the second is the minimum field width.

[[email protected] shelllearning]$ printf '%s = %d\n' Lines $LINES
Lines = 49
[[email protected] shelllearning]$ printf '%-10.10s = %4.2f\n' 'GigaHerz' 1.92735
GigaHerz   = 1.93
[[email protected] shelllearning]$

1.4 Writing Output Without the Newline

Using printf ——just leave of the ending \n in your format string. with echo use the -n option.

[[email protected] shelllearning]$ printf "%s %s" next promt
next promt[[email protected] shelllearning]$ 
[[email protected] shelllearning]$ echo -n prompt
prompt[[email protected] shelllearning]$ 
[[email protected] shelllearning]$ 

1.5 Saving Output from a Command

Use the > symbol to tell the shell to redirect the output into a file.

The cat command get its name from the longer word concatenation. The cat command concatenates  the output from the several files listed on its command line.

[[email protected] shelllearning]$ echo fill it up
fill it up
[[email protected] shelllearning]$ echo fill it up > file.txt | cat file.txt
fill it up
[[email protected] shelllearning]$ echo fill it up one  > first.half
[[email protected] shelllearning]$ echo fill it up two  > second.half
[[email protected] shelllearning]$ cat first.half second.half > whole.file | cat whole.file
fill it up one
fill it up two
[[email protected] shelllearning]$ 

1.6 Saving Output to Other Files

Use more of  a pathname when you redirect the output.

[[email protected] test]$ echo some more data > /tmp/echo.out
[[email protected] test]$ # or
[[email protected] test]$ echo some more data > ../../over.here
[[email protected] test]$ 

1.7 Saving Output from the ls Command

Use the -C option on ls when you redirect the output.

[[email protected] test]$ echo you are a  > a.out
[[email protected] test]$ echo he is a congfigure > cong.txt
[[email protected] test]$ echo my name is def > def.conf
[[email protected] test]$ echo that is a good file > file.txt
[[email protected] test]$ echo there are more foods > more.txt
[[email protected] test]$ echo I like the zebra which is a animal > zebra.list
[[email protected] test]$ ls
a.out  cong.txt  def.conf  file.txt  more.txt  zebra.list
[[email protected] test]$ ls > /tmp/save.out | cat /tmp/save.out
[[email protected] test]$ cat /tmp/save.out
a.out
cong.txt
def.conf
file.txt
more.txt
zebra.list
[[email protected] test]$ ls -C > /tmp/save.out
[[email protected] test]$ cat /tmp/save.out
a.out  cong.txt  def.conf  file.txt  more.txt  zebra.list
[[email protected] test]$ ls -l
total 24
-rw-rw-r-- 1 maxwell maxwell 10 Jul 21 15:39 a.out
-rw-rw-r-- 1 maxwell maxwell 19 Jul 21 15:40 cong.txt
-rw-rw-r-- 1 maxwell maxwell 15 Jul 21 15:40 def.conf
-rw-rw-r-- 1 maxwell maxwell 20 Jul 21 15:40 file.txt
-rw-rw-r-- 1 maxwell maxwell 21 Jul 21 15:41 more.txt
-rw-rw-r-- 1 maxwell maxwell 35 Jul 21 15:42 zebra.list
[[email protected] test]$ 
[[email protected] test]$ 

1.8 Sending Both Output and Error Messages to Different Files.

Redirect output and error messages to different files

[[email protected] test]$ myprogram 1> messages.out 2> message.err

or more commonly

[[email protected] test]$ myprogram > messages.out 2> message.err

1.9 Sending Both Output and Error Messages to the Same File

Use the shell syntax to redirect standard error messages to the same place as standard output.

[[email protected] test]$ both >& outfile

# or

[[email protected] test]$ both &> outfile

&> or >& is a shortcut that simply sends both STDOUT and STDERR to the same place --exactly what we want to do.

1.10 Appending Rather Than Clobbering Output

[[email protected] test]$ ls
a.out  cong.txt  def.conf  file.txt  more.txt  zebra.list
[[email protected] test]$ ls > /tmp/ls.out
[[email protected] test]$ cd ../elsewhere
[[email protected] elsewhere]$ ls >> /tmp/ls.out
[[email protected] elsewhere]$ cd ../anotherdir
[[email protected] anotherdir]$ ls >> /tmp.ls.out
-bash: /tmp.ls.out: Permission denied
[[email protected] anotherdir]$ ls >> /tmp/ls.out
[[email protected] anotherdir]$ cat /tmp/ls.out
a.out
cong.txt
def.conf
file.txt
more.txt
zebra.list
a1.out
cong1.txt
def1.conf
file1.txt
more1.txt
zebra1.list
TestA.txt
TestB.txt
TestC.txt
[[email protected] anotherdir]$

1.11 Using Just the Beginning or End of a File

use the head or tail commands.head will output the first 10 lines and tail will output the last 10 lines of the given file.

1.12 Throwing Output Away

 Redirect the output to /dev/null as shown in these examples:

[[email protected] test]$ find / -name myfile -print 2> /dev/null
[[email protected] test]$ noisy >/dev/null 2>&1

1.13 Saving or Grouping Output from Several Commands

Use braces { } to group these commands together, then redirection applies to the output from all commands in the group.

[[email protected] test]$ { pwd; ls; cd ../elsewhere; pwd; ls; } > /tmp/all.out
[[email protected] elsewhere]$ (pwd; ls; cd ../elsewhere; pwd; ls;) > /tmp/alla.out
[[email protected] elsewhere]$ cat /tmp/all.out
/home/maxwell/shelllearning/test
a.out
asdfwednnrw2rs2esff.txt
cong.txt
def.conf
file.txt
lines
more.txt
zebra.list
/home/maxwell/shelllearning/elsewhere
a1.out
cong1.txt
def1.conf
file1.txt
more1.txt
zebra1.list
[[email protected] elsewhere]$ cat /tmp/alla.out
/home/maxwell/shelllearning/elsewhere
a1.out
cong1.txt
def1.conf
file1.txt
more1.txt
zebra1.list
/home/maxwell/shelllearning/elsewhere
a1.out
cong1.txt
def1.conf
file1.txt
more1.txt
zebra1.list
[[email protected] elsewhere]$ 

1.14 Connecting Two Programs by Using Output As Input

[[email protected] test]$ cat one.file another.file > /tmp/cat.out
[[email protected] test]$ sort < /tmp/cat.out
1
23
345
4567
56789
678910
a
ab
abc
abcd
abcde
abcdef
abcdefg
abcdefgh
abcdefghi
abcdefghij
adjajnsojqo
dfjajdjado
sadjajdfc
sdjdasfa
[[email protected] test]$ cat one.file another.file | sort 
1
23
345
4567
56789
678910
a
ab
abc
abcd
abcde
abcdef
abcdefg
abcdefgh
abcdefghi
abcdefghij
adjajnsojqo
dfjajdjado
sadjajdfc
sdjdasfa
[[email protected] test]$