conditions elif | sed -n q;d awk

cat rd

if [ -r "$1" ]; then

  echo "$1 is readable"

else

  echo "Error to read $1"

fi

$ bash rd file

file is readable

$ bash rd file1

Error to read file1

--------------

cat el

OS=`uname -s`

if [ "$OS" = "Linux" ]; then

  echo "This is Linux"

elif [ "$OS" = "Solaris" ]; then

  echo "This is Solaris"

else

  echo "This is Unix"

fi

$ bash el

This is Linux

--------------

if [ -e /etc/resolv.conf ]; then

-e stands for exists

-L stands for symbolic link

-S stands for socket

-p stands for pipe

-d stands for directory

 

--------------

#test symbolic link

$ cat sy

file=$1

if [ -L $file ]; then

  echo "this is symbolic link"

else

  echo "not identified file"

fi

$ bash sy filesym1

this is symbolic link

--------------

Replace particular line 3:

sed 3s/echo/read/g el > el3

Print line 3:

cat el | awk 'NR==3{print}'

Print column 3:

cat el | awk '{print $1}'

#print line number 2

sed '2q;d' file # method efficient for large files

#other methods, print lines 2 and 3

sed '2,3!d' file

#tell sed to delete all but the second and third lines

Alternative method for print lines from 2 to 4

#
sed -n 2,4p file

sed -n -e 1p -e 3,4p file

#print lines 1, 3 and 4

---------------------

cat file2

some text

new text

aaaa text

bbbb cccc

new ddddd

awk '$1=="new"{$2="NEW"}1' file2

some text

new NEW

aaaa text

bbbb cccc

new NEW

---------------------

$ cat comp

file1=$1

file2=$2

ls $file1 $file2 -il

if [ $file1 -ef $file2 ]; then

  echo "files are similar"

else

  echo "$file1 is not the same as $file2"

  diff -q $file1 $file2

    if [ "$?" -eq "0" ]; then

      echo "contents is the same"

    fi

fi
--------------------

$ bash comp file1 file3

2130443 -rw-r--r-- 2 x 10 Jul 23 13:00 file1

2130443 -rw-r--r-- 2 x 10 Jul 23 13:00 file3

files are similar

$ bash comp file1 file2

2130443 -rw-r--r-- 2 x 10 Jul 23 13:00 file1

2130444 -rw-r--r-- 1 x 10 Jul 23 13:00 file2

file1 is not the same as file2

contents is the same

---------------------

if [ new –nt old ]; then #true if new is newer than older file

if [ new –ot old ]; then #true if new is older than older file

---------------------

Bash compare 2 strings

If [ “$1”=”$2” ]; then

Bash shell, and other, will also accept a double ==, which is more in line with other languages

However traditional Bourne shell does not recognize this syntax. It’s best to use single = as that is understood everywhere

For inequality

if [ “$1”!=”$2” ]; then

---------------------

If you want to change a pathname that contains a slash - say /usr/local/bin to /common/bin - you could use the backslash to quote the slash:

sed 's/\/usr\/local\/bin/\/common\/bin/' <old >new
Gulp. Some call this a 'Picket Fence' and it's ugly. It is easier to read if you use an underline instead of a slash as a delimiter:

sed 's_/usr/local/bin_/common/bin_' <old >new
Some people use colons:

sed 's:/usr/local/bin:/common/bin:' <old >new
Others use the "|" character.

sed 's|/usr/local/bin|/common/bin|' <old >new