Ubuntu – Why does using double quotes to enclose awk’s action statements produce different results than when using single quotes to enclose them

awkcommand line

If I use single quotes to enclose awk's action statement, it works as expected:

$ dpkg -l | grep "linux\-[a-z]*\-" | awk {'print $2'}
linux-headers-3.13.0-27
linux-headers-3.13.0-27-generic
linux-headers-3.14.4-031404
linux-headers-3.14.4-031404-generic
linux-headers-generic
linux-image-3.13.0-27-generic
linux-image-3.14.4-031404-generic
linux-image-extra-3.13.0-27-generic
linux-image-generic
linux-libc-dev:i386
linux-sound-base

But if I use double quotes to enclose them, it prints the whole line instead of the particular column/field:

$ dpkg -l | grep "linux\-[a-z]*\-" | awk {"print $2"}
ii  linux-headers-3.13.0-27                    3.13.0-27.50                                           all          Header files related to Linux kernel version 3.13.0
ii  linux-headers-3.13.0-27-generic            3.13.0-27.50                                           i386         Linux kernel headers for version 3.13.0 on 32 bit x86 SMP
ii  linux-headers-3.14.4-031404                3.14.4-031404.201405130853                             all          Header files related to Linux kernel version 3.14.4
ii  linux-headers-3.14.4-031404-generic        3.14.4-031404.201405130853                             i386         Linux kernel headers for version 3.14.4 on 32 bit x86 SMP
ii  linux-headers-generic                      3.13.0.27.33                                           i386         Generic Linux kernel headers
ii  linux-image-3.13.0-27-generic              3.13.0-27.50                                           i386         Linux kernel image for version 3.13.0 on 32 bit x86 SMP
ii  linux-image-3.14.4-031404-generic          3.14.4-031404.201405130853                             i386         Linux kernel image for version 3.14.4 on 32 bit x86 SMP
ii  linux-image-extra-3.13.0-27-generic        3.13.0-27.50                                           i386         Linux kernel extra modules for version 3.13.0 on 32 bit x86 SMP
ii  linux-image-generic                        3.13.0.27.33                                           i386         Generic Linux kernel image
ii  linux-libc-dev:i386                        3.13.0-27.50                                           i386         Linux Kernel Headers for development
ii  linux-sound-base                           1.0.25+dfsg-0ubuntu4                                   all          base package for ALSA and OSS sound systems

Why is it so? Why does using double quotes to enclose awk's action statements produce different results than when using single quotes to enclose them? Can I do something so that enclosing using double quotes works like enclosing using single quotes?

Best Answer

If you use double quotes the $2 gets replaced by the shell before awk is called. As $2 is usually empty running

awk {"print $2"}

is the same as running

awk {"print "}

Use

awk {"print \$2"}

if you want to use double quotes.

`ps aux` includes the full command line (path and parameters), while pgrep only looks at the first 15 characters of the executable's names

ps aux returns the full command line of each process, while pgrep only looks at the names of the executables.

That means that grepping ps aux output will match anything that occurs in the path or the parameters of a process' binary: e.g. `

ps aux | grep php5 will match /usr/share/php5/i-am-a-perl-script.pl
but pgrep php5 won't

Take an example from my system -- only we'll use python instead of php5:

ps aux | grep python gives us:

izx       2348  0.0  0.7 514928 15644 ?        Sl   Jun24   0:00 /usr/bin/python /usr/lib/unity-lens-video/unity-lens-video
izx       2444  0.0  0.9 547392 18864 ?        Sl   Jun24   0:01 /usr/bin/python /usr/lib/unity-scope-video-remote/unity-scope-video-remote
root      2805  0.0  0.5  95436 12204 ?        S    Jun24   0:00 /usr/bin/python /usr/lib/system-service/system-service-d
izx       6272  0.0  2.9 664400 60320 ?        SNl  Jun24   1:16 /usr/bin/python /usr/bin/update-manager --no-focus-on-map
root     11729  0.0  0.9 180508 19516 ?        S    Jun25   0:00 python /usr/lib/software-properties/software-properties-dbus

But pgrep python returns only 11729, which you'll see from the above list is:

root     11729  0.0  0.9 180508 19516 ?        S    Jun25   0:00 python /usr/lib/software-properties/software-properties-dbus

Ubuntu – Why does running `time` and `/usr/bin/time` give different results

Because time and /usr/bin/time run different code. In order to determine it, do the following:

type time

It will show you that:

time is a shell keyword

Thus, it doesn't run /usr/bin/time if you simply call time.

They are different things...

Best Answer

Related Solutions

Command Line – Why ‘ps aux | grep x’ Gives Better Results Than ‘pgrep x’

ps aux includes the full command line (path and parameters), while pgrep only looks at the first 15 characters of the executable's names

Ubuntu – Why does running `time` and `/usr/bin/time` give different results

Related Question

`ps aux` includes the full command line (path and parameters), while pgrep only looks at the first 15 characters of the executable's names