在ubuntu系統上安裝指定版本的gfortran，具體步驟如下：
zhao@ubuntu:~$ sudo apt-get install gfortran-6
Reading package lists... Done
Building dependency tree
Reading state information... Done
The following packages were automatically installed and are no longer required:
libdumbnet1 libllvm7
Use 'sudo apt autoremove' to remove them.
**The following additional packages will be installed:**
*cpp-6 gcc-6 gcc-6-base libasan3 libgcc-6-dev libgfortran-6-dev libgfortran3*
Suggested packages:
gcc-6-locales gcc-6-multilib gcc-6-doc libgcc1-dbg libgomp1-dbg libitm1-dbg libatomic1-dbg libasan3-dbg
liblsan0-dbg libtsan0-dbg libubsan0-dbg libcilkrts5-dbg libmpx2-dbg libquadmath0-dbg gfortran-6-multilib
gfortran-6-doc libgfortran3-dbg libcoarrays-dev
The following NEW packages will be installed:
cpp-6 gcc-6 gcc-6-base gfortran-6 libasan3 libgcc-6-dev libgfortran-6-dev libgfortran3
0 upgraded, 8 newly installed, 0 to remove and 37 not upgraded.
Need to get 23.4 MB of archives.
After this operation, 89.6 MB of additional disk space will be used.
Do you want to continue? [Y/n]
zhao@ubuntu:~$ sudo apt-get install gfortran-6
Reading package lists... Done
Building dependency tree
Reading state information... Done
The following packages were automatically installed and are no longer required:
libdumbnet1 libllvm7
Use 'sudo apt autoremove' to remove them.
The following additional packages will be installed:
cpp-6 gcc-6 gcc-6-base libasan3 libgcc-6-dev libgfortran-6-dev libgfortran3
Suggested packages:
gcc-6-locales gcc-6-multilib gcc-6-doc libgcc1-dbg libgomp1-dbg libitm1-dbg libatomic1-dbg libasan3-dbg
liblsan0-dbg libtsan0-dbg libubsan0-dbg libcilkrts5-dbg libmpx2-dbg libquadmath0-dbg gfortran-6-multilib
gfortran-6-doc libgfortran3-dbg libcoarrays-dev
The following NEW packages will be installed:
cpp-6 gcc-6 gcc-6-base gfortran-6 libasan3 libgcc-6-dev libgfortran-6-dev libgfortran3
0 upgraded, 8 newly installed, 0 to remove and 37 not upgraded.
Need to get 23.4 MB of archives.
After this operation, 89.6 MB of additional disk space will be used.
Do you want to continue? [Y/n] y
Get:1 http://mirrors.aliyun.com/ubuntu bionic-updates/universe amd64 gcc-6-base amd64 6.5.0-2ubuntu1~18.04 [16.7 kB]
Get:2 http://mirrors.aliyun.com/ubuntu bionic-updates/universe amd64 cpp-6 amd64 6.5.0-2ubuntu1~18.04 [6,396 kB]
Get:3 http://mirrors.aliyun.com/ubuntu bionic-updates/universe amd64 libasan3 amd64 6.5.0-2ubuntu1~18.04 [313 kB]
Get:4 http://mirrors.aliyun.com/ubuntu bionic-updates/universe amd64 libgcc-6-dev amd64 6.5.0-2ubuntu1~18.04 [2,308 kB]
Get:5 http://mirrors.aliyun.com/ubuntu bionic-updates/universe amd64 gcc-6 amd64 6.5.0-2ubuntu1~18.04 [7,067 kB]
Get:6 http://mirrors.aliyun.com/ubuntu bionic-updates/universe amd64 libgfortran3 amd64 6.5.0-2ubuntu1~18.04 [270 kB]
Get:7 http://mirrors.aliyun.com/ubuntu bionic-updates/universe amd64 libgfortran-6-dev amd64 6.5.0-2ubuntu1~18.04 [303 kB]
Get:8 http://mirrors.aliyun.com/ubuntu bionic-updates/universe amd64 gfortran-6 amd64 6.5.0-2ubuntu1~18.04 [6,769 kB]
Fetched 23.4 MB in 16s (1,439 kB/s)
Selecting previously unselected package gcc-6-base:amd64.
(Reading database ... 184523 files and directories currently installed.)
Preparing to unpack .../0-gcc-6-base_6.5.0-2ubuntu1~18.04_amd64.deb ...
Unpacking gcc-6-base:amd64 (6.5.0-2ubuntu1~18.04) ...
Selecting previously unselected package cpp-6.
Preparing to unpack .../1-cpp-6_6.5.0-2ubuntu1~18.04_amd64.deb ...
Unpacking cpp-6 (6.5.0-2ubuntu1~18.04) ...
Selecting previously unselected package libasan3:amd64.
Preparing to unpack .../2-libasan3_6.5.0-2ubuntu1~18.04_amd64.deb ...
Unpacking libasan3:amd64 (6.5.0-2ubuntu1~18.04) ...
Selecting previously unselected package libgcc-6-dev:amd64.
Preparing to unpack .../3-libgcc-6-dev_6.5.0-2ubuntu1~18.04_amd64.deb ...
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Selecting previously unselected package gcc-6.
Preparing to unpack .../4-gcc-6_6.5.0-2ubuntu1~18.04_amd64.deb ...
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Selecting previously unselected package libgfortran3:amd64.
Preparing to unpack .../5-libgfortran3_6.5.0-2ubuntu1~18.04_amd64.deb ...
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Preparing to unpack .../6-libgfortran-6-dev_6.5.0-2ubuntu1~18.04_amd64.deb ...
Unpacking libgfortran-6-dev:amd64 (6.5.0-2ubuntu1~18.04) ...
Selecting previously unselected package gfortran-6.
Preparing to unpack .../7-gfortran-6_6.5.0-2ubuntu1~18.04_amd64.deb ...
Unpacking gfortran-6 (6.5.0-2ubuntu1~18.04) ...
Setting up gcc-6-base:amd64 (6.5.0-2ubuntu1~18.04) ...
Setting up cpp-6 (6.5.0-2ubuntu1~18.04) ...
Setting up libasan3:amd64 (6.5.0-2ubuntu1~18.04) ...
Setting up libgcc-6-dev:amd64 (6.5.0-2ubuntu1~18.04) ...
Setting up libgfortran3:amd64 (6.5.0-2ubuntu1~18.04) ...
Setting up libgfortran-6-dev:amd64 (6.5.0-2ubuntu1~18.04) ...
Setting up gcc-6 (6.5.0-2ubuntu1~18.04) ...
Setting up gfortran-6 (6.5.0-2ubuntu1~18.04) ...
Processing triggers for man-db (2.8.3-2ubuntu0.1) ...
Processing triggers for libc-bin (2.27-3ubuntu1) ...
zhao@ubuntu:~$
zhao@ubuntu:~$
安裝完成後，如何調用，如下：
zhao@ubuntu:~$
zhao@ubuntu:~$
zhao@ubuntu:~$ gfortran-6 --version
GNU Fortran (Ubuntu 6.5.0-2ubuntu1~18.04) 6.5.0 20181026
Copyright (C) 2017 Free Software Foundation, Inc.
This is free software; see the source for copying conditions. There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
zhao@ubuntu:~$ gfortran --version
Command 'gfortran' not found, but can be installed with:
sudo apt install gfortran
zhao@ubuntu:~$
zhao@ubuntu:~$
可見，如果安裝時，指定了版本，那麼調用時，也需要指定版本。