git – Show/diff parents of a merge commit

git cat-file -p <commit>

or

git show --pretty=raw <commit>

Both these show similar information: the committer, the parent(s), commit text and so on.

Now that you have the two parents of a merged request (see above tip), you can see all files changed between its two parents with a command like this:

git diff --name-only <sha1> <sha2>

Useful journald tweaks

Flush old logs in journalctl

By date or by size:

    sudo journalctl --vacuum-time=2d
    sudo journalctl --vacuum-size=500M

Tail journalctl

journalctl -f

For a specific service:

journalctl -u httpd -f

Store logs on disk

(from http://unix.stackexchange.com/questions/159221/how-display-log-messages-from-previous-boots-under-centos-7)

On CentOS 7, you have to enable the persistent storage of log messages:

# mkdir /var/log/journal
# systemd-tmpfiles --create --prefix /var/log/journal
# systemctl restart systemd-journald

Otherwise, the journal log messages are not retained between boots. This is the default on Fedora 19+.

Useful RPM/YUM commands

Sort RPMs by size

rpm -qa --queryformat '%{size} %{name}\n' | sort -rn | more

Extract rpm into current folder instead of installing:

rpm2cpio boost-system-1.53.0-23.el7.x86_64.rpm | cpio -idmv

Trace a binary or file to the RPM that installed it:

yum whatprovides /usr/lib64/libdbus-c++-1.so.0

or this:

rpm -qf /usr/lib64/libdbus-c++-1.so.0

Yum/dnf revert

If a yum remove wiped out several packages, do this:

• dnf history # note the id of the bad removal here
• dnf history undo 96

yum/dnf will reinstall all the packages that were removed in that id.

Dependencies of a package

This command shows what other packages need the queried package:

repoquery --whatrequires libunwind

Another way, without using repoquery:

rpm -q --whatrequires python-ipaddress

This command shows what other packages need to be installed for a queried package:

yum deplist nginx

Suppress echo in bash for reading passwords

In bash, while reading input from the user, if you want to suppress the echo on the screen (for sensitive inputs like passwords), do this:

stty -echo
read SECRETPASSWD
stty echo

See whitespace with cat

Use this:

cat -v -t -e <somefile>

• -e: Add a trailing $ at the end of a line.
• -t: Show tabs as ^I

Deleting files with odd names

There’s more than one way. Here’s one: find the inode with ls -i, then delete with:

find -inum <inode-number> -exec rm -i {} \;