How to Fix Linux apt 'Could Not Get Lock' Errors (Ubuntu)
sudo apt install says 'Could not get lock /var/lib/dpkg/lock-frontend'. On a real Ubuntu 24.04 server we made every common cause happen for real: Ubuntu's automatic security updates holding the lock, and a second admin's apt left at its yes/no question. We read who holds it, wait the right way, and show what the two forum shortcuts really do: deleting the lock file, and killing the process (dpkg was interrupted).
Could not get lock
You typed sudo apt install, and Ubuntu said: could not get lock. On this library's catalog server, the same install ran twice, moments apart. apt-get gave up at once. Plain apt waited fifteen seconds, then installed it. Same holder both times.
Ana's error names the lock, the process holding it, and a warning: removing the lock file is not a solution. Most forum answers say: delete it. Do that while apt is writing, and you can break the package database. We'll see how.
sudo apt-get install -y htopE: Could not get lock /var/lib/dpkg/lock-frontend. It is held by process 471 (unattended-upgr) N: Be aware that removing the lock file is not a solution and may break your system. E: Unable to acquire the dpkg frontend lock (/var/lib/dpkg/lock-frontend), is another process using it?
What the lock is
So what is the lock? apt is Ubuntu's installer, and underneath it, dpkg does the actual work. dpkg keeps a list of every installed package and its files: its database. Anything that installs or updates software must change it: apt, apt-get, the automatic security updates, or a second admin. Two writers at once would scramble it, so each must first take one lock. Whoever is first gets it. Everyone else gets: could not get lock.
Think of the one key to a shared stockroom. Whoever holds the key is inside, moving boxes. You don't break the door. You find out who has the key, and you wait for them to come out. The rest of this video is that, on a real server.
The quick fix: read it, then wait
The quick fix starts with reading the error. Every running program has a number, its process ID, and the error gives it: four hundred seventy-one, unattended upgrade. The ps command lists running programs, and shows what that one is: Ubuntu's automatic security updates. And systemctl, the tool for background services, says their daily job is still activating: at work right now. apt-get gave up. Guess first: what does plain apt do in the same spot? It waits. Waiting for cache lock, fifteen seconds, and then it installs.
ps -o pid,etime,cmd -p 471PID ELAPSED CMD 471 00:00 /usr/bin/python3 /usr/bin/unattended-upgrade
systemctl is-active apt-daily-upgrade.serviceactivating
sudo apt install -y htopWaiting for cache lock: Could not get lock /var/lib/dpkg/lock-frontend. It is held by process 471 (unattended-upgr)... 15s Reading package lists... Done Building dependency tree... Done Reading state information... Done The following additional packages will be installed: libnl-3-200 libnl-genl-3-200 Suggested packages: lm-sensors strace The following NEW packages will be installed: htop libnl-3-200 libnl-genl-3-200 0 upgraded, 3 newly installed, 0 to remove and 22 not upgraded. Need to get 240 kB of archives. After this operation, 791 kB of additional disk space will be used. Get:1 http://ports.ubuntu.com/ubuntu-ports noble-updates/main arm64 libnl-3-200 arm64 3.7.0-0.3build1.1 [57.5 kB] Get:2 http://ports.ubuntu.com/ubuntu-ports noble-updates/main arm64 libnl-genl-3-200 arm64 3.7.0-0.3build1.1 [12.2 kB] Get:3 http://ports.ubuntu.com/ubuntu-ports noble/main arm64 htop arm64 3.3.0-4build1 [171 kB] Fetched 240 kB in 1s (300 kB/s) debconf: delaying package configuration, since apt-utils is not installed Selecting previously unselected package libnl-3-200:arm64. (Reading database ... 7951 files and directories currently installed.) Preparing to unpack .../libnl-3-200_3.7.0-0.3build1.1_arm64.deb ... Unpacking libnl-3-200:arm64 (3.7.0-0.3build1.1) ... Selecting previously unselected package libnl-genl-3-200:arm64. Preparing to unpack .../libnl-genl-3-200_3.7.0-0.3build1.1_arm64.deb ... Unpacking libnl-genl-3-200:arm64 (3.7.0-0.3build1.1) ... Selecting previously unselected package htop. Preparing to unpack .../htop_3.3.0-4build1_arm64.deb ... Unpacking htop (3.3.0-4build1) ... Setting up libnl-3-200:arm64 (3.7.0-0.3build1.1) ... Setting up libnl-genl-3-200:arm64 (3.7.0-0.3build1.1) ... Setting up htop (3.3.0-4build1) ... Processing triggers for libc-bin (2.39-0ubuntu8.12) ...
Why do the two behave differently? apt is made for people at a keyboard. apt-get, the older one, is made for scripts, so its output never changes between versions. And when the lock is busy, apt waits, while apt-get gives up at once, unless you tell it to wait. So if a tutorial's apt-get line failed on Ana's server, typing it again with plain apt is a fine quick fix.
Who holds it on a fresh server
Why was it busy on a server nobody had touched? Ubuntu's own documentation says it. Security updates are installed automatically, by a package that is there by default. And if the server was off at the scheduled time, the job may run right after it starts, and other installs have to wait.
On the catalog server, it's a timer: six in the morning, plus up to an hour of random delay, and persistent, meaning a missed run happens as soon as the server is back. We started the same job by hand for this demo. The last lines of its log show why it held the lock: thirty packages to upgrade, then all upgrades installed, and the job is now inactive. Here it took seconds. On a server that has been off for months, it can take several minutes. Let it finish.
systemctl cat apt-daily-upgrade.timer# /usr/lib/systemd/system/apt-daily-upgrade.timer [Unit] Description=Daily apt upgrade and clean activities After=apt-daily.timer [Timer] OnCalendar=*-*-* 6:00 RandomizedDelaySec=60m Persistent=true [Install] WantedBy=timers.target
sudo tail -n 3 /var/log/unattended-upgrades/unattended-upgrades.log | cut -c 12-9517:49:35,979 INFO Packages that will be upgraded: apt coreutils diffutils dpkg gcc-1 17:49:35,979 INFO Writing dpkg log to /var/log/unattended-upgrades/unattended-upgrad 17:49:47,515 INFO All upgrades installed
systemctl is-active apt-daily-upgrade.serviceinactive
Who holds it: another terminal
Who else can hold the lock? On another day, Ana's error names plain apt. ps shows its command line and how long it has run: an install of build tools, started one minute ago. And who typed it? The sudo processes say: Ben. One more check: L S of lists who has a file open. In its F D column, the file handle, the W means apt holds the write lock.
sudo apt-get install -y treeE: Could not get lock /var/lib/dpkg/lock-frontend. It is held by process 1454 (apt) N: Be aware that removing the lock file is not a solution and may break your system. E: Unable to acquire the dpkg frontend lock (/var/lib/dpkg/lock-frontend), is another process using it?
ps -o pid,tty,etime,cmd -p 1454PID TT ELAPSED CMD 1454 pts/2 01:15 apt install build-essential
ps -o ruser,etime,cmd -C sudoRUSER ELAPSED CMD ben 01:15 sudo apt install build-essential ben 01:15 sudo apt install build-essential
sudo lsof /var/lib/dpkg/lock-frontendCOMMAND PID USER FD TYPE DEVICE SIZE/OFF NODE NAME apt 1454 root 4uW REG 0,63 0 959045 /var/lib/dpkg/lock-frontend
Ben's terminal explains it. apt asked: do you want to continue? And Ben walked away. A question waiting for an answer still holds the lock. Ben answers no, and Ana's install goes straight through.
sudo apt install build-essentialReading package lists... Done Building dependency tree... Done Reading state information... Done The following additional packages will be installed: binutils binutils-aarch64-linux-gnu binutils-common bzip2 cpp cpp-13 cpp-13-aarch64-linux-gnu cpp-aarch64-linux-gnu dirmngr dpkg-dev fakeroot fontconfig-config fonts-dejavu-core fonts-dejavu-mono g++ g++-13 g++-13-aarch64-linux-gnu g++-aarch64-linux-gnu gcc gcc-13 gcc-13-aarch64-linux-gnu gcc-13-base gcc-aarch64-linux-gnu gnupg gnupg-l10n gnupg-utils gpg gpg-agent gpg-wks-client gpgconf gpgsm keyboxd libalgorithm-diff-perl libalgorithm-diff-xs-perl libalgorithm-merge-perl libaom3 libasan8 libatomic1 libbinutils libbrotli1 libbsd0 libc-dev-bin libc-devtools libc6-dev libcc1-0 libcrypt-dev libctf-nobfd0 libctf0 libdeflate0 libdpkg-perl libfakeroot libfile-fcntllock-perl libfontconfig1 libfreetype6 libgcc-13-dev libgd3 libgdbm-compat4t64 libgdbm6t64 libgomp1 libgprofng0 libheif-plugin-aomdec libheif-plugin-aomenc libheif1 libhwasan0 libisl23 libitm1 libjansson4 libjbig0 libjpeg-turbo8 libjpeg8 libksba8 libldap-common libldap2 liblerc4 liblocale-gettext-perl liblsan0 libmpc3 libmpfr6 libperl5.38t64 libpng16-16t64 libsasl2-2 libsasl2-modules libsasl2-modules-db libsframe1 libsharpyuv0 libstdc++-13-dev libtiff6 libtsan2 libubsan1 libwebp7 libx11-6 libx11-data libxau6 libxcb1 libxdmcp6 libxpm4 linux-libc-dev lto-disabled-list make manpages manpages-dev patch perl perl-modules-5.38 pinentry-curses rpcsvc-proto Suggested packages: binutils-doc gprofng-gui bzip2-doc cpp-doc gcc-13-locales cpp-13-doc dbus-user-session libpam-systemd pinentry-gnome3 tor debian-keyring gcc-13-doc gcc-multilib autoconf automake libtool flex bison gdb gcc-doc gdb-aarch64-linux-gnu parcimonie xloadimage gpg-wks-server scdaemon glibc-doc git bzr libgd-tools gdbm-l10n libheif-plugin-libde265 libheif-plugin-x265 libheif-plugin-ffmpegdec libheif-plugin-jpegdec libheif-plugin-jpegenc libheif-plugin-j2kdec libheif-plugin-j2kenc libheif-plugin-rav1e libheif-plugin-svtenc libsasl2-modules-gssapi-mit | libsasl2-modules-gssapi-heimdal libsasl2-modules-ldap libsasl2-modules-otp libsasl2-modules-sql libstdc++-13-doc make-doc man-browser ed diffutils-doc perl-doc libterm-readline-gnu-perl | libterm-readline-perl-perl libtap-harness-archive-perl pinentry-doc The following NEW packages will be installed: binutils binutils-aarch64-linux-gnu binutils-common build-essential bzip2 cpp cpp-13 cpp-13-aarch64-linux-gnu cpp-aarch64-linux-gnu dirmngr dpkg-dev fakeroot fontconfig-config fonts-dejavu-core fonts-dejavu-mono g++ g++-13 g++-13-aarch64-linux-gnu g++-aarch64-linux-gnu gcc gcc-13 gcc-13-aarch64-linux-gnu gcc-13-base gcc-aarch64-linux-gnu gnupg gnupg-l10n gnupg-utils gpg gpg-agent gpg-wks-client gpgconf gpgsm keyboxd libalgorithm-diff-perl libalgorithm-diff-xs-perl libalgorithm-merge-perl libaom3 libasan8 libatomic1 libbinutils libbrotli1 libbsd0 libc-dev-bin libc-devtools libc6-dev libcc1-0 libcrypt-dev libctf-nobfd0 libctf0 libdeflate0 libdpkg-perl libfakeroot libfile-fcntllock-perl libfontconfig1 libfreetype6 libgcc-13-dev libgd3 libgdbm-compat4t64 libgdbm6t64 libgomp1 libgprofng0 libheif-plugin-aomdec libheif-plugin-aomenc libheif1 libhwasan0 libisl23 libitm1 libjansson4 libjbig0 libjpeg-turbo8 libjpeg8 libksba8 libldap-common libldap2 liblerc4 liblocale-gettext-perl liblsan0 libmpc3 libmpfr6 libperl5.38t64 libpng16-16t64 libsasl2-2 libsasl2-modules libsasl2-modules-db libsframe1 libsharpyuv0 libstdc++-13-dev libtiff6 libtsan2 libubsan1 libwebp7 libx11-6 libx11-data libxau6 libxcb1 libxdmcp6 libxpm4 linux-libc-dev lto-disabled-list make manpages manpages-dev patch perl perl-modules-5.38 pinentry-curses rpcsvc-proto 0 upgraded, 107 newly installed, 0 to remove and 22 not upgraded. Need to get 86.7 MB of archives. After this operation, 321 MB of additional disk space will be used. Do you want to continue? [Y/n] n Abort.
Why deleting the lock file is wrong
Now the forum shortcut. There are two lock files: lock front end for apt, and lock for dpkg, the engine underneath. With nothing running, they're still there, empty, and L S of finds nobody holding them. The file is just the hook the key hangs on. The lock is a running process holding it, and Linux drops it the moment that process ends.
ls -l /var/lib/dpkg/lock*-rw-r----- 1 root root 0 Nov 13 17:51 /var/lib/dpkg/lock -rw-r----- 1 root root 0 Apr 23 2024 /var/lib/dpkg/lock-frontend
sudo lsof /var/lib/dpkg/lock-frontend
So deleting the file while apt runs doesn't stop anything. The next apt makes a fresh file, locks that one, and walks in while the first is still changing the database. Two writers, one database: exactly what the lock was there to prevent. That's why apt's own warning says it may break your system.
What killing the holder costs
The other shortcut is to kill the process that holds the lock. Ben runs his install again, this time for real. Ana, blocked, takes the process ID from her error and kills it with kill dash nine, which stops a process with no chance to clean up: someone dragged out of the stockroom mid-box. Ben's terminal says: killed. Guess first: what does Ana's next install say? dpkg was interrupted.
sudo apt-get install -y jqE: Could not get lock /var/lib/dpkg/lock-frontend. It is held by process 1505 (apt) N: Be aware that removing the lock file is not a solution and may break your system. E: Unable to acquire the dpkg frontend lock (/var/lib/dpkg/lock-frontend), is another process using it?
sudo kill -9 1505
sudo apt-get install -y jqE: dpkg was interrupted, you must manually run 'sudo dpkg --configure -a' to correct the problem.
The lock is free, but Ben's install stopped halfway, and apt refuses to touch the database until that's repaired. dpkg dash dash configure dash a finishes what was left. dpkg dash dash audit checks for broken packages, and still lists one in a mess, half unpacked when the kill came. Running Ben's install once more puts it back, and the audit comes up empty. Recoverable here, but a kill during a big upgrade can leave far more to repair.
sudo dpkg --configure -a
sudo dpkg --auditThe following packages are in a mess due to serious problems during installation. They must be reinstalled for them (and any packages that depend on them) to function properly: liblocale-gettext-perl (no description available)
sudo dpkg --audit
In scripts: wait for the lock
One place where waiting doesn't happen by itself: scripts. Ana's setup script for the library's server uses apt-get, as scripts should, for its stable output, and Ben has an upgrade question open again. The script fails on its first line and stops, and whatever came after never runs.
cat setup.sh#!/bin/bash set -e sudo apt-get install -y curl echo "setup done"
bash setup.shE: Could not get lock /var/lib/dpkg/lock-frontend. It is held by process 2970 (apt) N: Be aware that removing the lock file is not a solution and may break your system. E: Unable to acquire the dpkg frontend lock (/var/lib/dpkg/lock-frontend), is another process using it?
The fix is one option, passed with a lowercase dash O: dpkg lock timeout, in seconds. Now apt-get waits, as plain apt does, but only that long. Ben answers his question, the lock comes free after about thirteen seconds, and the script finishes: setup done.
cat setup.sh#!/bin/bash set -e sudo apt-get -o DPkg::Lock::Timeout=300 install -y curl echo "setup done"
bash setup.shWaiting for cache lock: Could not get lock /var/lib/dpkg/lock-frontend. It is held by process 2970 (apt) Reading package lists... Done Building dependency tree... Done Reading state information... Done The following additional packages will be installed: libcurl4t64 libnghttp2-14 libpsl5t64 librtmp1 libssh-4 publicsuffix The following NEW packages will be installed: curl libcurl4t64 libnghttp2-14 libpsl5t64 librtmp1 libssh-4 publicsuffix 0 upgraded, 7 newly installed, 0 to remove and 22 not upgraded. Need to get 1070 kB of archives. After this operation, 2979 kB of additional disk space will be used. Get:1 http://ports.ubuntu.com/ubuntu-ports noble-updates/main arm64 libnghttp2-14 arm64 1.59.0-1ubuntu0.4 [74.8 kB] Get:2 http://ports.ubuntu.com/ubuntu-ports noble/main arm64 libpsl5t64 arm64 0.21.2-1.1build1 [57.4 kB] Get:3 http://ports.ubuntu.com/ubuntu-ports noble/main arm64 publicsuffix all 20231001.0357-0.1 [129 kB] Get:4 http://ports.ubuntu.com/ubuntu-ports noble/main arm64 librtmp1 arm64 2.4+20151223.gitfa8646d.1-2build7 [57.8 kB] Get:5 http://ports.ubuntu.com/ubuntu-ports noble-updates/main arm64 libssh-4 arm64 0.10.6-2ubuntu0.5 [192 kB] Get:6 http://ports.ubuntu.com/ubuntu-ports noble-updates/main arm64 libcurl4t64 arm64 8.5.0-2ubuntu10.16 [337 kB] Get:7 http://ports.ubuntu.com/ubuntu-ports noble-updates/main arm64 curl arm64 8.5.0-2ubuntu10.16 [222 kB] Fetched 1070 kB in 1s (940 kB/s) debconf: delaying package configuration, since apt-utils is not installed Selecting previously unselected package libnghttp2-14:arm64. (Reading database ... 17353 files and directories currently installed.) Preparing to unpack .../0-libnghttp2-14_1.59.0-1ubuntu0.4_arm64.deb ... Unpacking libnghttp2-14:arm64 (1.59.0-1ubuntu0.4) ... Selecting previously unselected package libpsl5t64:arm64. Preparing to unpack .../1-libpsl5t64_0.21.2-1.1build1_arm64.deb ... Unpacking libpsl5t64:arm64 (0.21.2-1.1build1) ... Selecting previously unselected package publicsuffix. Preparing to unpack .../2-publicsuffix_20231001.0357-0.1_all.deb ... Unpacking publicsuffix (20231001.0357-0.1) ... Selecting previously unselected package librtmp1:arm64. Preparing to unpack .../3-librtmp1_2.4+20151223.gitfa8646d.1-2build7_arm64.deb ... Unpacking librtmp1:arm64 (2.4+20151223.gitfa8646d.1-2build7) ... Selecting previously unselected package libssh-4:arm64. Preparing to unpack .../4-libssh-4_0.10.6-2ubuntu0.5_arm64.deb ... Unpacking libssh-4:arm64 (0.10.6-2ubuntu0.5) ... Selecting previously unselected package libcurl4t64:arm64. Preparing to unpack .../5-libcurl4t64_8.5.0-2ubuntu10.16_arm64.deb ... Unpacking libcurl4t64:arm64 (8.5.0-2ubuntu10.16) ... Selecting previously unselected package curl. Preparing to unpack .../6-curl_8.5.0-2ubuntu10.16_arm64.deb ... Unpacking curl (8.5.0-2ubuntu10.16) ... Setting up libpsl5t64:arm64 (0.21.2-1.1build1) ... Setting up libnghttp2-14:arm64 (1.59.0-1ubuntu0.4) ... Setting up librtmp1:arm64 (2.4+20151223.gitfa8646d.1-2build7) ... Setting up libssh-4:arm64 (0.10.6-2ubuntu0.5) ... Setting up publicsuffix (20231001.0357-0.1) ... Setting up libcurl4t64:arm64 (8.5.0-2ubuntu10.16) ... Setting up curl (8.5.0-2ubuntu10.16) ... Processing triggers for libc-bin (2.39-0ubuntu8.12) ... setup done
And for servers you start only now and then, the same Ubuntu page shows a second choice. Open the apt daily upgrade timer we saw earlier with systemctl edit, and set persistent to false, and a missed run waits for its next scheduled time instead of starting the moment the server boots. The updates still come, just not in your way.
Recap
So, could not get lock: read who holds it, and wait, or answer the question left open. Never delete the lock file, and don't kill the holder. If something was killed anyway, dpkg configure dash a. And in scripts, give apt-get a lock timeout.
If your install finally went through, subscribing brings more real fixes from real servers like this one.
Every lock in this video was held by a real process on a real Ubuntu server, and every command and the lab files are linked in the description. Thanks for watching Cloud Explainers.
Sources
- apt(8) on Ubuntu 24.04
- apt.conf(5), DPkg::Lock::Timeout (seconds to wait for the dpkg lock)
- dpkg(1), --configure -a and --audit
- Ubuntu Server documentation, Automatic updates (installed by default; may run at startup and make other installs wait)
- fcntl(2): a record lock is released when the process that holds it ends
Demos: a container on our own Mac standing in for a library's server; the library and its admins are made up. The security updates and every lock were real. Watch the full video on YouTube.
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