Background
I’m using Ubuntu on Windows 11 WSL2 and installed qemu via apt. However, the qemu available through apt was quite old, so I decided to build the latest version from source code. I uninstalled the existing qemu and installed the newly built latest version of qemu.
When I ran qemu-system-x86_64 --version to check if qemu was installed, I got the following error:
-bash: /usr/bin/qemu-system-x86_64: No such file or directory
Thought Process Leading to the Solution
The environment variables seemed fine.
echo $PATH
/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin: ...
Was qemu-system-x86_64 actually installed? I ran the following command to check:
which qemu-system-x86_64
/usr/local/bin/qemu-system-x86_64
“/usr/local/bin/”…? The earlier error message was looking for “/usr/bin/qemu-system-x86_64”, so it seems the correct location of the command wasn’t being recognized.
When I asked AI, it suggested creating a symbolic link from “/usr/bin/qemu-system-x86_64” to “/usr/local/bin/qemu-system-x86_64”, and indeed the command started working. However, I didn’t think this was a fundamental solution.
Root Cause Identified
While investigating further, I discovered a new fact. Apparently, in bash, once a command is executed, the full path of the command executable is recorded in a hash table. From then on, bash refers to this hash table instead of searching for the executable.
I suspected this might be the cause and displayed the contents of the hash table:
hash
1 /usr/bin/find
21 /usr/bin/qemu-system-x86_64
14 /usr/bin/ls
3 /usr/bin/make
2 /usr/bin/chown
19 /usr/bin/sudo
Bingo! This hash table was probably causing bash to look for “/usr/bin/qemu-system-x86_64” during command execution.
When I first installed the old version of qemu, it was probably installed to “/usr/bin/qemu-system-x86_64”.
Since I ran qemu-system-x86_64 --version several times while using the old version, it was recorded in the hash table, and even after the new version was installed to “/usr/local/bin/qemu-system-x86_64”, bash was still looking for “/usr/bin/qemu-system-x86_64”.
Solution
By running the following command, qemu-system-x86_64 started working correctly:
PATH="$PATH"
This may seem meaningless as it replaces the current environment variable with itself, but this resets the command cache.
I later learned that running hash -r can empty the hash table contents, so that would work too.
References
【Linux】PATHについて #Linux - Qiita Linuxコマンドはどうやって実行されている？環境変数PATHが設定される流れを調べてみた - Devplatform blog コマンド hash bash のハッシュテーブル管理用コマンド。bash の内部コマンド。 - UNIX/Linuxの部屋 hashコマンドの使い方