Why can't add the header in the df's output?

Get all processes whose name contain firefox and exclude grep process,it is no use to show all processes here,omit many lines. ps aux | grep [f]irefox debian 7069 1.0 4.4 3134148 359168 ? Sl 11:58 0:12 /usr/lib/firefox-esr/firefox-esr debian 7128 0.0 0.4 223884 36824 ? Sl 11:58 0:00 /usr/lib/firefox-esr/firefox-esr -contentproc -parentBuildID 20241118130310 -prefsLen 28341 -prefMapSize 249085 -appDir /usr/lib/firefox-esr/browser {0c853969-95e1-4db0-9e95-eeaee3d4f814} 7069 true socket The output does not contain header info in ps's command ,in order to get the header,add head -n1 after the pipe. ps aux |(head -n 1 ;grep [f]irefox) USER PID %CPU %MEM VSZ RSS TTY STAT START TIME COMMAND debian 7069 0.9 4.4 3134148 361740 ? Sl 11:58 0:13 /usr/lib/firefox-esr/firefox-esr debian 7128 0.0 0.4 223884 36824 ? Sl 11:58 0:00 /usr/lib/firefox-esr/firefox-esr -contentproc -parentBuildID 20241118130310 -prefsLen 28341 -prefMapSize 249085 -appDir /usr/lib/firefox-esr/browser {0c853969-95e1-4db0-9e95-eeaee3d4f814} 7069 true socket Other bash command: df Filesystem 1K-blocks Used Available Use% Mounted on udev 3977796 0 3977796 0% /dev tmpfs 804900 1356 803544 1% /run /dev/sdb1 460349516 143209832 293681076 33% / tmpfs 4024488 100444 3924044 3% /dev/shm tmpfs 5120 16 5104 1% /run/lock df | grep shm tmpfs 4024488 101536 3922952 3% /dev/shm df |(head -n1; grep shm) Filesystem 1K-blocks Used Available Use% Mounted on Why can't get the below output when to execute df |(head -n1; grep shm)? df |(head -n1; grep shm) Filesystem 1K-blocks Used Available Use% Mounted on tmpfs 4024488 101536 3922952 3% /dev/shm Why grep in "ps aux |(head -n 1 ;grep [f]irefox)" can get lines to match? As expert user10489 point out: The command df |(head -n1; grep shm) does this: df generates some output head takes all of the output, prints the first line, and then quits throwing away all the rest of what it read. There is no output left for grep to take as input. Another post can explore it in depth: cat > raw.txt <Gilles Quénot get a simple solution--simple solution { head -2; sort -n; } {} get input redirect by ,if head -2; run as user10489 say:head takes all of the output, prints the first line, and then quits throwing away all the rest of what it read.,why sort -n have lines to sort with? The result would be ID DESCRIPTION ----- -------------- No lines sorted!!!