Use sponge utility to read standard input and safely write it out to the same file.
Prerequisites
Install moreutils package to use sponge utility.
$ sudo apt install moreutils
Case #1
Inspect fstab configuration file.
$ cat /etc/fstab
# /etc/fstab: static file system information. # # Use 'blkid' to print the universally unique identifier for a # device; this may be used with UUID= as a more robust way to name devices # that works even if disks are added and removed. See fstab(5). # # <file system> <mount point> <type> <options> <dump> <pass> /dev/mapper/vgubuntu-root / ext4 errors=remount-ro 0 1 # /boot was on /dev/sda1 during installation UUID=45aa534e-49f6-4593-9257-64de25d2da14 /boot ext4 defaults 0 2 /dev/mapper/vgubuntu-swap_1 none swap sw 0 0
Use it alter file contents, for example remove swap file system.
$ grep -v swap /etc/fstab | sudo sponge /etc/fstab
Inspect fstab configuration file again, swap file system was removed.
$ cat /etc/fstab
# /etc/fstab: static file system information. # # Use 'blkid' to print the universally unique identifier for a # device; this may be used with UUID= as a more robust way to name devices # that works even if disks are added and removed. See fstab(5). # # <file system> <mount point> <type> <options> <dump> <pass> /dev/mapper/vgubuntu-root / ext4 errors=remount-ro 0 1 # /boot was on /dev/sda1 during installation UUID=45aa534e-49f6-4593-9257-64de25d2da14 /boot ext4 defaults 0 2
Case #2
Inspect sample log file.
$ cat /var/log/kern.log.20201011
[...] Oct 11 16:13:34 desktop kernel: [3325726.332658] blk_update_request: I/O error, dev sdc, sector 244514032 op 0x0:(READ) flags 0x80700 phys_seg 2 prio class 0 Oct 11 16:13:34 desktop kernel: [3325726.334528] blk_update_request: I/O error, dev sdc, sector 244513792 op 0x0:(READ) flags 0x0 phys_seg 1 prio class 0 Oct 11 16:13:34 desktop kernel: [3325726.334531] Buffer I/O error on dev sdc1, logical block 30560128, async page read Oct 13 19:01:50 desktop kernel: [3508622.151379] audit: type=1400 audit(1602608510.241:470): apparmor="STATUS" operation="profile_replace" info="same as current profile, skipping" profile="unconfined" name="snap-update-ns.terraform" pid=374426 comm="apparmor_parser" Oct 13 19:01:50 desktop kernel: [3508622.159218] audit: type=1400 audit(1602608510.249:471): apparmor="STATUS" operation="profile_replace" info="same as current profile, skipping" profile="unconfined" name="snap.terraform.terraform" pid=374427 comm="apparmor_parser" [...]
Truncate it, preserve original user/group and keep only I/O error messages
$ grep -i "i/o error" /var/log/kern.log.20201011 | sudo -u syslog -g adm sponge /var/log/kern.log.20201011
Log file was altered.
$ sudo cat /var/log/kern.log.20201011
Oct 11 16:13:34 desktop kernel: [3325726.332658] blk_update_request: I/O error, dev sdc, sector 244514032 op 0x0:(READ) flags 0x80700 phys_seg 2 prio class 0 Oct 11 16:13:34 desktop kernel: [3325726.334528] blk_update_request: I/O error, dev sdc, sector 244513792 op 0x0:(READ) flags 0x0 phys_seg 1 prio class 0 Oct 11 16:13:34 desktop kernel: [3325726.334531] Buffer I/O error on dev sdc1, logical block 30560128, async page read
Reverse this log file.
$ tac /var/log/kern.log.20201011 | sudo -u syslog -g adm sponge /var/log/kern.log.20201011
Lines are in reverse order.
$ cat /var/log/kern.log.20201011
Oct 11 16:13:34 desktop kernel: [3325726.334531] Buffer I/O error on dev sdc1, logical block 30560128, async page read Oct 11 16:13:34 desktop kernel: [3325726.334528] blk_update_request: I/O error, dev sdc, sector 244513792 op 0x0:(READ) flags 0x0 phys_seg 1 prio class 0 Oct 11 16:13:34 desktop kernel: [3325726.332658] blk_update_request: I/O error, dev sdc, sector 244514032 op 0x0:(READ) flags 0x80700 phys_seg 2 prio class 0
Case #3
Look at this simple temperature log file.
$ cat temp.log
#${TEMPLATE_DATE}:${TEMPLATE_TEMP}
1600133692:24
1600937387:27
1601132241:23
1602135324:25
1603131093:28
Use sponge and envsubst utilities to append new entry.
$ grep TEMPLATE temp.log | TEMPLATE_DATE=$(date +%s) TEMPLATE_TEMP=22 envsubst | tr -d '#' | sponge -a temp.log
New entry was added to the log file.
$ cat temp.log
#${TEMPLATE_DATE}:${TEMPLATE_TEMP}
1600133692:24
1600937387:27
1601132241:23
1602135324:25
1603131093:28
1603135757:22