update comment

Use ctrl+d instead of ctrl+c

README.md

@@ -1145,6 +1145,29 @@ date --date @1615852800
 
 ```
 
+##### Print current time point for N days ago or N days after
+```bash
+# print current date first (for the following example)
+date +"%F %H:%M:%S"
+# 2023-03-11 16:17:09
+
+# print the time that is 1 day ago
+date -d"1 day ago" +"%F %H:%M:%S"
+# 2023-03-10 16:17:09
+
+# print the time that is 7 days ago
+date -d"7 days ago" +"%F %H:%M:%S"
+# 2023-03-04 16:17:09
+
+# print the time that is a week ago
+date -d"1 week ago" +"%F %H:%M:%S"
+# 2023-03-04 16:17:09
+
+# add 1 day to date
+date -d"-1 day ago" +"%F %H:%M:%S"
+# 2023-03-12 16:17:09
+```
+
 ##### wait for random duration (e.g. sleep 1-5 second, like adding a jitter)
 ```bash
 sleep $[ ( $RANDOM % 5 ) + 1 ]
@@ -2837,12 +2860,28 @@ var=$((var+1))
 cat filename|rev|cut -f1|rev
 ```
 
-##### Cat to a file
+##### Create or replace a file with contents
 ```bash
 cat >myfile
 let me add sth here
-exit by control + c
+# exit with ctrl+d
-^C
+
+# or using tee
+tee myfile
+let me add sth else here
+# exit with ctrl+d
+```
+
+##### Append to a file with contents
+```bash
+cat >>myfile
+let me add sth here
+# exit with ctrl+d
+
+# or using tee
+tee -a myfile
+let me add sth else here
+# exit with ctrl+d
 ```
 
 ##### Clear the contents of a file (e.g. filename)
@@ -3240,9 +3279,9 @@ fallocate -l 10G 10Gigfile
 
 ##### Create dummy file of certain size (e.g. 200mb)
 ```bash
-dd if=/dev/zero of=//dev/shm/200m bs=1024k count=200
+dd if=/dev/zero of=/dev/shm/200m bs=1024k count=200
 # or
-dd if=/dev/zero of=//dev/shm/200m bs=1M count=200
+dd if=/dev/zero of=/dev/shm/200m bs=1M count=200
 
 # Standard output:
 # 200+0 records in
@@ -3255,9 +3294,29 @@ dd if=/dev/zero of=//dev/shm/200m bs=1M count=200
 watch -n 1 wc -l filename
 ```
 
+##### Use Bash Strict Mode
+```bash
+# These options can make your code safer but, depending on how your pipeline is written, it might be too aggressive 
+# or it might not catch the errors that you are interested in
+
+# for reference see https://gist.github.com/mohanpedala/1e2ff5661761d3abd0385e8223e16425
+#               and https://mywiki.wooledge.org/BashPitfalls#set_-euo_pipefail
+
+set -o errexit      # exit immediately if a pipeline returns a non-zero status
+set -o errtrace     # trap ERR from shell functions, command substitutions, and commands from subshell
+set -o nounset      # treat unset variables as an error
+set -o pipefail     # pipe will exit with last non-zero status, if applicable
+set -Eue -o pipefail  # shorthand for above (pipefail has no short option)
+```
+
 ##### Print commands and their arguments when execute (e.g. echo `expr 10 + 20 `)
 ```bash
 set -x; echo `expr 10 + 20 `
+# or
+set -o xtrace; echo `expr 10 + 20 `
+
+# to turn it off..
+set +x
 ```
 
 ##### Print some meaningful sentences to you (install fortune first)