1/13: Course overview + the shell

Exercises

1

echo $SHELL

2

mkdir /tmp/missing

3

man touch

4

touch /tmp/missing/semester

5

echo '#!/bin/sh' >> /tmp/missing/semester
echo 'curl --head --silent https://missing.csail.mit.edu' >> /tmp/missing/semester

When I try to use echo "#!/bin/sh" >> /tmp/missing/semester, something strange occured, after I typed echo "#!/bin/sh" then I press the space key and found I cannot enter any more character with the following prompt:

bash: !/bin/bash: event not found

From the Bash quoting manual page:

• Escape Character	How to remove the special meaning from a single character.
• Single Quotes	How to inhibit all interpretation of a sequence of characters.
• Double Quotes	How to suppress most of the interpretation of a sequence of characters.
• ANSI-C Quoting	How to expand ANSI-C sequences in quoted strings.
• Locale Translation	How to translate strings into different languages.

命令行下，双引号里面用 !，Shell 会以为要执行历史展开，从而可能导致上面的报错（或者执行历史展开）。

6

/tmp/missing/semester

bash: /tmp/missing/semester: Permission denied

From ArchWiki, we can use the ls command's -l option to view the permissions (or file mode) set for the contents of a directory:

ls -l /tmp/missing/semester

-rw-rw-r-- 1 xyu xyu 61 Sep 10 21:09 /tmp/missing/semester

owner/group/others

7

sh /tmp/missing/semester

Why does this work, while ./semester didn’t?

When we type commands in shell, the first word is seen as the name of executable (if it is not absolute or relative path，shell will seek it in system path). However, we know that the semester script is not executable from the permissions listed above while sh is executable we can found in system path. For sh to "execute" the script, it only needs to be able to read the file.

8

man chmod

9

chmod + x/tmp/missing/semester

-rwxrwxr-x 1 xyu xyu 61 Sep 10 21:09 /tmp/missing/semester

Now the script is executable.

10

/tmp/missing/semester | sed -n '6p' > ~/last-modified.txt
cat ~/last-modified.txt

last-modified: Sat, 03 Sep 2022 12:25:26 GMT

11

sudo find -L /sys/class/power_supply -maxdepth 3 -name 'BAT*'
cat /sys/class/power_supply/BAT0/capacity_level
cat /sys/class/thermal/thermal_zone*/temp