Scripts: loops

for over a list, wildcards or "$@", while and until, while read to read a file line by line, break and continue: tracing a loop and avoiding the trap of names containing spaces.

25 cards
4 topics
Free

Study this deck

The outline, topic by topic

Each topic is weighted the way the exam weighs it.

Concepts
Questions
Sort and link
Scenarios

A few cards from the deck

For loop: what is it?
Show the answer
A loop that runs its body once per word of a list fixed at the start, storing that word in its variable on each iteration.
for i in 1 2 3 runs three iterations; for i in $(seq 3 6) runs four, with 3, 4, 5 then 6.
What does this script print?

```bash
i=0
while [ $i -lt 5 ]; do
i=$((i + 2))
done
echo $i
`
- 6
- 5
- 4
- 2, 4 then 6
Show the answer
6
i takes 0, 2, 4 then 6. The condition is only tested at the start of each iteration: at 4 it is true, i becomes 6, and on the next iteration it is false.
The body of a for loop sits between … and ….
Show the answer
The body of a for loop sits between do and done.
do opens the body and done closes it, for for, while and until alike.
while and until loops: what is it?
Show the answer
Loops driven by a condition command: one repeats as long as it returns 0, the other repeats until it returns 0.
while [ $n -gt 0 ] and until [ $n -le 0 ] describe the same loop: only the wording of the condition is inverted.
A renaming script breaks on "my report.txt", treated as two files. It starts with for f in $(ls *.txt). Which fix is enough?
- Write for f in $(ls -1 *.txt), which puts one name per line
- Replace for with while, which does not split words
- Use single quotes: for f in '$(ls *.txt)'
- Write for f in *.txt and quote "$f" in the loop body
Show the answer
Write for f in *.txt and quote "$f" in the loop body
The splitting comes from the result of $(ls) being re-read by the shell. The wildcard supplies whole names. Quoting "$f" avoids a second split in mv.

Study this deck