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Essentials of the Bash Shell: Loops and String Manipulation for Programming Linguists

In this lecture, we cover the fundamental concepts of using loops and string manipulation in the Bash shell. We explore for-loops, while-loops, and until-loops, with hands-on examples of creating backup files and manipulating command-line parameters. Additionally, we discuss arithmetic expansion, pathname expansion, and string manipulation techniques such as substring extraction and file renaming. These skills are essential for programming linguists to automate tasks and handle text efficiently in their projects.

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Essentials of the Bash Shell: Loops and String Manipulation for Programming Linguists

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  1. LING 408/508: Programming for Linguists Lecture 5 September 9th

  2. Administrivia • Reminder • BMI homework due tomorrow night

  3. Today's Topic • Another lecture on the bash shell …

  4. loops • For-loop: • for VAR [in LIST]; do …; done • Example: • create backup copies of files • (let's create a few .JPG files first) • for i in *.jpg; do echo "$i" ; done • for i in *.jpg; do cp "$i" "$i".orig ; done

  5. loops • while-loop: • while COND; do …; done • break (out of while-loop) #!/bin/bash line="" while true do read -p "Next word: " word line="$line $word" if [ ${#line} -gt 30 ] then break fi done echo ${#line}:$line exit 0 • until COND; do …; done

  6. Positional Parameters • command line to script (or function): • $0, $1, etc. • $# number of parameters passed • $* all parameters as a single word • $@ each parameter is a quoted string • shift removes one parameter for arg in "$*" do done for arg in "$@" do done

  7. Example • Sum all numbers supplied on commmand line #!/bin/bash sum=0 while [ $# -ne 0 ] do ((sum=sum+$1)) shift done echo $sum exit 0 shsum.sh 1 2 3 4 5 15 sh sum.sh 1 2 3 shsum.sh1 1 shsum.sh 0

  8. Expansion • Arithmetic expansion: • $(( … expression ..)) • x=$(($x+1)) • Pathname expansion (aka globbing): • similar (but not the same) as regular expressions • * (wild card string) • ? (wild card character) • [ab] (a or b) • [^ab] (not (a or b)) • (curly) Brace expansion: • mkdir ~/class/examples/{ex1,ex2} • echo x{1,2,3,4} Use command ls in conjunction with globbing

  9. String manipulation • String length: • ${#var} • Substring: • ${string:position} starting at position (0,1,2…), (-N) from the end • ${string:position:length} • Delete prefix: • ${string#substring} shortest match • ${string##substring} longest match • Delete suffix: • ${string%substring} shortest match • ${string%%substring} longest match • Substring substitution: • ${string/substring/replacement} replace first match • ${string//substring/replacement} replace all matches • Prefix substitution: ${string/#substring/replacement} • Suffix substitution: ${string/%substring/replacement}

  10. File extension renaming Shell script: #!/bin/bash if [ $# -ne 2 ]; then echo “usage: ext1 ext2” exit 1 fi for filename in *.$1 # Traverse list of files ending with 1st argument. do mv "$filename" "${filename%$1}$2" done exit 0 create a subdirectory with some .JPG files sh../rename_ext.sh JPG jpg

  11. File renaming • Example: • append a suffix -1 to all jpg files • for f in *.jpg; do mv "$f" "${f/./-1.}"; done • Levels of quoting: $ echo text ~/*.txt {a,b} $(echo foo) $((2+2)) $USER text /home/me/ls-output.txt a b foo 4 me $ echo "text ~/*.txt {a,b} $(echo foo) $((2+2)) $USER" text ~/*.txt {a,b} foo 4 me $ echo 'text ~/*.txt {a,b} $(echo foo) $((2+2)) $USER' text ~/*.txt {a,b} $(echo foo) $((2+2)) $USER http://linuxcommand.org

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